Hierarchical skeletal architecture and ecological tradeoffs in Pacific Northwest sea stars

Hierarchical skeletal architecture and ecological tradeoffs in Pacific Northwest sea stars | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Hierarchical skeletal architecture and ecological tradeoffs in Pacific Northwest sea stars Karly Cohen, Cassandra Donatelli, Adam Summers, Mo Turner This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8739467/v1 This work is licensed under a CC BY 4.0 License Status: Under Revision Version 1 posted 11 You are reading this latest preprint version Abstract Sea star armor comes in the form of a highly articulated endoskeleton made up of individual elements called ossicles. Many descriptive studies have been conducted on the basic patterning of sea star skeletons, with differences in ossicle shape forming the basis of some echinoderm phylogenies. However, ossicle function is not related only to individual element morphology, but rather the whole system. In this study, we use micro-computed tomography (CT) to describe and compare skeletal anatomy of nine sea star species from the Salish Sea, Washington, USA. We quantified 14 morphological traits and tested whether or not they were predictors of ecology. We expected to see that differences in the amount of armoring (relative volume of skeleton) arise from varying arrangement and shape of ossicles across distinct regions of the body. For broad comparability, we grouped skeletal elements into five basic types of ossicles. The amount of skeletal armoring across the body varied by at least an order of magnitude across species and differed in its distribution across ossicle types. Heavily armored sea stars invest in larger, boxy body wall ossicles, whereas a reduction in armor volume was often paired with more intricately-shaped body wall ossicles and an increase in the number and complexity of spines. ossicles starfish sea star skeleton echinoderm armor Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Introduction Armoring evolves frequently in nature due to its role as protection, offense, camouflage, and structural support (Edmunds, 1974 ). Biological armor can be found across phyla, manifesting as shells, scales, thickened skin, spines, horns, and endo- or exoskeletons, each of which is accompanied by its own suite of tradeoffs (Cohen et al., 2025 ; Hoover et al., 2023 ; Lakowitz et al., 2008 ; Laver et al., 2020 ; Taylor & Patek, 2010 ; Woodruff et al., 2022 ; Yang et al., 2012 ). In echinoderms, those tradeoffs are expressed not only in mineral investment, but in how the body wall itself constrains (and enables) locomotion, feeding, and habitat use, making body-wall mechanics a key determinant of ecological opportunity (O’Neill, 1989 ). For invertebrates, the most common material used for armoring is calcium carbonate (CaCO 3 ; Vermeij, 1989 ). The stiffness and toughness of calcium carbonate structures can be enhanced by addition of other materials like iron or chitin, while the shape and interactions between elements will also affect the behavior of the aggregate structure. Increasing armor mass (coverage, thickness, or density) requires energy investment and is heavier (Kruppert et al., 2020 ; Lakowitz et al., 2008 ), so increased protection may come at the expense of speed, maneuverability and flexibility. For example, hermit crabs are slower and burn more energy to move when burdened with the protection of a snail shell than when they are naked (Herreid & Full, 1986 ). Increased armor often comes at the cost of maneuverability and flexibility, with movement only possible at junctions between armor plates (Hazerli & Richter, 2020 ). Sea stars (Asteroidea, Echinodermata) are exclusively benthic as adults and can be found from tropical to polar latitudes and from mid intertidal zones to the deepest ocean trenches (Birkeland, n.d.; Dayton et al., 1974 ; McClintock, 1994 ; Zhang et al., 2024 ). They exhibit a wide range in diet, from scavengers and suspension feeders to active predators, cannibals, and specialist consumers of other sea star species (Lawrence, 2013 ). Sea stars armor is a highly articulated endoskeleton made up of small calcium carbonate elements called ossicles. It creates a scaffolding just below the dermis that supports and protects the internal organs (O’Neill, 1989 ). Stars are usually heavily armored and are considered slow or even sedentary. However, many sea stars are active predators, with the fastest species, Pycnopodia helianthoides , reaching speeds of up to 1.5m per minute (Po et al., 2024 ; Mauzey et al., 1968 ). The endoskeleton creates a scaffolding just below the dermis that supports and protects the internal organs (O’Neill, 1989 ). In many of their ecosystems, sea stars are the apex predator and, in some cases, a keystone species (Paine, 1966 , 1969 ), so understanding how armor shape and arrangement affects defensibility and behavior is essential to our understanding of their ecology and distribution. Sea star skeletons are composed of numerous discrete calcitic elements termed ossicles, which together form a highly articulated internal endoskeleton embedded within the dermis. Each ossicle is a stereom—an open, lattice-like microstructure of calcite—whose external shape, thickness, and pattern of articulation with neighboring elements vary systematically across body regions and among taxa. Ossicles are traditionally described and classified based on their position, serial arrangement, and associated structures (such as spines or pedicellariae), leading to regionally defined series including ambulacral (AMO), adambulacral (ADO), carinal (CO), reticular (RO, sometimes defined more broadly as abactinal), and marginal (MO) (Blake, 1987 ; Blowes et al., 2017 ; Eylers, 1976 ; Schwertmann et al., 2019 ; Fig. 1). The adambulacral and ambulacral ossicles together form the ambulacral groove, creating a concave space on the oral side of the body that houses the soft tissue tube feet. The carinal, reticular, and marginal ossicles together make up the aboral body wall of the endoskeleton (Schwertmann et al., 2019 ). The carinal ossicles are a central row of ossicles that run along the top of the aboral side of the body wall from ray tip to the center of the sea star. Marginal ossicles are a collection of ossicles that connect the body wall to the ambulacral groove. The remaining meshwork of the body wall consists of reticular ossicles. Although the presence, differentiation, and homology of particular ossicle series can vary among clades and through ontogeny, these elements are generally treated as repeating anatomical units whose size, shape, and spatial distribution collectively determine skeletal architecture rather than functioning as isolated structures (Blake 1987 ; Fau & Villier 2018, 2020 ). Many forcipulataceans (especially asteriids) have pedicellariae (PED), small pinching claws supported by a flexible stalk, covering the aboral surface of the body wall (Fau & Villier 2020 ; Blake, 1987 ). The claws but not the stalks of the pedicellariae are made of calcium carbonate and therefore are visible as floating ossicles in an x-ray or CT scan (Fig. 1). Descriptive studies on the basic patterning of sea star skeletons and the shapes of individual ossicles highlight that their shapes differ substantially and can be diagnostic of particular taxa (e.g. Blowes et al., 2017 ; Fau & Villier, 2020 , 2024; Gale, 2018 ; Tomholt et al., 2023 ). In this study, we use micro-computed tomography x-ray imaging (µCT) to measure and compare morphological differences in the endoskeleton of nine sea star species from the Salish Sea in the northwest of Washington, USA. We qualitatively describe general patterns of shapes and arrangements of the ossicles and spines. Because asteroid skeletons are assembled from a shared set of ossicle “building blocks” that can be arranged and elaborated in many ways, we use µCT to shift from descriptive comparisons toward repeatable, quantitative characterization of skeletal architecture. We emphasize that our goal is not to redescribe classical taxonomy or propose new diagnoses, but rather to harmonize three-dimensional skeletal traits into a comparative character matrix that can be mapped onto ecology and phylogeny. In practice, this requires collapsing some lineage-specific terminology and fine-scale subtypes into higher-level trait bins that are broadly homologous and measurable across taxa (e.g., ambulacral, adambulacral, marginal, carinal, and reticular/abactinal skeletal elements, Fau & Villier 2024). Doing so allows us to ask predictive questions about how lineages working with the same skeletal components balance feeding, mobility, protection, offense, and camouflage across ecological contexts.In this study we aim to: 1) describe the skeletal morphology of nine species of sea stars 2) calculate armor ratios across the body, and 3) quantify how armoring is related to behavior and ecology. Methods Specimen collection and micro-computed tomography We surveyed nine species of sea stars representing six families: Solasteridae ( Crossaster papposus , Solaster stimpsoni ), Asteriidae ( Pisaster ochraceus , Evasterias troschelii , Leptasterias sp. complex), Pterasteridae ( Pteraster tesselatus ), Dermasteriidae ( Dermasterias imbricata ), Echinasteridae ( Henricia sp. complex), and Mediasteridae ( Mediaster aequalis ). These taxa span a range of skeletal architectures that have been recognized in classical morphological treatments for over a century (e.g. Fisher 1911, 1928, 1930), including differences in ossicle shape, articulation, and spine number. As expected given their shared evolutionary history, closely related species within families exhibit similarities in skeletal characters; this study explicitly evaluates how such phylogenetically structured morphology compares to ecological similarity across co-occurring taxa. Sea stars were collected in the San Juan archipelago (San Juan County, WA, USA) from tidepools by hand and the subtidal environment by SCUBA diving and trawling. Sea stars were relaxed in magnesium chloride, then put in 10% formaldehyde for 24 hours, after which they were rinsed in water and dehydrated to 70% ethanol for storage. Sea stars were µCT scanned wrapped in cheesecloth wetted with 70% ethanol. All specimens were µCT scanned with a Bruker SkyScan 1173, using the high-resolution 2k detector, a 1mm aluminum filter, and voltages ranging from 80-133kV and amperages ranging from 40–80µA (Table 1). Groups of whole specimens were wrapped in damp cheesecloth and placed together into a 3D printed plastic cylinder. The entire cylinder was wrapped in plastic film to ensure it was airtight, then mounted on a rotating stage in the scanner and scanned with voxel size ranging from 30–35µm. Once the whole specimen scans were completed, approximately 10mm of a ray tip from each individual was removed using a scalpel and scanned again at a voxel size of 8µm for high resolution images of the arms. Images were reconstructed with nRecon (Bruker Systems) and visualized with 3D Slicer (Fedorov et al., 2012 ). Table 1 µCT scan specifications using a Bruker SkuScan 1173 (high-resolution 2k detector and a 1mm aluminum filter) for each of the 9 sea star species, scanned at the scale of the whole sea star and a tip of one ray. Species Resolution (um) Voltage (kV) Power (uA) Exposure (ms) Crossaster papposus 29.1 55 143 1475 Dermasterias imbricata 35.7 65 123 1185 Evasterias troschelii 29.1 55 143 1475 Henricia sp. 35.7 65 123 1185 Leptasterias sp. 35.7 65 123 1185 Mediaster aequalis 31 65 123 1165 Pisaster ochraceus 35.7 65 123 1185 Pteraster tesselatus 35.7 65 123 1185 Solaster stimpsoni 31 65 123 1165 Ray tips of every species 8.9 65 123 1135 Segmentation and measurements Skeletal morphology and body tissue were segmented in the open source platform 3D Slicer (Fedorov et al., 2012 ) using grayscale intensity thresholding, grow seeds, and manual slice-by-slice editing in the segment editor module. In addition to creating segments of the different regions (body wall and ambulacral groove) and types of ossicles, we also isolated a cross-sectional unit of the ray representing one set of repeated ossicles (here referred to as a “ray band”). Volume (mm 3 ) of each segment was measured using the Segment Statistics module. The total specimen volume included tissue, skeleton, and the internal body cavity. Since the specimens differed in absolute size, it was necessary to quantify traits that would enable comparison among species without a confounding effect of size, e.g. more total ossicle volume in larger specimens. Size correction was carried out in most cases by calculating ratios among body parts, which then emphasized shape rather than size characteristics (Table 2). Ray length for each specimen was measured from the tip of the longest ray to the center of the central disc. The central disc radius was