Contrasting cryptofaunal responses to seabird nutrient inputs illuminate coral reef productivity pathways

Contrasting cryptofaunal responses to seabird nutrient inputs illuminate coral reef productivity pathways | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 24 September 2025 V2 Latest version Share on Contrasting cryptofaunal responses to seabird nutrient inputs illuminate coral reef productivity pathways Authors : Laura-Li Jeannot 0000-0002-1999-7563 [email protected] , Ruth Dunn , Joyce Velos , Gareth Williams , Cassandra Benkwitt , Nicholas Graham 0000-0002-0304-7467 , and Simon Brandl Authors Info & Affiliations https://doi.org/10.22541/au.175821493.32427292/v2 290 views 237 downloads Contents Abstract Abstract Introduction Methods Results Discussion Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Nutrients moving across ecosystems can boost productivity, stability, and resilience. Their integration through food webs is often mediated by small, abundant taxa channelling subsidies from primary producers to higher trophic levels. On coral reefs, cryptobenthic fishes and invertebrates could fill these roles, yet their responses to allochthonous nutrients are poorly understood. We compared cryptofaunal communities and associated trophic dynamics across islands with or without seabird-derived nutrients in the Chagos Archipelago, Indian Ocean. Most cryptobenthic fishes and some invertebrates showed nutrient enrichment. Community patterns revealed strong asymmetries: cryptobenthic fish biomass doubled in nutrient-rich environments, with a tenfold increase in larger piscivorous fish productivity, while cryptic invertebrate biomass stagnated, with reduced invertivore productivity. Life-history traits likely enable cryptobenthic fishes to exploit subsidies more efficiently, intensifying pressure on invertebrates in nutrient-rich environments. Our findings reveal how nutrients are routed through cryptofaunal pathways, underscoring the far-reaching impacts of cross-ecosystem nutrient vectors on trophic functioning. Differential cryptofaunal responses to seabird nutrient inputs illuminate coral reef productivity pathways L.-L. Jeannot 1* , R.E. Dunn 1,2 , Velos, J. 3 , G. J. Williams 4 , C.E. Benkwitt 1 , N. A. J. Graham 1 , S. J. Brandl 3 1 Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK 2 Centre d’Ecologie Fonctionnelle et Evolutive, 1919 Route de Mende, Montpellier, 34293, France 3 Department of Marine Science, The University of Texas at Austin, Marine Science Institute, Port Aransas, Texas, USA 4 School of Ocean Sciences, Bangor University, Menai Bridge, Anglesey LL59 5AB, UK *Corresponding author: [email protected] Email addresses of all authors: • Laura-Li Jeannot: [email protected] (institutional,) / [email protected] (for future/post-publication correspondence) • Ruth E Dunn: [email protected] • Joyce Velos: [email protected] • Gareth J Williams: [email protected] • Casey E Benkwitt: [email protected] • Nick AJ Graham: [email protected] • Simon J Brandl: [email protected] Statement of authorship: NAJG, SJB, CEB, RED and LLJ conceptualized the study. LLJ, RED, and CEB collected data. JV performed stable isotope laboratory work. LLJ performed modelling work, analysed output data, and wrote the first draft of the manuscript. All authors contributed substantially to revisions. Data accessibility statement: Data and code supporting the results are available at https://anonymous.4open.science/status/seabirds_cryptofauna-11E8 (anonymized for review; to be made publicly available later). Short running title: Seabirds spur reef production via cryptofauna (45 characters with spaces) Up to 10 keywords: Seabird guano, nutrient subsidies, coral reef ecology, nutrient enrichment, stable isotope analysis, reef invertebrates, cryptofauna, cryptobenthic fishes Type of article: Letter Number of words in the abstract: 150 Number of words in the main text: 4,998 Number of words in each text box: No text boxes are included in this manuscript. Number of references: 79 Number of figures: 5 excluding graphical abstract. Number of tables: All tables are in the supplement. Number of text boxes: All text boxes are in the supplement. Abstract Nutrients moving across ecosystems can boost productivity, stability, and resilience. Their integration through food webs is often mediated by small, abundant taxa channelling subsidies from primary producers to higher trophic levels. On coral reefs, cryptobenthic fishes and invertebrates could fill these roles, yet their responses to allochthonous nutrients are poorly understood. We compared cryptofaunal communities and associated trophic dynamics across islands with or without seabird-derived nutrients in the Chagos Archipelago, Indian Ocean. Most cryptobenthic fishes and some invertebrates showed nutrient enrichment. Community patterns revealed strong asymmetries: cryptobenthic fish biomass doubled in nutrient-rich environments, with a tenfold increase in larger piscivorous fish productivity, while cryptic invertebrate biomass stagnated, with reduced invertivore productivity. Life-history traits likely enable cryptobenthic fishes to exploit subsidies more efficiently, intensifying pressure on invertebrates in nutrient-rich environments. Our findings reveal how nutrients are routed through cryptofaunal pathways, underscoring the far-reaching impacts of cross-ecosystem nutrient vectors on trophic functioning. Graphical abstract Introduction Nutrient flow, both across ecosystems and through food webs, is critical for sustaining productivity, especially for oligotrophic ecosystems. On nutrient-limited coral reefs that surround isolated atolls and