measured from the interradius (junction where two arms meet the central disc) to the center of the sea star. Table 2 Fourteen morphological traits defined in this study across varying scale of measurement, from whole specimen to individual ossicles for nine species of sea stars. Scale Armor Ratios Ossicle size and organization Whole specimen Skeleton : whole body volume Central disc radius : longest ray length Ray (whole) Skeleton : whole ray volume No. of rays Ray (cross-section) Surface area : volume No. of spine clusters / ray band Ambulacral groove Ambulacral groove : ray skeleton No. of spines / adambulacral ossicle Body wall Carinal (ossicle + spine) : whole ray band volume Presence/absence of pedicellariae Spines All spines : whole ray band volume No. of spines / cluster Mouth Oral spine length : central disc radius No. of spines / pair of oral plates Skeletal traits We examined the µCT-scan of each species and qualitatively described differences in endoskeletal arrangement of ossicles in terms of body wall organization, body wall ossicle shape, and spines (Table 4). We quantified numerical trait values to assess the number and robustness of skeletal components at different scales, ranging from individual ossicles to the whole organism (Table 2, Table S.1-S.2). Specifically, we measured the skeletal volume of each species as a proxy for armor investment. In addition to total body armor ratio, we examined armoring in four ways: (1) the relative armoring of ray tips, (2) the relative armoring around the ambulacral groove, (3) the surface area-to-volume ratio of ossicles within a ray band as an index of skeletal surface complexity, and (4) the external shape characterized by ray length relative to central disc size. For mouth traits, we analyzed the number of and length of oral spines (length standardized to body size). Asteroid skeletal terminology varies across clades and historical treatments, and individual ossicle series may be subdivided differently depending on taxonomic context. To enable consistent cross-taxon comparison, we standardized nomenclature to major, widely recognized ossicle systems (ambulacral, adambulacral, marginal, carinal, and reticular/abactinal body-wall ossicles) and scored traits at the level of ossicle geometry, packing, articulation, and spine support that can be reliably extracted from µCT volumes. Where published sources or clade-specific traditions recognize additional subcategories (e.g., specialized abactinal derivatives such as paxillae, or multiple plate fields within the body wall), we collapsed those into broader bins when the finer distinctions could not be applied consistently across all taxa. This approach intentionally prioritizes repeatability and comparability over taxonomic redescription, producing a character matrix suited for hypothesis testing and prediction rather than diagnosis. Comparison of morphological, ecological, and phylogenetic distance matrices We calculated three distance matrices across the nine species based on morphological traits, ecological traits, and phylogenetic relationships. Ecological traits (diet, habitat use, and known predators) were compiled from multiple literature sources (Flowers & Foltz, 2001 ; Kozloff et al., 1996 ; Mauzey et al., 1968 ; Rodenhouse & Guberlet, 1946 ) and converted into discrete categories to reflect broad ecological similarity rather than fine-scale niche partitioning. Importantly, habitat categories were intended to capture typical habitat use rather than absolute depth limits, as several taxa commonly associated with intertidal environments (e.g. Pisaster , Evasterias , Leptasterias ) are known to occur across a wider depth range in parts of their geographic distribution. For the ecological (Table 3) and morphological (Table 2) traits, dissimilarities were calculated using Gower’s distances (Laliberté et al., 2014) for each pairwise species comparison, an approach appropriate for mixed categorical and continuous data. Phylogenetic distances were based on a consensus molecular phylogeny incorporating nuclear (18S rDNA, 28S rDNA, histone H3) and mitochondrial markers (12S rDNA, 16S rDNA, tRNA cluster, cytochrome c oxidase I) (Janies et al., 2011 ). These distances are based on branch lengths from published, time-calibrated molecular phylogenies, rather than on raw pairwise genetic distances. Specifically, distances represent the summed branch length separating each pair of taxa. Because Mediaster aequalis was not included in this tree, we used an additional published phylogeny (Mah & Foltz, 2011 ) containing that species and overlapping taxa to estimate relative phylogenetic distances. Correlations between morphological distance and ecological and phylogenetic distance matrices were tested statistically. Table 3 Ecological traits defined in this study for nine species of sea stars. Ecological distances were calculated from 12 binary traits representing diet, habitat, and predators. Categories Binary Columns: each trait scored as 0/1 for each species Diet Cnidarians, sponge, gastropods, bivalves, barnacles, tunicates, echinoderms Habitat Rock, cobble, sand/mud Predator Echinoderms, (removed gulls due to high correlation with “intertidal”) Depth Intertidal Statistical analysis of skeletal and ecological traits We use linear discriminant analysis (LDA) to determine which skeletal traits best predict ecological traits (diet, habitat, and depth) (R Core Team, 2020; Ripley & Venables, 2009 ; Fig. 13). Diet was separated into three categories based on prey type: soft-bodied only, hard-bodied only, and a mix of both; habitat was separated into two categories: rocky or mixed; depth was separated into two categories: intertidal and subtidal (Table S.3). Many of the skeletal traits were highly correlated (Table S.4), so we used six of the fourteen skeletal traits (Table S.1-S.2) in our LD models to avoid collinearity (Wei & Simko, 2010 ). In addition to the LDA, we ran pairwise t-tests on all fourteen skeletal traits to quantify significant differences between diet, habitat, and depth groups (Fig. 7). Results Overall comparative morphology We identified three general patterns of ossicle organization across the nine species (Figs. 2–4). First, several taxa exhibited an evenly distributed, grid-like abactinal skeleton in which ossicles form a relatively regular, tessellated field across the body wall (e.g., M. aequalis , Fig. 2A′–M′). Second, other taxa showed ossicles arranged as one or more discrete, longitudinal series that extend from the ray tip toward the central disc, producing clear ray-wise patterning (e.g., an abactinal/cranial-aligned series in Leptasterias sp ., Fig. 3N–Z). Third, some species possessed a reticular abactinal skeleton, where ossicles are organized as an interconnected meshwork that defines polygonal “cells” (e.g., C. papposus , Fig. 4A–M). Consistent with recent descriptions of sea star skeletal architecture emphasizing modular, hierarchical organization of stereo elements, these patterns reflect differences in how ossicles are packaged into either (i) broadly uniform fields, (ii) ray-parallel series, or (iii) reticulate networks that couple adjacent ossicles across the body wall. Across species, the shape and size distribution of ossicles comprising the abactinal and actinal body wall ranged from relatively uniform elements (e.g., Henricia sp ., Fig. 3A–M) to heterogeneous, size-structured networks in which larger “nodes” are linked by smaller, elongated connecting ossicles (e.g., P. ochraceus , Fig. 4N–Z). This latter condition produces a more explicitly hierarchical mesh—large ossicles acting as structural hubs connected by narrower struts—whereas the more uniform condition yields a comparatively regular tiling of similarly sized elements. All species possessed spines associated with abactinal and/or adambulacral ossicles, except D. imbricata , in which spines were restricted to the adambulacral series bordering the ambulacral groove (Fig. 2N–Z). In the remaining species, spines were either distributed broadly and consistently across the most abactinal body-wall ossicles (e.g., P. tesselatus , Fig. 4A′–M′) or localized to select larger ossicles, including abactinal and/or marginal elements (e.g., E. troschelii, Fig. 2A–M; Table 4). Table 4 Qualitative comparison of endoskeletal arrangement. Descriptions of patterns observed across three scales of ossicle organization and the corresponding sea star species that exhibited that pattern. Scale Pattern Species Body wall organization 1. One or more distinct rows of ossicles running from ray tip to central disc Evasterias, Leptasterias, Pisaster 2. Grid-like, evenly distributed Henricia, Mediaster, Pteraster, Solaster 3. Meshwork of ossicles form regular polygons Crossaster, Dermasterias Body wall ossicle shape 1. Uniform shape and size throughout body wall Henricia, Leptasterias, Mediaster, Pteraster, Solaster 2. A few larger ossicles connected to each other by a webbing of thin, elongated ossicles Crossaster, Evasterias, Leptasterias, Pisaster Spines 1. No spines on the body Dermasterias 2. Spines on select, larger ossicles Crossaster, Evasterias, Pisaster 3. Spines on every ossicle Henricia, Leptasterias, Pteraster, Solaster Armor volume ratios Ossicles varied significantly in all calculated armor metrics (Table S.1). Total body armoring (volume of skeleton relative to total body volume) ranged from 3% to 39%, with P. tesselatus having the least amount of armor and M. aequalis and Leptasterias sp. having the most (37 and 39% respectively). For all nine species, the ray tips were more heavily armored than the body as a whole (range 27–69%). In the least armored sea star, the abactinal and lateral body wall skeleton made up 38% of the mineral, while the ambulacral and adambulacral skeleton accounted for the remaining 62%, while in more heavily armored stars the dorsal and lateral made up as much as 77% of the mineral (Fig. 5). This increase in body wall ossicles is paired with a decrease in surface area to volume ratio, calculated from the cross-sectional ray band. As armoring (skeletal investment) increases across the body, the overall shape becomes less convoluted. In general, body shape shifts from short to long rays relative to the length of the central disc. Oral spine length (relative to radius of the central disc) is not correlated with number of oral spines per pair of oral plates, however the two sea stars with the greatest number of rays ( C. papposus and S. stimpsoni ) also had the greatest number of oral spines per pair of oral plates. Linear discriminant (LD) analysis The LD models showed that skeletal traits are sufficient to differentiate between depth, habitat, and diet (Fig. 6). In our depth model, the number of oral spines contributes most to the separation between intertidal and subtidal species with subtidal stars having more oral spines than intertidal stars (Table 5, Fig. 6A). In our habitat model, the number of oral spines, total body armor ratio, and ray-to-disc ratio contribute most to the separation between sea stars that live on rocky substrate and sea stars that live on mixed substrate (Table 5, Fig. 6B). Stars that live on rocky substrate have fewer oral spines, less armor, and shorter arms than stars that live on mixed substrates like cobble, sand, or mud. In our diet model, LD 1 accounted for 98.86% of separation and split seastars that prey on soft bodied organisms from sea stars that prey on hard-bodied organisms and those that had a mixed diet (Fig. 6C). Stars that feed on soft prey have fewer arms and more armor in the ambulacral groove than stars that eat hard prey. Conversely, stars that eat hard prey have more arms and less armor in their ambulacral grove. LD 2 accounts for only 1.14% of the separation between groups and is driven by the number of oral spines(Table 5). Table 5 Loadings for three LD models. Traits that had the largest effect on separation between groups are bold. Morphological traits Depth Habitat Diet LD1 LD1 LD1 LD2 Number of rays -0.0449 0.4165 -5.1139 -1.122 Number of oral spines / pair of oral plates 2.71124 -1.6587 2.81844 2.07809 Skeleton : whole body volume ratio 0.05698 -1.2835 -0.0341 0.87208 Central disc radius : longest ray length ratio -0.3367 -1.2812 1.81177 -0.2971 Ray skeleton : whole ray volume ratio 0.50699 0.58854 2.34827 -1.2469 Ambulacral groove : ray skeleton ratio 0.36851 -0.4981 4.39688 -1.4021 Pairwise t-tests showed that the only traits with significant differences between groups were branching (p = 0.023), number of oral spines (p < 0.001), and number of spines on the adambulacral ossicles (p = 0.01) between intertidal and subtidal groups (Fig. 7). Phylogeny and ecology We compared pairwise differences in skeletal traits among nine sea star species against phylogenetic distances (from Janies et al., 2011 and Mah & Foltz, 2011 ). Morphological variation in armor traits showed a weak but significant correlation with phylogenetic distance (Pearson’s correlation: r = 0.44, p = 0.007; Fig. 7). Three species, E. troschelii , P. ochraceus , and Leptasterias sp., showed pairwise similarities in both phylogeny and morphology. In contrast, these species exhibited high phylogenetic divergence from Henricia sp., despite displaying convergent morphological traits. Morphological distance also had a positive but weak correlation with ecological distance (p = 0.03, C = 0.37; Fig. 7). Correlation of phylogenetic and ecological distance (p = 0.02, C = 0.39) was high relative to how well ecological distance correlated with morphological relationships but low relative to how well phylogenetic distance correlated with morphological relationships. Discussion Like trabecular bone, the support structures of woody plants, and the foamy exterior rind of citrus fruits, the endoskeleton of sea stars is a classic hierarchical structure with morphological variation at levels above the diagnostic shapes of particular ossicles (Fratzl & Weinkamer, 2007 ). The diversity in skeletal structure and ossicle arrangement across sea star species reflects their varied ecological strategies and functional needs. The slime star, P. tesselatus , for example, has reduced ossicles and long spines that support a flexible external body wall (supradorsal membrane), likely allowing the animal to swell and exude viscous slime for defense when threatened (Nance, 1981 ; Rodenhouse & Guberlet, 1946 ). In contrast, D. imbricata and M. aequalis share a pattern of voluminous, imbricated ossicles along their ray edges, reinforcing their flexible body walls to capture and hold soft prey, providing both adaptability and structural support. Henricia sp. Are generally considered suspension feeders, with some studies suggesting predation on sea sponges (Cossi et al., 2021; Shield and Witman 1993; Ferguson 1969 ; Chichvarkin et al., 2019). They have small uniform ossicles which allow flexibility, aiding in a range of behaviors from locomotion to defense, and their widespread habitat range suggests uniform ossicle shapes may support adaptability to various environmental conditions. These morphological differences suggest that closely related species may evolve distinct structures to adapt to their specific environments. Of course, in some cases, phylogeny rules as in the three asteriids in our dataset, P. ochraceus , E. troschelii , and Leptasterias sp., which all have similar ossicle organization forming a grid-like pattern with a distinct carinal ridge and pedicellariae covering the entire body wall. The grid-like skeletal pattern with protruding spines enhances stiffness in some directions, increasing protection from wave action. Five arms represent the ancestral and most common body plan in sea stars, but multiple lineages have independently evolved increases in arm number. In our dataset, species with more than five arms are associated with predatory strategies that involve manipulating or overpowering hard or resistant prey such as bivalves, other echinoderms, and crustaceans (Fig. 13.C) (Mauzey et al., 1968 ; Rahman et al., 2018). However, increased arm number is not universally linked to macrophagy or durophagy across Asteroidea. Notable counterexamples include members of the order Brisingida, which possess numerous elongate arms but are suspension feeders adapted to deep-sea environments, and Acanthaster species, which use multiple arms during feeding but specialize on coral tissue rather than mechanically resistant prey (Emson & Chesher, 1976; Lawrence, 2013 ). Our data show that other factors such as number of oral spines may be more associated with depth and filter feeding (Fig. 13.A). Rather than serving as a simple proxy for predatory intensity, increased arm number may expand the functional envelope of sea stars by enhancing force distribution, prey handling surface area, or behavioral flexibility depending on ecological context. These interactions are also likely to be time-dependent: dorsal body-wall tissue exhibits strong stress relaxation consistent with linear viscoelastic behavior, meaning the effective stiffness of the body wall can shift over seconds to minutes under sustained loading. This provides a mechanistic route by which similar skeletal architectures could support different behaviors depending on neuromuscular state and loading history (O’Neill, 1989 ). In addition to increased arm number, stars that feed on hard prey also have less armoring in their ambulacral groove. Less armor means more flexibility and an easier time conforming to extremely rigid prey items (Fig. 13C). This flexibility may grant stars mechanical leverage while trying to pry open difficult to access prey. This interpretation aligns with growing interest in the biomechanics of the tube foot system, where recent work has emphasized the importance of coordination, speed, and force generation during prey capture and locomotion (Hennebert et al., 2010 ; Heydari et al., 2020 ; Ellers et al., 2024 ; Po et al., 2024 ). Although direct performance data remain limited, our findings suggest that variation in ambulacral ossicle number, density, and interdigitation provides a structural framework that may underlie differences in functional capacity. As quantitative measurements of tube foot performance become available, mapping these data onto three-dimensional skeletal architecture will be essential for testing how arm number and internal morphology jointly shape feeding performance. We see two strategies for defense embedded in these ossicle data. Some stars are defended like tortoises with boxy, heavily mineralized ossicles, while others are porcupine-like in their spikiness. The tradeoff we see between ossicle density—both in volume and proximity—and the presence of spines is extreme, with skeletal armoring varying by an order of magnitude or more in certain species (Fig. 12). We propose that this metaphor may extend beyond a tradeoff in morphology to an explanation of locomotor performance with the tortoise stars being slower and less agile than those with a pincushion style of defense. For example, one of the fastest sea stars, the many armed sea star ( Pycnopodia helianthoides ) is speedy, spry, and covered in spiny ossicles (Mauzey et al., 1968 ; Montgomery, 2014 ). This is similar to the crown-of-thorns ( Acanthaster planci ), another spiny, fast-moving porcupine of a sea star. In contrast, the doughboy seastar ( Choriaster granulatus ) is a tortoise – smooth, stiff-bodied, and slow-moving denizen of sandy substrates (Montgomery & Palmer, 2012 ; Moran, 1988 ). Locomotion, predator-prey dynamics, and other ontogenetic pressures complicate the relationship between ossicle morphology and ecological factors (Fig. 15). For instance, the types of predators that feed on sea stars may shape ossicle investment more than their habitat (Eigen et al., 2022 ; Montgomery, 2014 ; Rudykh et al., 2015 ; Van Veldhuizen & Oakes, 1981 ). If a star is preyed upon by a fast-moving, pinching crab, it is likely to evolve different armor compared to a star whose main predator is a conspecific. Conversely, in predatory sea stars, the demands of efficient locomotion and flexibility during feeding may be associated with convergent ossicle patterning across lineages, even where underlying phylogenetic relationships differ (Fau & Villier, 2020 ). Micro–computed tomography (µCT) has emerged as a powerful imaging tool for resolving internal morphology in three dimensions, yet its application in invertebrate systems has only begun to expand in earnest within the past five years. This lag reflects genuine technical challenges that are far less pronounced in vertebrate imaging, including the prevalence of hydrostatic skeletons, weakly mineralized or compositionally heterogeneous tissues, limited affinity of contrast agents for many invertebrate materials, and the frequent collapse or distortion of critical anatomical features during fixation and dehydration. These issues have historically constrained the use of µCT for invertebrates to descriptive studies or isolated anatomical reconstructions rather than comparative, character-based analyses (Ziegler, 2019 ; Blowes et al., 2017 ). Here, we demonstrate that µCT can be used not only to visualize sea star skeletal morphology, but also to generate repeatable, quantifiable character matrices that distinguish phylogenetically conserved features from those that are more evolutionarily labile. By resolving ossicle architecture, articulation, and spatial organization in three dimensions, µCT enables the construction of datasets that are directly comparable across taxa and amenable to hypothesis-driven analyses of armor evolution and function. Importantly, three-dimensional imaging also provides a platform for exploring functional trade-offs that are otherwise difficult to infer from external morphology alone. CT-derived models allow explicit consideration of how sea stars balance mobility, protection, and predation risk, including how skeletal elements constrain or accommodate soft-tissue insertion, flexibility, and mechanical compromise. Such data open new avenues for testing how protective structures evolve in systems where defense, locomotion, and feeding are tightly integrated rather than modular. Finally, the value of µCT data increases substantially when scans are archived, reused, and integrated into open, comparative frameworks. Vertebrate biology has benefited enormously from centralized, open-access CT repositories (e.g. MorphoSource and the oVert initiative), and our results underscore the importance of extending similar infrastructure to invertebrate systems. Declarations Research Highlights Although most sea stars share basic ossicle types, the distribution, size, and shape vary across species, reflecting a tradeoff in armoring. These differences correlate more strongly with phylogenetic relationships than with ecological factors. Author Contribution M.S., C.M.D., and K.E.C. contributed to specimen collection and data acquisition. K.E.C., M.S., and C.M.D. conducted data analysis, generated figures, and led manuscript drafting and revision. A.P.S. contributed to study design, data interpretation, manuscript editing, and provided funding, equipment, and technical support. All authors approved the final manuscript. Acknowledgement Thank you to the Karl Liem Imaging Facility at Friday Harbor Laboratories for allowing me access and use of the imaging equipment. Stephen and Ruth Wainwright Endowed Fellowship. Anne Hof Blinks Fellowship. NSF DBI-2301407 to CMD and NSF DBI-2301406 to APS. Data Availability All data supporting the findings of this study are available from the corresponding authors upon reasonable request and are archived on GitHub. References Birkeland C (n.d.) (ed) The influence of echinoderms on coral-reef communities. 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Zoosymposia15 Additional Figures Figure numbers 12, 13, and 15 are not available with this version. Additional Declarations No competing interests reported. Supplementary Files SupplementalTables.xlsx Cite Share Download PDF Status: Under Revision Version 1 posted Editorial decision: Revision requested 22 Feb, 2026 Reviews received at journal 20 Feb, 2026 Reviews received at journal 17 Feb, 2026 Reviews received at journal 16 Feb, 2026 Reviewers agreed at journal 14 Feb, 2026 Reviewers agreed at journal 13 Feb, 2026 Reviewers agreed at journal 03 Feb, 2026 Reviewers invited by journal 01 Feb, 2026 Editor assigned by journal 30 Jan, 2026 Submission checks completed at journal 30 Jan, 2026 First submitted to journal 30 Jan, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8739467","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":583140004,"identity":"2e3c111e-f912-4d7f-82cf-7097ef13219e","order_by":0,"name":"Karly Cohen","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAq0lEQVRIiWNgGAWjYBACCSBmBmI5MI+HFC3GpGtJbCBai2R788PHBRV30jfcSGB88LaNCC3SPMeMjWeceZYL1MJsOJcYLXISOWzSvG2HczfcTgAxiNEi/wao8t/hdIPbCey/idIiLcED1NJwOAGohY2ZKC2SPWnGxjzHDhvOvP+wWXLOOSK0SBw//PAxT81heb4zhw9+eFNGhBYkwNhAmvpRMApGwSgYBbgBAI+tM5FoPLaoAAAAAElFTkSuQmCC","orcid":"","institution":"University of Washington","correspondingAuthor":true,"prefix":"","firstName":"Karly","middleName":"","lastName":"Cohen","suffix":""},{"id":583140005,"identity":"ed165ecf-8837-4c14-a258-4c358fcda9bb","order_by":1,"name":"Cassandra Donatelli","email":"","orcid":"","institution":"University of Washington","correspondingAuthor":false,"prefix":"","firstName":"Cassandra","middleName":"","lastName":"Donatelli","suffix":""},{"id":583140006,"identity":"2c028af0-1bb7-4ffb-acfb-a28e062715f7","order_by":2,"name":"Adam Summers","email":"","orcid":"","institution":"University of Washington","correspondingAuthor":false,"prefix":"","firstName":"Adam","middleName":"","lastName":"Summers","suffix":""},{"id":583140007,"identity":"ac31ce48-356f-4f98-b7e7-ec4465e381d1","order_by":3,"name":"Mo Turner","email":"","orcid":"","institution":"University of Washington","correspondingAuthor":false,"prefix":"","firstName":"Mo","middleName":"","lastName":"Turner","suffix":""}],"badges":[],"createdAt":"2026-01-30 09:11:59","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8739467/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8739467/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":101730850,"identity":"3078f393-39de-46d0-ba91-7c53944d6a5b","added_by":"auto","created_at":"2026-02-03 06:05:44","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":2642594,"visible":true,"origin":"","legend":"\u003cp\u003eSea star ray skeletons comprise five fundamental types of ossicles: ambulacral ossicles (AMO), adambulacral ossicles (ADO), marginal ossicles (MO), reticular ossicles (RO, sometimes defined more broadly as abactinal), and carinal ossicles (CO). The presence of pedicellariae (PED) and spines (SPs) is species specific.