islands, seabirds act as key cross-ecosystem nutrient vectors, feeding in the open ocean and depositing nitrogen- and phosphorus-rich guano where they roost and nest (Benkwitt et al. 2022; Gove et al. 2016; Graham et al. 2018; Honig & Mahoney 2016; McCauley et al. 2012; Otero et al. 2018). These nutrients leach into adjacent marine environments, fertilizing reefs and boosting productivity across trophic levels, underscoring the central role seabird-derived nutrients play in structuring reef ecosystems (Benkwitt et al. 2021a, 2023). Bottom-up enrichment can propagate through food webs via multiple trophic pathways (Dunn et al. 2025), yet the mechanisms through which added nutrients are funnelled up food webs and influence fish biomass and productivity are only partially understood. As a direct result of primary producer nutrient enrichment, herbivorous reef fishes consistently show elevated growth and biomass associated with seabird presence (Benkwitt et al. 2021b; Graham et al. 2018). Higher-level consumers also respond to nutrient-driven changes: reefs with seabird nutrient input also support greater biomass of carnivorous fishes (Benkwitt et al. 2019; Graham et al. 2018). In contrast, little is known about how nutrient availability affects underlying prey communities, and the specific trophic routes by which seabird-derived nutrients propagate through reef food webs. A primary mechanism for the integration of seabird nutrients is likely through the benthic community, where otherwise nutrient limited primary producers (e.g. turf algae) take up seabird-derived nutrients (Benkwitt et al. 2025b; Graham et al. 2018). Organisms strongly associated with the benthos that derive energy from its primary producers may therefore act as critical vectors in the transfer of seabird nutrients to higher trophic levels. A key, yet often overlooked, component of reef pathways are cryptofauna – small-bodied, hidden reef organisms (typically < 5 cm) that reside within crevices, rubble, and coral structure. These include cryptobenthic fishes and small motile invertebrates (e.g., crustaceans, mollusks, worms sensu lato ), which collectively dominate reef biodiversity and biomass (Glynn & Enochs 2011; Plaisance et al. 2021). Both guilds are highly productive, ecologically important, and their strong association with benthic habitats for shelter and food suggest that they play a key part in integrating and transferring seabird nutrients through food webs (Goberdhan et al. 2024; Plaisance et al. 2021; Stella et al. 2011). In particular, cryptobenthic fishes’ short lifespans and early maturation result in extremely rapid turnover, making them a cornerstone of energy transfer from lower trophic levels to piscivores (Brandl et al. 2018, 2019). Cryptic invertebrates, likewise, are a hyper-diverse assemblage that sustains invertivorous fishes and contribute significantly to reef food web stability (Kamen et al. 2024; Kramer et al. 2015). Yet, their small sizes and inconspicuousness lead to logistical difficulties associated with sampling, and both guilds are typically studied separately. Little is known about the relative responses to nutrient inputs of cryptobenthic fishes and invertebrate communities inhabiting the same microhabitats. A significant gap therefore remains in understanding the ecological dynamics driving cryptic fishes and invertebrates, which, despite co-occurring in up to two-thirds of reef habitats (Takada et al. 2007), have rarely been investigated together. The role of small consumers in nutrient and energy fluxes exhibits strong environmental context specificity and may therefore differ between nutrient-rich and nutrient-poor habitats (Brandl et al. 2025). Evidence suggests both guilds respond to seabird-derived nutrient input: both cryptobenthic fish and invertebrate species can exhibit elevated δ 15 N near seabird colonies indicative of seabird-nutrient uptake, and cryptobenthic fishes and macro-invertebrates are larger in nutrient-rich environments (Appoo et al. 2023, 2024; Healing et al. 2024; Jeannot et al. 2025). However, they differ fundamentally in their life-history, ecology, anatomy, nutritional value, and are used by different suites of predators, and potentially represent a potential dichotomy in how nutrients are cycled from lower trophic levels into larger consumers. Unravelling the relative contributions of both carnivorous trophic pathways – piscivory and invertivory – to overall reef productivity and how these might be affected by nutrient availability is key to further our understanding of reef functioning. Here, we compare the use of environmentally available nutrients in cryptobenthic reef fishes and cryptic invertebrate communities and evaluate how they influence associated piscivorous and invertivorous trophic pathways. We focus on a remote reef system, the Chagos Archipelago in the Indian Ocean under high (rat-free, seabird-rich islands) vs. low (rat-infested, seabird poor islands) nutrient conditions, and ask: do nutrient-enriched reefs favour both communities concurrently, or one over the other? How does seabird-nutrient uptake affect cryptobenthic fish and invertebrate community biomass, abundance, and structure? And, if present, how do shifts in cryptofauna influence predator biomass and productivity of piscivores and invertivores? By examining these dynamics, we clarify the role of nutrient subsidies for the flow of energy through reef food webs – particularly the balance between fish- and invertebrate-mediated energy pathways – and to improve our understanding of how changes in nutrient availability might impact coral reef functioning. Methods Study site This study was conducted across the remote atolls of the Chagos Archipelago, Indian Ocean, a large, isolated and largely uninhabited marine protected area with no direct human impacts on the studied atolls. Despite being a globally important area for seabird populations (Carr et al. 2023), the presence of invasive predatory black rats ( Rattus rattus ) has significantly depleted seabirds on some islands where rats were introduced several hundred years ago. Seabird density is therefore ~760 times greater on rat-free islands, with ~ 250 times greater seabird-derived nitrogen fertilising the reef surrounding these islands and promoting the growth of corals, herbivorous damselfish and parrotfish (Benkwitt et al. 2021b, 2023; Graham et al. 2018). We used these differences in seabird-derived nutrient subsidies between islands with and without rats to test how nutrient inputs from seabirds affect cryptofauna. We collected cryptobenthic fishes and cryptic mobile invertebrates from 17 to 31 October 2023 from 7 islands (3 seabird-rich and 4 seabird-poor) spanning three atolls: Salomon, Peros Banhos, and the Great Chagos Bank (Fig. 1). All cryptofauna was collected on the lagoonal side of the islands except for the rat-infested Eagle Island (Great Chagos Bank) where environmental conditions in the lagoon prohibited sampling. All individuals were caught on SCUBA at an average depth of 5.9 ± 1.9 m, in reef crest and back reef slope habitats within 150 m from islands. Field sampling Each sampling station consisted of a coral outcrop selected to be similar in shape, isolation, and size (150 – 260 cm Curved Surface Length (CSL); 1.4 to 4.3 m 2 surface area) and sampling suitability for cryptobenthic fish and invertebrate sampling. Sampling followed well-established cryptofauna collection methods (Ackerman & Bellwood 2002; Brandl et al. 2018; Stier & Leray 2014) and is further detailed in Supplementary Text S1. In total, cryptobenthic fishes and invertebrates were sampled across 23 sampling stations (seabird-rich location: 12 stations, seabird-poor location: 11 stations; n = 2-4 per island; Table S1). To compare visually conspicuous piscivorous and invertivorous fish communities, Underwater Visual Censuses (UVC) were conducted both in 2021 and 2023 following Benkwitt et al. (2019) and Graham et al. (2018). Stable isotope analyses All individuals were photographed, weighed, measured, and identified to the highest taxonomic level possible. This was at species level for fish and family level (90.3% of all sampled invertebrates) and above (9.7%) for invertebrates, due to comparatively limited taxonomic resolution for small invertebrates in the region. Average size and weight per taxa and invertebrate length metrics are presented in Table S2. Seven cryptobenthic fish species – including six gobies and one carnivorous dottyback ( Chlidichthys chagosensis ) - and seven cryptic invertebrate taxa were selected for stable isotope analysis due to high abundance and presence in both seabird-rich and seabird-poor islands (Table S3). For full details of laboratory procedures see Supplementary Text S2. Data analysis To explore δ 15 N across different taxa as a function of island status (fixed effect, two levels: seabird-rich and rat-infested) and island (nested within atoll as a random effect to account for local variation in δ 15 N), we fit Gaussian Bayesian linear mixed-effects models. Separate models were first fit for each guild, and interactions with invertebrate taxa and cryptobenthic fish species respectively were then included to investigate taxonomic level influence on δ 15 N. We further explored the interaction between island status and cryptobenthic fish or invertebrate length on δ 15 N to investigate differential enrichment in larger, higher-metabolism individuals. Length was log-transformed and centered to account for allometric scaling and to reduce collinearity between length and its interaction with island status. Sampling depth and reef habitat were similar within each atoll and were therefore accounted for using this model structure. Isotopic niches were investigated using the SIBER package (Jackson et al. 2011). We fitted ellipses using JAGS, computed percent overlap, and calculated the standard ellipse area corrected for small samples (SEAc) for both cryptobenthic fish and invertebrate communities across seabird-rich and rat-infested islands. Models were run using 2 chains of 20,000 iterations, 1,000 burn-in samples, and a thinning factor of 10. We compared the density and biomass of cryptic invertebrate and cryptobenthic fishes across seabird-rich and rat-infested islands, standardized by CSL-derived sampling area, using log-link Gamma Bayesian linear mixed-effects models. Three stations were characterized by high-swell conditions that affected sampling and were therefore removed from community analyses due to resulting concerns about the comprehensiveness of the collection, resulting in a total of 20 stations (seabird-rich location: 10 stations, rat-infested location: 10 stations; Table S1) for this analysis. Models were also run at cryptobenthic fish family level in addition to species level, with a small constant set to the order of magnitude of the smallest non-zero observation. To explore the effect of seabirds on the outcrop-level ratio of cryptobenthic fishes to invertebrates, two lognormally distributed Bayesian mixed-effect models were run with ratios of fish to invertebrate density and biomass as response variables, and island status and island within atoll as fixed and random effects respectively. Effects on community composition were further investigated using a Redundancy Analysis (RDA) with island status and atoll as constraints. A SIMPER (Similarity Percentage) analysis based on abundance using the vegan package (Oksanen et al. 2001) was used to identify taxa primarily responsible for differences between rat-infested and seabird-rich islands. To investigate whether seabird nutrients influenced the proportion of fish or invertebrates consumed by a cryptobenthic mesopredator, we estimated the reliance of the dottyback Chlidichthys chagosensis on either source. This was done using a two-source dual-biotracer (δ 15 N and δ 13 C) isotope mixing models with the R package MixSIAR (Stock et al. 2018). Only > 30 mm C. chagosensis were selected due to the high reliance of smaller individuals on δ 13 C-enriched resources (i.e. planktonic resources) (Fig. S1). Prey sources were other cryptobenthic fishes and cryptic invertebrates. We chose a trophic discrimination factor (TDF) of 0.4‰ ± 1.3 SD for δ 13 C which is considered widely applicable in aquatic food webs (Post 2002). To visualize ontogenetic changes in diet, C. chagosensis’ total length was used as a continuous variable. Model parameters were set using the pre-defined “normal” parameter set, corresponding to three chains of 100,000 iterations with 50,000 burn-in samples and a thinning interval of 50. To investigate potential biomass and productivity differences in larger reef fish communities, larger fish recorded from UVC data were partitioned into feeding groups (Sandin and Williams, 2010) and biomass was calculated using length-weight relationships from Fishbase Bayesian estimators (Froese et al. 2014; Froese & Pauly 2000). We computed yearly productivity based on Benkwitt et al. (2020), Brandl et al. (2019), and Morais and Bellwood, (2020). Species trait data was extracted from Froese and Pauly (2000) and Morais and Bellwood (2018), and sea surface temperature was set at 28°C which corresponds to the average sea surface temperature in the Chagos Archipelago (Benkwitt et al. 2020; Sheppard et al. 2012). Finally, biomass and productivity at each transect were integrated in log-link Gamma Bayesian models to evaluate the effect of seabird nutrients, with island within atoll and year as random effects. All Bayesian models were run with four chains and 2,000 iterations including 1,000 burn-in samples, using brms v 2.22.0 (Bürkner 2017). Model validation was performed using visual evaluation of chain convergence and posterior predictive checks. Models were run with default priors unless specified in Table S4, in which case prior predictive checks were performed. All analyses were done in R 4.5.0 (R Core Team 2025). Results Seven cryptobenthic fish families were found across the three atolls (Gobiidae, Tripterygiidae, Blenniidae, Syngnathidae, Pseudochromidae, Apogonidae, and Gobiesocidae), with 703 fish across 64 species from the 20 stations that were retained for community analyses. For invertebrates, we found 618 individuals across 35 taxa. Both cryptobenthic fish and invertebrates had average sizes short of two centimetres, and invertebrates were slightly smaller and lighter than cryptobenthic fish overall (Table S2). Seabird nutrient assimilation Both cryptobenthic fish (Fig. 2.a) and invertebrates (Fig. 2.c) showed moderate evidence of nutrient enrichment near seabird islands (85% and 81% posterior probability of higher δ 15 N near seabird islands, respectively), with significant variability at higher taxonomic resolution within each group. Within cryptobenthic fishes, posterior probabilities of higher δ 15 N ranged from 74% to 95%, with only Eviota nebulosa presenting no clear effect (40% posterior probability; Fig. 2.e, Table S5). Three invertebrate taxa also showed moderate to strong evidence (> 75%) of higher δ 15 N near seabird islands: trapeziid (93% posterior probability), and Porcellanid (80% posterior probability) crabs, and brittle stars (ophiuroids) (90% posterior probability), while all others ranged from 28% (galatheid squat lobsters) to 66% (palaemonid shrimps) posterior probabilities (Fig. 2.f). Body size influenced δ 15 N enrichment differently according to guild: seabird presence had a positive effect on this relationship for cryptobenthic fishes, with a 26% stronger correlation between length and δ 15 N for islands with seabirds, suggesting that larger cryptobenthic fishes display higher enrichment in δ 15 N near seabird islands (>99% posterior probability of positive effect of seabird presence on the correlation between length and δ 15 N; Fig. 2.b). No such overall trend was detected in invertebrates, where there was little to no observable effect of seabird presence on the strength of the correlation between δ 15 N and length (69% posterior probability; Fig. 2.d), suggesting larger size and higher metabolic demand correlates with higher δ 15 N assimilation near seabird islands for cryptobenthic fishes only. Isotopic niche variation The isotopic niches between cryptobenthic fishes and cryptic invertebrates overlapped regardless of island status. However, the proportional overlap differed according to treatment (98.4% overlap near seabird-rich islands; 72.8% overlap near rat-infested islands; Fig. 3.c). This was due to the corrected standard ellipse area (SEAc) of cryptobenthic fishes shrinking by 14.6% and the isotopic niche of cryptic invertebrates expanding by 61.4% near seabird-rich islands (Fig. 3.a, Fig. 3.b; Table S6). Cryptofaunal biomass, density, and community composition There was strong evidence of higher cryptobenthic fish biomass near seabird islands (96% posterior probability) and moderate evidence of higher density (84% posterior probability). Estimated cryptobenthic fish