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8739467/v1/8384d50c3e84aeeecf9e863d.png"},{"id":101730851,"identity":"7fb4f441-3b48-47a1-acfe-da3ece10cbe6","added_by":"auto","created_at":"2026-02-03 06:05:44","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":2689755,"visible":true,"origin":"","legend":"\u003cp\u003eµCT endoskeletal architecture and ossicle organization in three sea star species. Panels are arranged by species and follow the same viewing sequence to enable direct comparison: A–M, \u003cem\u003eEvasterias troschelii\u003c/em\u003e; N–Z, \u003cem\u003eDermasterias imbricata\u003c/em\u003e; A′–M′, \u003cem\u003eMediaster aequalis\u003c/em\u003e. Whole-animal µCT volume renderings are shown in aboral and oral view (A–B, N–O, A′–B′). Arm close-ups with the transect line used for segmentation are presented in C, P, and C′, while magnified oral views highlighting the ambulacral groove and mouth ossicles are shown in D, Q, and D′. Color-segmented arm volumes (E–G, R–T, E′–G′) distinguish major ossicle classes and the organization of the ambulacral groove. Insets and cross-sectional views (H–J, U–W, H′–J′) illustrate ossicle packing and articulation within the arm wall. Whole-animal renderings with a red reference line (K, X, K′) indicate the plane from which arm segmentations were derived. Fine-scale segmentations of representative ossicles and external spines are shown in L–M, Y–Z, and L′–M′. Segmentation colors denote skeletal regions and elements as follows: green, arm skeleton; pink, ambulacral groove; yellow, single longitudinal arm row; purple, single row of ambulacral groove ossicles; cyan, cardinal ossicle; beige, external spine; and magenta, pedicellariae. Across taxa, these panels highlight differences in ossicle uniformity, longitudinal row development, and spine distribution while maintaining consistent orientation and scale.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8739467/v1/8c462fb70f5ae38f27115fee.png"},{"id":101754026,"identity":"129282bc-62a2-4d89-8ae7-74dc58a7f31b","added_by":"auto","created_at":"2026-02-03 10:41:23","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":2926198,"visible":true,"origin":"","legend":"\u003cp\u003eµCT endoskeletal architecture and ossicle organization in three sea star species. Panels are arranged by species and follow the same viewing sequence to enable direct comparison: A–M, \u003cem\u003eHenricia\u003c/em\u003e sp.; N–Z, \u003cem\u003eLeptasterias\u003c/em\u003e sp.; A′–M′, \u003cem\u003eSolaster stimpsoni\u003c/em\u003e. Whole-animal µCT volume renderings are shown in aboral and oral view (A–B, N–O, A′–B′). Arm close-ups with the transect line used for segmentation are presented in C, P, and C′, while magnified oral views highlighting the ambulacral groove and mouth ossicles are shown in D, Q, and D′. Color-segmented arm volumes (E–G, R–T, E′–G′) distinguish major ossicle classes and the organization of the ambulacral groove. Insets and cross-sectional views (H–J, U–W, H′–J′) illustrate ossicle packing and articulation within the arm wall. Whole-animal renderings with a red reference line (K, X, K′) indicate the plane from which arm segmentations were derived. Fine-scale segmentations of representative ossicles and external spines are shown in L–M, Y–Z, and L′–M′. Segmentation colors denote skeletal regions and elements as follows: green, arm skeleton; pink, ambulacral groove; yellow, single longitudinal arm row; purple, single row of ambulacral groove ossicles; cyan, cardinal ossicle; beige, external spine; and magenta, pedicellariae.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8739467/v1/3d5661c0c12c9ff99a24f2f0.png"},{"id":101730849,"identity":"6e46fafb-c1dc-49d1-a9c2-986263ad3847","added_by":"auto","created_at":"2026-02-03 06:05:44","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":2807845,"visible":true,"origin":"","legend":"\u003cp\u003eµCT endoskeletal architecture and ossicle organization in three sea star species. Panels are arranged by species and follow the same viewing sequence to enable direct comparison: A–M, \u003cem\u003eCrossaster papposus\u003c/em\u003e; N–Z, \u003cem\u003ePisaster ochraceus\u003c/em\u003e; A′–M′, \u003cem\u003ePteraster tesselatus\u003c/em\u003e. Whole-animal µCT volume renderings are shown in aboral and oral view (A–B, N–O, A′–B′). Arm close-ups with the transect line used for segmentation are presented in C, P, and C′, while magnified oral views highlighting the ambulacral groove and mouth ossicles are shown in D, Q, and D′. Color-segmented arm volumes (E–G, R–T, E′–G′) distinguish major ossicle classes and the organization of the ambulacral groove. Insets and cross-sectional views (H–J, U–W, H′–J′) illustrate ossicle packing and articulation within the arm wall. Whole-animal renderings with a red reference line (K, X, K′) indicate the plane from which arm segmentations were derived. Fine-scale segmentations of representative ossicles and external spines are shown in L–M, Y–Z, and L′–M′. Segmentation colors denote skeletal regions and elements as follows: green, arm skeleton; pink, ambulacral groove; yellow, single longitudinal arm row; purple, single row of ambulacral groove ossicles; cyan, cardinal ossicle; beige, external spine; and magenta, pedicellariae.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8739467/v1/765ba633754ab690fd6d1668.png"},{"id":101754477,"identity":"175d9f69-54af-4fcc-afd0-6109edf5dd58","added_by":"auto","created_at":"2026-02-03 10:42:40","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":70174,"visible":true,"origin":"","legend":"\u003cp\u003eWhole body armor ratio vs ambulacral skeleton armor ratio. Color represents branching with 0 being the lightest blue and 10 being the darkest blue.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8739467/v1/74f4adba18bfed1d925650f0.png"},{"id":101730844,"identity":"82cdf988-e6a2-418c-841c-7c73d5f9df5c","added_by":"auto","created_at":"2026-02-03 06:05:44","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":24964,"visible":true,"origin":"","legend":"\u003cp\u003eLinear discriminant analysis showing grouping by depth (intertidal vs subtidal, A), habitat (mixed vs rocky, B), and diet (hard, mixed, and soft bodied prey, C). A and B show only LD 1 histograms of two-axis models. C shows a scatter plot of a three-axis model.\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-8739467/v1/2bc215f774be37fe0955bff4.png"},{"id":101730847,"identity":"69b71e16-8c86-4d28-8f66-227d2928e686","added_by":"auto","created_at":"2026-02-03 06:05:44","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":142436,"visible":true,"origin":"","legend":"\u003cp\u003eSignificant pairwise relationships. Intertidal species had fewer spines per spine cluster (A),number of oral ossicles per pair of oral plates (B), and spines per adambulacral ossicle (C) than subtidal species. Error bars show means and standard deviations. Points are jittered for clarity.\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-8739467/v1/f17c67dc9e2a0b99e36f182a.png"},{"id":101755747,"identity":"d479cb34-bc95-4dfa-a649-fad0b7bec6ec","added_by":"auto","created_at":"2026-02-03 10:54:32","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":11412974,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8739467/v1/a06daf0c-7e8e-41db-82db-00695b9e041f.pdf"},{"id":101730846,"identity":"36afe814-c626-44b7-b4d0-ee22d66bb11d","added_by":"auto","created_at":"2026-02-03 06:05:44","extension":"xlsx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":115379,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementalTables.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-8739467/v1/bb202c2acfcf77da42e4a67e.xlsx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Hierarchical skeletal architecture and ecological tradeoffs in Pacific Northwest sea stars","fulltext":[{"header":"Introduction","content":"\u003cp\u003eArmoring evolves frequently in nature due to its role as protection, offense, camouflage, and structural support (Edmunds, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e1974\u003c/span\u003e). Biological armor can be found across phyla, manifesting as shells, scales, thickened skin, spines, horns, and endo- or exoskeletons, each of which is accompanied by its own suite of tradeoffs (Cohen et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2025\u003c/span\u003e; Hoover et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Lakowitz et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Laver et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Taylor \u0026amp; Patek, \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Woodruff et al., \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Yang et al., \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). In echinoderms, those tradeoffs are expressed not only in mineral investment, but in how the body wall itself constrains (and enables) locomotion, feeding, and habitat use, making body-wall mechanics a key determinant of ecological opportunity (O\u0026rsquo;Neill, \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e1989\u003c/span\u003e). For invertebrates, the most common material used for armoring is calcium carbonate (CaCO\u003csub\u003e3\u003c/sub\u003e; Vermeij, \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e1989\u003c/span\u003e). The stiffness and toughness of calcium carbonate structures can be enhanced by addition of other materials like iron or chitin, while the shape and interactions between elements will also affect the behavior of the aggregate structure. Increasing armor mass (coverage, thickness, or density) requires energy investment and is heavier (Kruppert et al., \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Lakowitz et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2008\u003c/span\u003e), so increased protection may come at the expense of speed, maneuverability and flexibility. For example, hermit crabs are slower and burn more energy to move when burdened with the protection of a snail shell than when they are naked (Herreid \u0026amp; Full, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e1986\u003c/span\u003e). Increased armor often comes at the cost of maneuverability and flexibility, with movement only possible at junctions between armor plates (Hazerli \u0026amp; Richter, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eSea stars (Asteroidea, Echinodermata) are exclusively benthic as adults and can be found from tropical to polar latitudes and from mid intertidal zones to the deepest ocean trenches (Birkeland, n.d.; Dayton et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e1974\u003c/span\u003e; McClintock, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e1994\u003c/span\u003e; Zhang et al., \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). They exhibit a wide range in diet, from scavengers and suspension feeders to active predators, cannibals, and specialist consumers of other sea star species (Lawrence, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). Sea stars armor is a highly articulated endoskeleton made up of small calcium carbonate elements called ossicles. It creates a scaffolding just below the dermis that supports and protects the internal organs (O\u0026rsquo;Neill, \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e1989\u003c/span\u003e). Stars are usually heavily armored and are considered slow or even sedentary. However, many sea stars are active predators, with the fastest species, \u003cem\u003ePycnopodia helianthoides\u003c/em\u003e, reaching speeds of up to 1.5m per minute (Po et al., \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Mauzey et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e1968\u003c/span\u003e). The endoskeleton creates a scaffolding just below the dermis that supports and protects the internal organs (O\u0026rsquo;Neill, \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e1989\u003c/span\u003e). In many of their ecosystems, sea stars are the apex predator and, in some cases, a keystone species (Paine, \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e1966\u003c/span\u003e, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e1969\u003c/span\u003e), so understanding how armor shape and arrangement affects defensibility and behavior is essential to our understanding of their ecology and distribution.