biomass was 3.91 g/m 2 (95% HDI: [0.01; 11.66]) near seabird islands, more than 2.2 times the estimated 1.76 g/m 2 (95% HDI: [0.01; 4.33]) near rat-infested islands (Fig. 4.a). This increase was mostly driven by gobies and triplefins (Fig. S2; Fig. S3). A weaker but opposite pattern emerged for invertebrates, which had a higher likelihood of greater or equal biomass and density near rat-infested islands (81% and 66% posterior probability respectively; Fig. 4.b). Ophiuroid, and trapeziid and porcellanid crab biomasses were lowest near seabird islands (Fig. S4). Likewise, in terms of relative biomass and abundance, the ratio of cryptobenthic fishes to invertebrates was consistently higher near seabird islands (97% posterior probability of higher biomass), with an average ratio of cryptobenthic fish biomass to invertebrate biomass of 0.9:1 near rat-infested islands to 5.2:1 near seabird-rich islands, indicating differences in cryptofauna community composition according to island status (Fig. 4.c). Both cryptic fish and invertebrate communities differed across the two treatments. There was species-level compositional dissimilarity (p-value Island status = 0.024, p-value Atoll = 0.001, R 2 adj = 0.183) for cryptobenthic fishes, and taxa-level dissimilarity (p-value Island status = 0.015, p-value Atoll = 0.123, R 2 adj = 0.129) for cryptic invertebrates. Five cryptobenthic fish species, including four goby species ( Amblygobius semicinctus, A. albomaculatus, Pleurosicya mossambica, P. plicata ), significantly contributed to the dissimilarities between treatments (Table S7), and were more abundant near seabird-rich islands. Three cryptic invertebrate taxa, including trapeziid and porcellanid crabs were all consistently more abundant near rat-infested islands (Table S8). Higher trophic level biomass and productivity The influence of seabird presence on higher trophic levels’ resource use was illustrated by the cryptobenthic predatory fish Chlidichthys chagosensis , which was found in the same coral bommies as the other cryptobenthic fishes and cryptobenthic invertebrates. C. chagosensis fed predominantly on invertebrate food sources irrespective of rat presence, yet relied more on fish as a resource near seabird islands (7.5% instead of 3.4%). This shift was related to total length, with larger individuals relying increasingly on fish (up to 19.4% of total diet by maximum total length measured near seabird islands; up to 9.5% near rat-infested islands; Fig. S5). Posterior estimates revealed substantial increases in both biomass and productivity of visually conspicuous piscivorous fishes near seabird-rich islands (> 99% posterior probability). Piscivorous fish biomass and productivity were enhanced by 86% and 58% respectively on the log scale (Fig. 5.a, Fig. 5.b), with an estimated biomass of 178.27 kg/ha (95% HDI: [29.49; 385.32]) near seabird-rich islands, corresponding to a tenfold increase compared to 16.83 kg/ha (95% HDI: [4.46; 35.08]) near rat-infested islands. In contrast, there was moderate evidence (81% and 84% posterior probability respectively) that invertivorous fishes exhibited a reduction in both biomass and productivity near seabird islands (15% and 10% lower respectively on the log scale) (Fig. 5.c, Fig. 5.d). Discussion Allochthonous nutrient inputs can influence the trophic pathways that sustain consumer communities within an ecosystem. We investigated how seabird-derived nutrient subsidies influence coral reef cryptofauna through whole-community collections of cryptobenthic fishes and cryptic invertebrates. Stable isotope analyses revealed assimilation of seabird-derived nitrogen by several cryptofaunal consumers. Yet, cryptobenthic fishes and invertebrates exhibited divergent trends in biomass and community structure: fish populations and biomass increased under nutrient enrichment, whereas motile invertebrate abundance and biomass showed weak and opposite trends. This divergence suggests that bottom-up fertilization by seabirds does not uniformly elevate all components of the food web, but instead triggers complex interactions likely modulated by life-history characteristics, feeding strategies, and predator–prey dynamics. In turn, as cryptofaunal interactions mediate uptake and trophic transfer to larger fishes, this asymmetry may propagate up the food web: larger nominal piscivores exhibited stark increases in biomass and productivity in the presence of nutrient enrichment, contrary to nominal invertivores. Collectively, these results suggest that seabird-vectored nutrients enhance reef productivity by boosting the abundance and biomass of cryptobenthic fishes, which in turn act as efficient conduits that shuttle energy to higher trophic levels that are adapted to feeding on fish prey. Assimilation of Seabird-Derived Nutrients by Cryptofauna Seabird-derived nutrients are assimilated into the coral reef cryptofaunal food web, but this appears to depend strongly on consumer identity. On islands with abundant seabird colonies and high guano input, several cryptobenthic fish species and invertebrate taxa had markedly elevated δ 15 N values relative to those from seabird-depleted islands. This is consistent with previous studies documenting seabird nutrient uptake across diverse aquatic invertebrates (e.g., isopods, amphipods, gastropods, decapods; (Appoo et al. 2024; Dunn et al. 2025; Healing et al. 2024; Kolb et al. 2010) and cryptobenthic fish assemblages (Jeannot et al. 2025). Here, isotopic enrichment was observed in several Eviota species, the goby Trimma haima , and cryptobenthic invertebrates (Ophiuroidea, Trapeziidae, Porcellanidae). However, elevated δ¹⁵N is not