\u003c/p\u003e \u003cp\u003eSea star skeletons are composed of numerous discrete calcitic elements termed ossicles, which together form a highly articulated internal endoskeleton embedded within the dermis. Each ossicle is a stereom\u0026mdash;an open, lattice-like microstructure of calcite\u0026mdash;whose external shape, thickness, and pattern of articulation with neighboring elements vary systematically across body regions and among taxa. Ossicles are traditionally described and classified based on their position, serial arrangement, and associated structures (such as spines or pedicellariae), leading to regionally defined series including ambulacral (AMO), adambulacral (ADO), carinal (CO), reticular (RO, sometimes defined more broadly as abactinal), and marginal (MO) (Blake, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e1987\u003c/span\u003e; Blowes et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Eylers, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e1976\u003c/span\u003e; Schwertmann et al., \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Fig.\u0026nbsp;1). The adambulacral and ambulacral ossicles together form the ambulacral groove, creating a concave space on the oral side of the body that houses the soft tissue tube feet. The carinal, reticular, and marginal ossicles together make up the aboral body wall of the endoskeleton (Schwertmann et al., \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). The carinal ossicles are a central row of ossicles that run along the top of the aboral side of the body wall from ray tip to the center of the sea star. Marginal ossicles are a collection of ossicles that connect the body wall to the ambulacral groove. The remaining meshwork of the body wall consists of reticular ossicles. Although the presence, differentiation, and homology of particular ossicle series can vary among clades and through ontogeny, these elements are generally treated as repeating anatomical units whose size, shape, and spatial distribution collectively determine skeletal architecture rather than functioning as isolated structures (Blake \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e1987\u003c/span\u003e; Fau \u0026amp; Villier 2018, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Many forcipulataceans (especially asteriids) have pedicellariae (PED), small pinching claws supported by a flexible stalk, covering the aboral surface of the body wall (Fau \u0026amp; Villier \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Blake, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e1987\u003c/span\u003e). The claws but not the stalks of the pedicellariae are made of calcium carbonate and therefore are visible as floating ossicles in an x-ray or CT scan (Fig.\u0026nbsp;1). Descriptive studies on the basic patterning of sea star skeletons and the shapes of individual ossicles highlight that their shapes differ substantially and can be diagnostic of particular taxa (e.g. Blowes et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Fau \u0026amp; Villier, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2020\u003c/span\u003e, 2024; Gale, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Tomholt et al., \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2023\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn this study, we use micro-computed tomography x-ray imaging (\u0026micro;CT) to measure and compare morphological differences in the endoskeleton of nine sea star species from the Salish Sea in the northwest of Washington, USA. We qualitatively describe general patterns of shapes and arrangements of the ossicles and spines. Because asteroid skeletons are assembled from a shared set of ossicle \u0026ldquo;building blocks\u0026rdquo; that can be arranged and elaborated in many ways, we use \u0026micro;CT to shift from descriptive comparisons toward repeatable, quantitative characterization of skeletal architecture. We emphasize that our goal is not to redescribe classical taxonomy or propose new diagnoses, but rather to harmonize three-dimensional skeletal traits into a comparative character matrix that can be mapped onto ecology and phylogeny. In practice, this requires collapsing some lineage-specific terminology and fine-scale subtypes into higher-level trait bins that are broadly homologous and measurable across taxa (e.g., ambulacral, adambulacral, marginal, carinal, and reticular/abactinal skeletal elements, Fau \u0026amp; Villier 2024). Doing so allows us to ask predictive questions about how lineages working with the same skeletal components balance feeding, mobility, protection, offense, and camouflage across ecological contexts.In this study we aim to: 1) describe the skeletal morphology of nine species of sea stars 2) calculate armor ratios across the body, and 3) quantify how armoring is related to behavior and ecology.\u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eSpecimen collection and micro-computed tomography\u003c/h2\u003e \u003cp\u003eWe surveyed nine species of sea stars representing six families: Solasteridae (\u003cem\u003eCrossaster papposus\u003c/em\u003e, \u003cem\u003eSolaster stimpsoni\u003c/em\u003e), Asteriidae (\u003cem\u003ePisaster ochraceus\u003c/em\u003e, \u003cem\u003eEvasterias troschelii\u003c/em\u003e, \u003cem\u003eLeptasterias\u003c/em\u003e sp. complex), Pterasteridae (\u003cem\u003ePteraster tesselatus\u003c/em\u003e), Dermasteriidae (\u003cem\u003eDermasterias imbricata\u003c/em\u003e), Echinasteridae (\u003cem\u003eHenricia\u003c/em\u003e sp. complex), and Mediasteridae (\u003cem\u003eMediaster aequalis\u003c/em\u003e). These taxa span a range of skeletal architectures that have been recognized in classical morphological treatments for over a century (e.g. Fisher 1911, 1928, 1930), including differences in ossicle shape, articulation, and spine number. As expected given their shared evolutionary history, closely related species within families exhibit similarities in skeletal characters; this study explicitly evaluates how such phylogenetically structured morphology compares to ecological similarity across co-occurring taxa.\u003c/p\u003e \u003cp\u003eSea stars were collected in the San Juan archipelago (San Juan County, WA, USA) from tidepools by hand and the subtidal environment by SCUBA diving and trawling. Sea stars were relaxed in magnesium chloride, then put in 10% formaldehyde for 24 hours, after which they were rinsed in water and dehydrated to 70% ethanol for storage. Sea stars were \u0026micro;CT scanned wrapped in cheesecloth wetted with 70% ethanol. All specimens were \u0026micro;CT scanned with a Bruker SkyScan 1173, using the high-resolution 2k detector, a 1mm aluminum filter, and voltages ranging from 80-133kV and amperages ranging from 40\u0026ndash;80\u0026micro;A (Table\u0026nbsp;1). Groups of whole specimens were wrapped in damp cheesecloth and placed together into a 3D printed plastic cylinder. The entire cylinder was wrapped in plastic film to ensure it was airtight, then mounted on a rotating stage in the scanner and scanned with voxel size ranging from 30\u0026ndash;35\u0026micro;m. Once the whole specimen scans were completed, approximately 10mm of a ray tip from each individual was removed using a scalpel and scanned again at a voxel size of 8\u0026micro;m for high resolution images of the arms. Images were reconstructed with nRecon (Bruker Systems) and visualized with 3D Slicer (Fedorov et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2012\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003e\u0026micro;CT scan specifications using a Bruker SkuScan 1173 (high-resolution 2k detector and a 1mm aluminum filter) for each of the 9 sea star species, scanned at the scale of the whole sea star and a tip of one ray.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSpecies\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eResolution (um)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eVoltage (kV)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003ePower (uA)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eExposure (ms)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cem\u003eCrossaster papposus\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e29.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e55\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e143\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1475\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cem\u003eDermasterias imbricata\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e35.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e65\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e123\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1185\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cem\u003eEvasterias troschelii\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e29.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e55\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e143\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1475\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cem\u003eHenricia sp.\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e35.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e65\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e123\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1185\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cem\u003eLeptasterias sp.\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e35.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e65\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e123\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1185\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cem\u003eMediaster aequalis\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e31\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e65\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e123\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1165\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cem\u003ePisaster ochraceus\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e35.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e65\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e123\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1185\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cem\u003ePteraster tesselatus\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e35.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e65\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e123\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1185\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cem\u003eSolaster stimpsoni\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e31\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e65\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e123\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1165\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRay tips of every species\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e8.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e65\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e123\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1135\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eSegmentation and measurements\u003c/h3\u003e\n\u003cp\u003eSkeletal morphology and body tissue were segmented in the open source platform 3D Slicer (Fedorov et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2012\u003c/span\u003e) using grayscale intensity thresholding, grow seeds, and manual slice-by-slice editing in the segment editor module. In addition to creating segments of the different regions (body wall and ambulacral groove) and types of ossicles, we also isolated a cross-sectional unit of the ray representing one set of repeated ossicles (here referred to as a \u0026ldquo;ray band\u0026rdquo;). Volume (mm\u003csup\u003e3\u003c/sup\u003e) of each segment was measured using the Segment Statistics module. The total specimen volume included tissue, skeleton, and the internal body cavity. Since the specimens differed in absolute size, it was necessary to quantify traits that would enable comparison among species without a confounding effect of size, e.g. more total ossicle volume in larger specimens. Size correction was carried out in most cases by calculating ratios among body parts, which then emphasized shape rather than size characteristics (Table\u0026nbsp;2). Ray length for each specimen was measured from the tip of the longest ray to the center of the central disc. The central disc radius was measured from the interradius (junction where two arms meet the central disc) to the center of the sea star.