always synonymous with the presence or absence of seabird bottom-up effects (Benkwitt et al. 2021b). Similarly, despite evidence for cryptofauna-wide enrichment, we found inconsistent δ¹⁵N signals among cryptobenthic fish and invertebrate taxa, echoing findings on larger, visually conspicuous fishes elsewhere (Benkwitt et al. 2025a). Seabird-nutrient assimilation therefore appears to be mediated by a combination of traits and spatial dynamics that result in heterogeneous uptake across taxa. The heterogeneity in nutrient uptake among taxa likely reflects both dietary differences and fine-scale interactions between species’ diets and their nutritional landscape, with structurally complex habitats such as coral crevices serving as retention sites. Hydrodynamic conditions may influence how nutrients accumulate locally: rapidly flushing reefs may homogenize nearshore nutrient landscapes and result in uniform δ 15 N enhancement elsewhere (i.e. fringing reefs, Jeannot et al., 2025). On the other hand, areas with longer residence times and limited mixing such as the lagoons of the Chagos Archipelago (Rayner & Drew 1984) may amplify spatial heterogeneity and create nutrient micro-hotspots that exacerbate species and higher taxa-level divergences. Indeed, peak seabird-nutrient enrichment occurs in low-flow shallow areas due to localised seafloor curvature, where organisms that inhabit complex microhabitats (i.e. biofilms and filter-feeders) can then sequester dissolved nutrients (Stuart et al. 2025). In line with this, the grazing of the epilithic algal matrix and its detrital aggregates can be an important seabird-nutrient incorporation pathway for intertidal cryptobenthic fishes and invertebrates (Andrades et al. 2024). Cryptofauna are uniquely positioned to exploit these spatially constrained resources as their small body sizes and microhabitat specialization allow access to nutrient hotspots inaccessible to larger consumers. Our results suggest that the degree to which cryptofauna can access and assimilate seabird-derived nutrients is influenced by their mobility and microhabitat preferences. Species restricted to microhabitats where seabird-subsidized primary producers or detritus may accumulate (e.g., coral branches, reef crevices) likely exhibit stronger isotopic enrichment, a pattern amplified by cryptofauna’ limited home ranges (<1–5 m² for cryptobenthic fishes; (Depczynski & Bellwood 2004). For example, E. distigma and E. prasina , which showed the strongest evidence of δ¹⁵N enrichment, occupy distinct reef zones, and are both associated with coral or rock crevice habitats (Herler 2007). In contrast, E. guttata and E. sebreei , which displayed weaker enrichment, occur across broader depth ranges, dwell on coral surfaces, and therefore may incorporate less enriched resources from mixed nutrient regimes (Herler 2007; Randall 1995). Similarly, the cryptobenthic invertebrates that showed the strongest seabird nutrient isotopic signal were those with specialist habitat preferences and diets that may have favoured seabird nutrient uptake. In particular, coral-associated trapeziid crabs seldom move far from their host colony and directly consume live tissue or mucus from corals subsidized by guano in this system (Benkwitt et al. 2023; Rinkevich et al. 1991; Stella et al. 2011; Stimson 1990). Likewise, Ophiuroidea are bottom-dwellers with diverse feeding modes that facilitate selective nutrient uptake, and they occupy reef crevices where seabird-enriched resources may accumulate (Stöhr et al. 2012). In the case of cryptobenthic invertebrates, differences in seabird nutrient assimilation can also be confounded by our limited taxonomic resolution that may mask species-specific responses. For example, the high variance in δ 15 N for palaemonids and squat lobsters (Galatheidae) correlates with the wide range of trophic strategies within those families (De Grave et al. 2021; Lovrich & Thiel 2011). Isotopic responses to seabird subsidies therefore likely reflect an interplay of microhabitat specificity and local hydrodynamics resulting in patchy nutrient distributions and species-specific isotopic profiles. Interguild Predation and Competition Several lines of evidence suggest that seabird-derived nutrients boost cryptobenthic fish communities more than invertebrates across our study system. The stronger correlation between δ 15 N and cryptobenthic fish total length near seabird islands implies two potential, non-mutually exclusive scenarios: (1) as fishes grow, they accumulate greater amounts of seabird-derived nutrients; and (2) fishes grow faster near seabird islands, as previously demonstrated in two larger, non-cryptobenthic fish species (Benkwitt et al. 2021b; Graham et al. 2018). In addition, Eviota distigma, which presented the strongest δ 15 N responses to seabird presence, was also associated with the most significant biomass enhancement near seabird islands. On the other hand, trapeziids and ophiuroids, which also exhibited higher δ 15 N, both showed significantly lower biomass near seabird islands. This suggests that while some invertebrates ingest seabird-derived nutrients, they are unable to translate it into biomass, as opposed to some cryptobenthic fishes. These patterns hold true at guild level, with higher biomass of cryptobenthic fishes and a fivefold increase in the proportion of cryptobenthic fish biomass to cryptic invertebrate biomass near seabird islands. Fishes and invertebrates’ ecological responses are intricately linked and, in some cases, interdependent, with some species of gobies, alpheid shrimps, and, in a rare case, a porcellanid crab, forming unique symbioses (Werding et al. 2016). Yet, our results are consistent with other studies demonstrating divergent responses across multiple ecosystems (Jackson & Harvey 1993; Lin et al. 2021; Wyżga et al. 2014), with competition and predation often driving these responses. Our findings provide further evidence that both competitive interactions and top-down forces modulate the bottom-up effects of nutrient enrichment in the cryptofaunal community. Competition is strongest in horizontal communities made up of sympatric, ecologically similar, and comparably sized individuals. Here, variation in isotopic niche overlap and widths suggests that interguild competition drives access to resources. Greater levels of competition and resource scarcity, as would be observed in the nitrogen- and phosphorus-limited waters near rat-infested islands, can result in diet specialization (Andrades et al. 2019; Pelage et al. 2022; Pianka 1974), and we indeed observed smaller isotopic niche overlap between fish and invertebrate guilds around islands with rats. On the other hand, in nutrient-rich environments near seabird islands, niche expansion is asymmetrical and driven solely by invertebrates. Dominant competitors can alter subordinate competitors’ niche width by driving them to exploit alternative, lower-quality resources (Abbey-Lee et al. 2013; Belant et al. 2010; Codron et al. 2011; Sebastián et al. 2015), and the nearly 52% difference in invertebrate niche width near seabird islands therefore suggests that small, motile invertebrates may be subordinate to dominant cryptobenthic fishes in this system. This is further supported by the fact that, in nutrient-poor conditions, cryptobenthic fishes retain access to high- δ 15 N resources. Cryptobenthic fishes’ life-history traits make them a prime candidate for rapid exploitation of a localized resource and explain why they might exert competitive dominance over invertebrates in these systems: rapid growth, sustained reproduction, abundant larvae, and heavily localized larval recruitment ensure that they can quickly expand their population when resources become abundant. By contrast, marine invertebrates encompass a much wider range of life-history traits, with highly variable larval dispersal distances (Huber 1985; Levin 2006; Scheltema 1986; Shanks 2009) and typically non-linear growth rates. Small marine invertebrates have been associated with lower mass-scaled metabolic rates (Banse 1982), contrary to cryptobenthic fishes’ high metabolic rates which present a competitive advantage in nutrient-rich environments (Clarke 1992). Near seabirds, the high metabolic and nutrient demands of cryptobenthic fishes’ can be matched through uptake of seabird-enriched resources, allowing them to maintain high growth and reproduction, to possibly outcompete or potentially prey on cryptic invertebrates for space or resources. Several of the cryptobenthic fishes we captured are also invertivorous: gobies, triplefins, clingfishes, apogonids, and pseudochromids are all known to feed on small or micro-invertebrates (Brandl et al. 2018), which are in fact the dominant prey in C. chagosensis’ diet. The increase in biomass of triplefins and gobies may therefore have been a driving factor in limiting invertebrate success. The fact that trapeziid and porcellanid crab biomass declined near seabird islands - in spite of corals being well-documented recipients of seabird nutrients in this system (Benkwitt et al. 2023; Savage 2019), and coral-associated decapod crustacean response being strongly tied to coral health (Glynn et al. 1985) - further suggests that additional pressures are exerted onto invertebrate communities. Whether cryptobenthic fishes or other invertivorous organisms actively select nutrient-enriched prey remains to be determined, but it is noteworthy that the invertebrates with the strongest δ 15 N signature were also characterized by the strongest decline in biomass near seabird islands. Here, we therefore suspect that any bottom-up benefit that cryptic invertebrates might have gained from guano was outweighed by increased predation and competition pressures. Trophic Channelling Toward Larger Consumers Our results have important implications for how seabird-derived nutrients ultimately propagate to higher trophic levels on coral reefs. Here, the dottybacks Chlidichthys chagosensis, small reef predators that typically feed on invertebrates and small fishes (Ashworth et al. 2014; McCormick & Holmes 2006; Palacios-Narváez et al. 2024), switched from invertebrate prey to fish prey, exhibiting a twofold increase in cryptobenthic fish consumption near seabird islands . C. chagosensis remained predominantly invertivorous, yet its increased fish consumption suggests that enhanced cryptobenthic fish biomass and availability can also cause mixed carnivorous species to consider more abundant and readily available prey. By altering the balance of cryptofaunal communities in favour of fishes over invertebrates, nutrient enrichment directs energy flow into the pathways that lead to larger piscivorous predators. Our observations are consistent with previous studies recording higher biomass of piscivores, with the second largest effect size following herbivores, and a larger effect size than invertivores’ near seabird islands in the Chagos Archipelago (Benkwitt et al. 2019; Graham et al. 2018). Similar to invertebrates, invertivorous fish represent a broad feeding group with a wide range of diets and may therefore present more variable responses to prey availability (Parravicini et al. 2020); invertebrate predation may also be dominated by invertivorous cryptobenthic fishes, whose spatial proximity offer more rapid access to prey than larger invertivorous fishes. Here, tenfold biomass gains in piscivorous fishes far exceeded cryptobenthic fishes’ twofold increase. It is worth noting that cryptobenthic fishes’ contribution to reef trophodynamics far outweigh what their standing biomass might suggest due to exceptionally fast biomass turnover rates. Even modest increases in their biomass can therefore trigger disproportionate gains in higher trophic levels (Brandl et al. 2019). Some of the increase in piscivorous biomass is also likely attributable to predation on visually conspicuous fishes, and underwater visual censuses only captures some predators that specialise on cryptobenthic fishes (average predator size = 3.65 cm; average piscivorous fish size here = 35.45 cm; (Mihalitsis et al. 2022)). Our evidence is ultimately indirect, and targeted experiments, gut content and isotope analysis of piscivores are likely needed to quantify the importance of cryptobenthic fishes within predatory fish diets. Ecologically, our findings highlight the importance of cross-ecosystem nutrient vectors in enhancing coral reef function and their mediation by community composition and interactions, underscoring the importance of cryptobenthic organisms in this linkage. Conserving seabird populations and nesting habitats directly therefore benefits adjacent marine ecosystems, shaping and promoting robust fish communities that can fuel food webs. Figure list Fig. 1: Map of the sampling locations. Numbers within parentheses represent the number of sampling stations per island, and +1 indicates that samples from an additional sampling station were used for stable isotope analysis only. See Table S1 for further details about sampling sites (location, seabird density, curved surface length of sampled area). Fig. 2: Effects of seabird presence on cryptofauna δ¹⁵N. Density curves and caterpillar plots show δ¹⁵N distributions for cryptobenthic (a) fish and (c) invertebrate guilds, grouped by seabird presence. Caterpillar plots represent fitted values from Bayesian linear models, with 50% (bold lines) and 95% (thin lines) credible intervals based on 1,000 posterior draws. Relationship between δ¹⁵N and length for (b) fish and (d) invertebrates, grouped by seabird presence. Shaded bands represent 50% credible intervals, and individual dots correspond to raw data points. Inset plots represent posterior draws of the percentage of slope increase near seabird islands. Contrasts between seabird-rich and rat-infested islands by cryptobenthic (e) fish species and (f) invertebrate taxa. Fig. 3: (a) Isotopic biplots of δ 13 C and δ 15 N for cryptobenthic fish and invertebrates grouped by seabird presence. Error bars represent standard deviation, and ellipses are drawn using the normal distribution at a 95% confidence level. (b) Posterior distributions of Bayesian Standard Ellipse Areas (per mille 2 ), estimated using posterior draws from a multivariate normal distribution using the SIBER package. (c) Posterior distributions of percentage overlap between cryptobenthic fish and invertebrate isotopic niches based on 4,000 draws. Fig. 4. Posterior predictive distributions for (a) cryptobenthic fish biomass and density, (b) cryptic invertebrate biomass and density and (c) fish to invertebrate ratios. Points represent median estimates, and lines 95 and 50% highest posterior density intervals. Fig. 5: Posterior distributions of (a) biomass and (b) productivity of visually conspicuous piscivorous fishes and (c) biomass and (d) productivity of visually conspicuous invertivorous fishes. Density curves and caterpillar plots (50% and 95% credible intervals) represent fitted values from Bayesian linear models based on 1,000 posterior draws. Acknowledgements We thank the United Kingdom Foreign and Commonwealth Office and the British Indian Ocean Territory Administration for granting us permission to undertake the research. This project was funded by the Bertarelli Foundation and contributed to the Bertarelli Programme in Marine Science. We would also like to thank Kate Stanton, Tom Everett, Yuneeda Oozeeraully, and Javier Gonzalez Barrios for their help during fieldwork, and Ryan Hladinyuk, Emma Chesley and Elisabeth Frasch for their help with stable isotope analysis. 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Keywords coral reef ecology cryptobenthic fishes cryptofauna nutrient enrichment nutrient subsidies reef invertebrates seabird guano stable isotope analysis Authors Affiliations Laura-Li Jeannot 0000-0002-1999-7563 [email protected] Lancaster University View all articles by this author Ruth Dunn Lancaster University View all articles by this author Joyce Velos The University of Texas at Austin View all articles by this author Gareth Williams Bangor University School of Ocean Sciences View all articles by this author Cassandra Benkwitt Lancaster University View all articles by this author Nicholas Graham 0000-0002-0304-7467 Lancaster University View all articles by this author Simon Brandl The University of Texas at Austin View all articles by this author Metrics & Citations Metrics Article Usage 290 views 237 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Laura-Li Jeannot, Ruth Dunn, Joyce Velos, et al. Contrasting cryptofaunal responses to seabird nutrient inputs illuminate coral reef productivity pathways. Authorea . 24 September 2025. DOI: https://doi.org/10.22541/au.175821493.32427292/v2 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download. For more information or tips please see 'Downloading to a citation manager' in the Help menu . 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