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eFourteen morphological traits defined in this study across varying scale of measurement, from whole specimen to individual ossicles for nine species of sea stars.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eScale\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eArmor Ratios\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eOssicle size and organization\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eWhole specimen\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSkeleton : whole body volume\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCentral disc radius : longest ray length\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRay (whole)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSkeleton : whole ray volume\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNo. of rays\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRay (cross-section)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSurface area : volume\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNo. of spine clusters / ray band\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAmbulacral groove\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAmbulacral groove : ray skeleton\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNo. of spines / adambulacral ossicle\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBody wall\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCarinal (ossicle\u0026thinsp;+\u0026thinsp;spine) : whole ray band volume\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003ePresence/absence of pedicellariae\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSpines\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAll spines : whole ray band volume\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNo. of spines / cluster\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMouth\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eOral spine length : central disc radius\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNo. of spines / pair of oral plates\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e\n\u003ch3\u003eSkeletal traits\u003c/h3\u003e\n\u003cp\u003eWe examined the \u0026micro;CT-scan of each species and qualitatively described differences in endoskeletal arrangement of ossicles in terms of body wall organization, body wall ossicle shape, and spines (Table\u0026nbsp;4). We quantified numerical trait values to assess the number and robustness of skeletal components at different scales, ranging from individual ossicles to the whole organism (Table\u0026nbsp;2, Table S.1-S.2). Specifically, we measured the skeletal volume of each species as a proxy for armor investment. In addition to total body armor ratio, we examined armoring in four ways: (1) the relative armoring of ray tips, (2) the relative armoring around the ambulacral groove, (3) the surface area-to-volume ratio of ossicles within a ray band as an index of skeletal surface complexity, and (4) the external shape characterized by ray length relative to central disc size. For mouth traits, we analyzed the number of and length of oral spines (length standardized to body size). Asteroid skeletal terminology varies across clades and historical treatments, and individual ossicle series may be subdivided differently depending on taxonomic context. To enable consistent cross-taxon comparison, we standardized nomenclature to major, widely recognized ossicle systems (ambulacral, adambulacral, marginal, carinal, and reticular/abactinal body-wall ossicles) and scored traits at the level of ossicle geometry, packing, articulation, and spine support that can be reliably extracted from \u0026micro;CT volumes. Where published sources or clade-specific traditions recognize additional subcategories (e.g., specialized abactinal derivatives such as paxillae, or multiple plate fields within the body wall), we collapsed those into broader bins when the finer distinctions could not be applied consistently across all taxa. This approach intentionally prioritizes repeatability and comparability over taxonomic redescription, producing a character matrix suited for hypothesis testing and prediction rather than diagnosis.\u003c/p\u003e\n\u003ch3\u003eComparison of morphological, ecological, and phylogenetic distance matrices\u003c/h3\u003e\n\u003cp\u003eWe calculated three distance matrices across the nine species based on morphological traits, ecological traits, and phylogenetic relationships. Ecological traits (diet, habitat use, and known predators) were compiled from multiple literature sources (Flowers \u0026amp; Foltz, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Kozloff et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1996\u003c/span\u003e; Mauzey et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e1968\u003c/span\u003e; Rodenhouse \u0026amp; Guberlet, \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e1946\u003c/span\u003e) and converted into discrete categories to reflect broad ecological similarity rather than fine-scale niche partitioning. Importantly, habitat categories were intended to capture typical habitat use rather than absolute depth limits, as several taxa commonly associated with intertidal environments (e.g. \u003cem\u003ePisaster\u003c/em\u003e, \u003cem\u003eEvasterias\u003c/em\u003e, \u003cem\u003eLeptasterias\u003c/em\u003e) are known to occur across a wider depth range in parts of their geographic distribution.\u003c/p\u003e \u003cp\u003eFor the ecological (Table\u0026nbsp;3) and morphological (Table\u0026nbsp;2) traits, dissimilarities were calculated using Gower\u0026rsquo;s distances (Lalibert\u0026eacute; et al., 2014) for each pairwise species comparison, an approach appropriate for mixed categorical and continuous data. Phylogenetic distances were based on a consensus molecular phylogeny incorporating nuclear (18S rDNA, 28S rDNA, histone H3) and mitochondrial markers (12S rDNA, 16S rDNA, tRNA cluster, cytochrome c oxidase I) (Janies et al., \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). These distances are based on branch lengths from published, time-calibrated molecular phylogenies, rather than on raw pairwise genetic distances. Specifically, distances represent the summed branch length separating each pair of taxa. Because \u003cem\u003eMediaster aequalis\u003c/em\u003e was not included in this tree, we used an additional published phylogeny (Mah \u0026amp; Foltz, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2011\u003c/span\u003e) containing that species and overlapping taxa to estimate relative phylogenetic distances. Correlations between morphological distance and ecological and phylogenetic distance matrices were tested statistically.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eEcological traits defined in this study for nine species of sea stars. Ecological distances were calculated from 12 binary traits representing diet, habitat, and predators.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"2\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCategories\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBinary Columns: each trait scored as 0/1 for each species\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDiet\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCnidarians, sponge, gastropods, bivalves, barnacles, tunicates, echinoderms\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHabitat\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eRock, cobble, sand/mud\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePredator\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eEchinoderms, (removed gulls due to high correlation with \u0026ldquo;intertidal\u0026rdquo;)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDepth\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eIntertidal\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e\n\u003ch3\u003eStatistical analysis of skeletal and ecological traits\u003c/h3\u003e\n\u003cp\u003eWe use linear discriminant analysis (LDA) to determine which skeletal traits best predict ecological traits (diet, habitat, and depth) (R Core Team, 2020; Ripley \u0026amp; Venables, \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Fig.\u0026nbsp;13). Diet was separated into three categories based on prey type: soft-bodied only, hard-bodied only, and a mix of both; habitat was separated into two categories: rocky or mixed; depth was separated into two categories: intertidal and subtidal (Table S.3). Many of the skeletal traits were highly correlated (Table S.4), so we used six of the fourteen skeletal traits (Table S.1-S.2) in our LD models to avoid collinearity (Wei \u0026amp; Simko, \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). In addition to the LDA, we ran pairwise t-tests on all fourteen skeletal traits to quantify significant differences between diet, habitat, and depth groups (Fig.\u0026nbsp;7).\u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003eOverall comparative morphology\u003c/h2\u003e \u003cp\u003eWe identified three general patterns of ossicle organization across the nine species (Figs.\u0026nbsp;2\u0026ndash;4). First, several taxa exhibited an evenly distributed, grid-like abactinal skeleton in which ossicles form a relatively regular, tessellated field across the body wall (e.g., \u003cem\u003eM. aequalis\u003c/em\u003e, Fig.\u0026nbsp;2A\u0026prime;\u0026ndash;M\u0026prime;). Second, other taxa showed ossicles arranged as one or more discrete, longitudinal series that extend from the ray tip toward the central disc, producing clear ray-wise patterning (e.g., an abactinal/cranial-aligned series in \u003cem\u003eLeptasterias sp\u003c/em\u003e., Fig.\u0026nbsp;3N\u0026ndash;Z). Third, some species possessed a reticular abactinal skeleton, where ossicles are organized as an interconnected meshwork that defines polygonal \u0026ldquo;cells\u0026rdquo; (e.g., \u003cem\u003eC. papposus\u003c/em\u003e, Fig.\u0026nbsp;4A\u0026ndash;M). Consistent with recent descriptions of sea star skeletal architecture emphasizing modular, hierarchical organization of stereo elements, these patterns reflect differences in how ossicles are packaged into either (i) broadly uniform fields, (ii) ray-parallel series, or (iii) reticulate networks that couple adjacent ossicles across the body wall.\u003c/p\u003e \u003cp\u003eAcross species, the shape and size distribution of ossicles comprising the abactinal and actinal body wall ranged from relatively uniform elements (e.g., \u003cem\u003eHenricia sp\u003c/em\u003e., Fig.\u0026nbsp;3A\u0026ndash;M) to heterogeneous, size-structured networks in which larger \u0026ldquo;nodes\u0026rdquo; are linked by smaller, elongated connecting ossicles (e.g., \u003cem\u003eP. ochraceus\u003c/em\u003e, Fig.\u0026nbsp;4N\u0026ndash;Z). This latter condition produces a more explicitly hierarchical mesh\u0026mdash;large ossicles acting as structural hubs connected by narrower struts\u0026mdash;whereas the more uniform condition yields a comparatively regular tiling of similarly sized elements.\u003c/p\u003e \u003cp\u003eAll species possessed spines associated with abactinal and/or adambulacral ossicles, except \u003cem\u003eD. imbricata\u003c/em\u003e, in which spines were restricted to the adambulacral series bordering the ambulacral groove (Fig.\u0026nbsp;2N\u0026ndash;Z). In the remaining species, spines were either distributed broadly and consistently across the most abactinal body-wall ossicles (e.g., \u003cem\u003eP. tesselatus\u003c/em\u003e, Fig.\u0026nbsp;4A\u0026prime;\u0026ndash;M\u0026prime;) or localized to select larger ossicles, including abactinal and/or marginal elements (e.g., E. troschelii, Fig.\u0026nbsp;2A\u0026ndash;M; Table\u0026nbsp;4).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eQualitative comparison of endoskeletal arrangement. Descriptions of patterns observed across three scales of ossicle organization and the corresponding sea star species that exhibited that pattern.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eScale\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePattern\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSpecies\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e\u003cb\u003eBody wall organization\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1. One or more distinct rows of ossicles running from ray tip to central disc\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cem\u003eEvasterias, Leptasterias, Pisaster\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2. Grid-like, evenly distributed\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cem\u003eHenricia, Mediaster, Pteraster, Solaster\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3. Meshwork of ossicles form regular polygons\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cem\u003eCrossaster, Dermasterias\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e\u003cb\u003eBody wall ossicle shape\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1. Uniform shape and size throughout body wall\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cem\u003eHenricia, Leptasterias, Mediaster, Pteraster, Solaster\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2. A few larger ossicles connected to each other by a webbing of thin, elongated ossicles\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cem\u003eCrossaster, Evasterias, Leptasterias, Pisaster\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e\u003cb\u003eSpines\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1. No spines on the body\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cem\u003eDermasterias\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2. Spines on select, larger ossicles\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cem\u003eCrossaster, Evasterias, Pisaster\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3. Spines on every ossicle\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cem\u003eHenricia, Leptasterias, Pteraster, Solaster\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eArmor volume ratios\u003c/h3\u003e\n\u003cp\u003eOssicles varied significantly in all calculated armor metrics (Table S.1). Total body armoring (volume of skeleton relative to total body volume) ranged from 3% to 39%, with \u003cem\u003eP. tesselatus\u003c/em\u003e having the least amount of armor and \u003cem\u003eM. aequalis\u003c/em\u003e and \u003cem\u003eLeptasterias sp.\u003c/em\u003e having the most (37 and 39% respectively). For all nine species, the ray tips were more heavily armored than the body as a whole (range 27\u0026ndash;69%). In the least armored sea star, the abactinal and lateral body wall skeleton made up 38% of the mineral, while the ambulacral and adambulacral skeleton accounted for the remaining 62%, while in more heavily armored stars the dorsal and lateral made up as much as 77% of the mineral (Fig.\u0026nbsp;5). This increase in body wall ossicles is paired with a decrease in surface area to volume ratio, calculated from the cross-sectional ray band. As armoring (skeletal investment) increases across the body, the overall shape becomes less convoluted. In general, body shape shifts from short to long rays relative to the length of the central disc. Oral spine length (relative to radius of the central disc) is not correlated with number of oral spines per pair of oral plates, however the two sea stars with the greatest number of rays (\u003cem\u003eC. papposus\u003c/em\u003e and \u003cem\u003eS. stimpsoni\u003c/em\u003e) also had the greatest number of oral spines per pair of oral plates.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eLinear discriminant (LD) analysis\u003c/h2\u003e \u003cp\u003eThe LD models showed that skeletal traits are sufficient to differentiate between depth, habitat, and diet (Fig.\u0026nbsp;6). In our depth model, the number of oral spines contributes most to the separation between intertidal and subtidal species with subtidal stars having more oral spines than intertidal stars (Table\u0026nbsp;5, Fig.\u0026nbsp;6A). In our habitat model, the number of oral spines, total body armor ratio, and ray-to-disc ratio contribute most to the separation between sea stars that live on rocky substrate and sea stars that live on mixed substrate (Table\u0026nbsp;5, Fig.\u0026nbsp;6B). Stars that live on rocky substrate have fewer oral spines, less armor, and shorter arms than stars that live on mixed substrates like cobble, sand, or mud. In our diet model, LD 1 accounted for 98.86% of separation and split seastars that prey on soft bodied organisms from sea stars that prey on hard-bodied organisms and those that had a mixed diet (Fig.\u0026nbsp;6C). Stars that feed on soft prey have fewer arms and more armor in the ambulacral groove than stars that eat hard prey. Conversely, stars that eat hard prey have more arms and less armor in their ambulacral grove. LD 2 accounts for only 1.14% of the separation between groups and is driven by the number of oral spines(Table\u0026nbsp;5).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab5\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 5\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eLoadings for three LD models. Traits that had the largest effect on separation between groups are bold.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMorphological traits\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eDepth\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eHabitat\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e \u003cp\u003eDiet\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cem\u003eLD1\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cem\u003eLD1\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cem\u003eLD1\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cem\u003eLD2\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNumber of rays\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-0.0449\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.4165\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e-5.1139\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-1.122\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNumber of oral spines / pair of oral plates\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003e2.71124\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e-1.6587\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2.81844\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cb\u003e2.07809\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSkeleton : whole body volume ratio\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.05698\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e-1.2835\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-0.0341\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.87208\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCentral disc radius : longest ray length ratio\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-0.3367\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e-1.2812\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.81177\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-0.2971\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRay skeleton : whole ray volume ratio\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.50699\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.58854\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2.34827\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-1.2469\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAmbulacral groove : ray skeleton ratio\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.36851\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-0.4981\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e4.39688\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-1.4021\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003ePairwise t-tests showed that the only traits with significant differences between groups were branching (p\u0026thinsp;=\u0026thinsp;0.023), number of oral spines (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001), and number of spines on the adambulacral ossicles (p\u0026thinsp;=\u0026thinsp;0.01) between intertidal and subtidal groups (Fig.\u0026nbsp;7).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003ePhylogeny and ecology\u003c/h2\u003e \u003cp\u003eWe compared pairwise differences in skeletal traits among nine sea star species against phylogenetic distances (from Janies et al., \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2011\u003c/span\u003e and Mah \u0026amp; Foltz, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). Morphological variation in armor traits showed a weak but significant correlation with phylogenetic distance (Pearson\u0026rsquo;s correlation: r\u0026thinsp;=\u0026thinsp;0.44, p\u0026thinsp;=\u0026thinsp;0.007; Fig.\u0026nbsp;7). Three species, \u003cem\u003eE. troschelii\u003c/em\u003e, \u003cem\u003eP. ochraceus\u003c/em\u003e, and \u003cem\u003eLeptasterias\u003c/em\u003e sp., showed pairwise similarities in both phylogeny and morphology. In contrast, these species exhibited high phylogenetic divergence from \u003cem\u003eHenricia\u003c/em\u003e sp., despite displaying convergent morphological traits. Morphological distance also had a positive but weak correlation with ecological distance (p\u0026thinsp;=\u0026thinsp;0.03, C\u0026thinsp;=\u0026thinsp;0.37; Fig.\u0026nbsp;7). Correlation of phylogenetic and ecological distance (p\u0026thinsp;=\u0026thinsp;0.02, C\u0026thinsp;=\u0026thinsp;0.39) was high relative to how well ecological distance correlated with morphological relationships but low relative to how well phylogenetic distance correlated with morphological relationships.\u003c/p\u003e \u003c/div\u003e"},{"header":"Discussion","content":"\u003cp\u003eLike trabecular bone, the support structures of woody plants, and the foamy exterior rind of citrus fruits, the endoskeleton of sea stars is a classic hierarchical structure with morphological variation at levels above the diagnostic shapes of particular ossicles (Fratzl \u0026amp; Weinkamer, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2007\u003c/span\u003e). The diversity in skeletal structure and ossicle arrangement across sea star species reflects their varied ecological strategies and functional needs. The slime star, \u003cem\u003eP. tesselatus\u003c/em\u003e, for example, has reduced ossicles and long spines that support a flexible external body wall (supradorsal membrane), likely allowing the animal to swell and exude viscous slime for defense when threatened (Nance, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e1981\u003c/span\u003e; Rodenhouse \u0026amp; Guberlet, \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e1946\u003c/span\u003e). In contrast, \u003cem\u003eD. imbricata\u003c/em\u003e and \u003cem\u003eM. aequalis\u003c/em\u003e share a pattern of voluminous, imbricated ossicles along their ray edges, reinforcing their flexible body walls to capture and hold soft prey, providing both adaptability and structural support. \u003cem\u003eHenricia\u003c/em\u003e sp. Are generally considered suspension feeders, with some studies suggesting predation on sea sponges (Cossi et al., 2021; Shield and Witman 1993; Ferguson \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e1969\u003c/span\u003e; Chichvarkin et al., 2019). They have small uniform ossicles which allow flexibility, aiding in a range of behaviors from locomotion to defense, and their widespread habitat range suggests uniform ossicle shapes may support adaptability to various environmental conditions. These morphological differences suggest that closely related species may evolve distinct structures to adapt to their specific environments. Of course, in some cases, phylogeny rules as in the three asteriids in our dataset, \u003cem\u003eP. ochraceus\u003c/em\u003e, \u003cem\u003eE. troschelii\u003c/em\u003e, and \u003cem\u003eLeptasterias\u003c/em\u003e sp., which all have similar ossicle organization forming a grid-like pattern with a distinct carinal ridge and pedicellariae covering the entire body wall. The grid-like skeletal pattern with protruding spines enhances stiffness in some directions, increasing protection from wave action.\u003c/p\u003e \u003cp\u003eFive arms represent the ancestral and most common body plan in sea stars, but multiple lineages have independently evolved increases in arm number. In our dataset, species with more than five arms are associated with predatory strategies that involve manipulating or overpowering hard or resistant prey such as bivalves, other echinoderms, and crustaceans (Fig.\u0026nbsp;13.C) (Mauzey et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e1968\u003c/span\u003e; Rahman et al., 2018). However, increased arm number is not universally linked to macrophagy or durophagy across Asteroidea. Notable counterexamples include members of the order Brisingida, which possess numerous elongate arms but are suspension feeders adapted to deep-sea environments, and \u003cem\u003eAcanthaster\u003c/em\u003e species, which use multiple arms during feeding but specialize on coral tissue rather than mechanically resistant prey (Emson \u0026amp; Chesher, 1976; Lawrence, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). Our data show that other factors such as number of oral spines may be more associated with depth and filter feeding (Fig.\u0026nbsp;13.A).\u003c/p\u003e \u003cp\u003eRather than serving as a simple proxy for predatory intensity, increased arm number may expand the functional envelope of sea stars by enhancing force distribution, prey handling surface area, or behavioral flexibility depending on ecological context. These interactions are also likely to be time-dependent: dorsal body-wall tissue exhibits strong stress relaxation consistent with linear viscoelastic behavior, meaning the effective stiffness of the body wall can shift over seconds to minutes under sustained loading. This provides a mechanistic route by which similar skeletal architectures could support different behaviors depending on neuromuscular state and loading history (O\u0026rsquo;Neill, \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e1989\u003c/span\u003e). In addition to increased arm number, stars that feed on hard prey also have less armoring in their ambulacral groove. Less armor means more flexibility and an easier time conforming to extremely rigid prey items (Fig.\u0026nbsp;13C). This flexibility may grant stars mechanical leverage while trying to pry open difficult to access prey. This interpretation aligns with growing interest in the biomechanics of the tube foot system, where recent work has emphasized the importance of coordination, speed, and force generation during prey capture and locomotion (Hennebert et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Heydari et al., \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Ellers et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Po et al., \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Although direct performance data remain limited, our findings suggest that variation in ambulacral ossicle number, density, and interdigitation provides a structural framework that may underlie differences in functional capacity. As quantitative measurements of tube foot performance become available, mapping these data onto three-dimensional skeletal architecture will be essential for testing how arm number and internal morphology jointly shape feeding performance.\u003c/p\u003e \u003cp\u003eWe see two strategies for defense embedded in these ossicle data. Some stars are defended like tortoises with boxy, heavily mineralized ossicles, while others are porcupine-like in their spikiness. The tradeoff we see between ossicle density\u0026mdash;both in volume and proximity\u0026mdash;and the presence of spines is extreme, with skeletal armoring varying by an order of magnitude or more in certain species (Fig.\u0026nbsp;12). We propose that this metaphor may extend beyond a tradeoff in morphology to an explanation of locomotor performance with the tortoise stars being slower and less agile than those with a pincushion style of defense. For example, one of the fastest sea stars, the many armed sea star (\u003cem\u003ePycnopodia helianthoides\u003c/em\u003e) is speedy, spry, and covered in spiny ossicles (Mauzey et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e1968\u003c/span\u003e; Montgomery, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). This is similar to the crown-of-thorns (\u003cem\u003eAcanthaster planci\u003c/em\u003e), another spiny, fast-moving porcupine of a sea star. In contrast, the doughboy seastar (\u003cem\u003eChoriaster granulatus\u003c/em\u003e) is a tortoise \u0026ndash; smooth, stiff-bodied, and slow-moving denizen of sandy substrates (Montgomery \u0026amp; Palmer, \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Moran, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e1988\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eLocomotion, predator-prey dynamics, and other ontogenetic pressures complicate the relationship between ossicle morphology and ecological factors (Fig.\u0026nbsp;15). For instance, the types of predators that feed on sea stars may shape ossicle investment more than their habitat (Eigen et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Montgomery, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Rudykh et al., \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Van Veldhuizen \u0026amp; Oakes, \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e1981\u003c/span\u003e). If a star is preyed upon by a fast-moving, pinching crab, it is likely to evolve different armor compared to a star whose main predator is a conspecific. Conversely, in predatory sea stars, the demands of efficient locomotion and flexibility during feeding may be associated with convergent ossicle patterning across lineages, even where underlying phylogenetic relationships differ (Fau \u0026amp; Villier, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eMicro\u0026ndash;computed tomography (\u0026micro;CT) has emerged as a powerful imaging tool for resolving internal morphology in three dimensions, yet its application in invertebrate systems has only begun to expand in earnest within the past five years. This lag reflects genuine technical challenges that are far less pronounced in vertebrate imaging, including the prevalence of hydrostatic skeletons, weakly mineralized or compositionally heterogeneous tissues, limited affinity of contrast agents for many invertebrate materials, and the frequent collapse or distortion of critical anatomical features during fixation and dehydration. These issues have historically constrained the use of \u0026micro;CT for invertebrates to descriptive studies or isolated anatomical reconstructions rather than comparative, character-based analyses (Ziegler, \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Blowes et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Here, we demonstrate that \u0026micro;CT can be used not only to visualize sea star skeletal morphology, but also to generate repeatable, quantifiable character matrices that distinguish phylogenetically conserved features from those that are more evolutionarily labile. By resolving ossicle architecture, articulation, and spatial organization in three dimensions, \u0026micro;CT enables the construction of datasets that are directly comparable across taxa and amenable to hypothesis-driven analyses of armor evolution and function.\u003c/p\u003e \u003cp\u003eImportantly, three-dimensional imaging also provides a platform for exploring functional trade-offs that are otherwise difficult to infer from external morphology alone. CT-derived models allow explicit consideration of how sea stars balance mobility, protection, and predation risk, including how skeletal elements constrain or accommodate soft-tissue insertion, flexibility, and mechanical compromise. Such data open new avenues for testing how protective structures evolve in systems where defense, locomotion, and feeding are tightly integrated rather than modular. Finally, the value of \u0026micro;CT data increases substantially when scans are archived, reused, and integrated into open, comparative frameworks. Vertebrate biology has benefited enormously from centralized, open-access CT repositories (e.g. MorphoSource and the oVert initiative), and our results underscore the importance of extending similar infrastructure to invertebrate systems.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eResearch Highlights\u003c/h2\u003e \u003cp\u003eAlthough most sea stars share basic ossicle types, the distribution, size, and shape vary across species, reflecting a tradeoff in armoring. These differences correlate more strongly with phylogenetic relationships than with ecological factors.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eM.S., C.M.D., and K.E.C. contributed to specimen collection and data acquisition. K.E.C., M.S., and C.M.D. conducted data analysis, generated figures, and led manuscript drafting and revision. A.P.S. contributed to study design, data interpretation, manuscript editing, and provided funding, equipment, and technical support. All authors approved the final manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThank you to the Karl Liem Imaging Facility at Friday Harbor Laboratories for allowing me access and use of the imaging equipment. Stephen and Ruth Wainwright Endowed Fellowship. Anne Hof Blinks Fellowship. NSF DBI-2301407 to CMD and NSF DBI-2301406 to APS.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eAll data supporting the findings of this study are available from the corresponding authors upon reasonable request and are archived on GitHub.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eBirkeland C (n.d.) (ed) The influence of echinoderms on coral-reef communities. Echinoderm Studies, 3, 1\u0026ndash;79\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBlake DB (1987) A classification and phylogeny of post-Palaeozoic sea stars (Asteroidea: Echinodermata). 2481\u0026ndash;528. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1080/00222938700771141\u003c/span\u003e\u003cspan address=\"10.1080/00222938700771141\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e. 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Zoosymposia15\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"},{"header":"Additional Figures","content":"\u003cp\u003eFigure numbers 12, 13, and 15 are not available with this version.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"zoomorphology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"zomo","sideBox":"Learn more about [Zoomorphology](http://link.springer.com/journal/435)","snPcode":"435","submissionUrl":"https://submission.nature.com/new-submission/435/3","title":"Zoomorphology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"ossicles, starfish, sea star, skeleton, echinoderm, armor","lastPublishedDoi":"10.21203/rs.3.rs-8739467/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8739467/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSea star armor comes in the form of a highly articulated endoskeleton made up of individual elements called ossicles. Many descriptive studies have been conducted on the basic patterning of sea star skeletons, with differences in ossicle shape forming the basis of some echinoderm phylogenies. However, ossicle function is not related only to individual element morphology, but rather the whole system. In this study, we use micro-computed tomography (CT) to describe and compare skeletal anatomy of nine sea star species from the Salish Sea, Washington, USA. We quantified 14 morphological traits and tested whether or not they were predictors of ecology. We expected to see that differences in the amount of armoring (relative volume of skeleton) arise from varying arrangement and shape of ossicles across distinct regions of the body. For broad comparability, we grouped skeletal elements into five basic types of ossicles. The amount of skeletal armoring across the body varied by at least an order of magnitude across species and differed in its distribution across ossicle types. Heavily armored sea stars invest in larger, boxy body wall ossicles, whereas a reduction in armor volume was often paired with more intricately-shaped body wall ossicles and an increase in the number and complexity of spines.\u003c/p\u003e","manuscriptTitle":"Hierarchical skeletal architecture and ecological tradeoffs in Pacific Northwest sea stars","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-02-03 06:05:37","doi":"10.21203/rs.3.rs-8739467/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-02-22T12:05:09+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-02-20T16:18:08+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-02-17T18:50:57+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-02-16T18:07:07+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"163168272370156846114684306252232692125","date":"2026-02-14T14:08:00+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"135603208527278761664319934136603601171","date":"2026-02-13T22:03:46+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"178576251017523703459265311258112326853","date":"2026-02-03T12:54:08+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-02-01T14:58:07+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-01-30T14:56:03+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-01-30T14:55:06+00:00","index":"","fulltext":""},{"type":"submitted","content":"Zoomorphology","date":"2026-01-30T08:36:32+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"zoomorphology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"zomo","sideBox":"Learn more about [Zoomorphology](http://link.springer.com/journal/435)","snPcode":"435","submissionUrl":"https://submission.nature.com/new-submission/435/3","title":"Zoomorphology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"0f43e1a8-d02a-4646-a90d-6f5a6b1f2084","owner":[],"postedDate":"February 3rd, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"in-revision","subjectAreas":[],"tags":[],"updatedAt":"2026-02-22T12:09:20+00:00","versionOfRecord":[],"versionCreatedAt":"2026-02-03 06:05:37","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8739467","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8739467","identity":"rs-8739467","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}