Assessment of the status and distribution of the poorly known nudibranch Madrella aurantiaca (Gastropoda: Nudibranchia: Madrellidae)

Assessment of the status and distribution of the poorly known nudibranch Madrella aurantiaca (Gastropoda: Nudibranchia: Madrellidae) | 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 Assessment of the status and distribution of the poorly known nudibranch Madrella aurantiaca (Gastropoda: Nudibranchia: Madrellidae) Bruno Almón, Jacinto Pérez, Juan Trigo, Miquel Pontes, Manuel Ballesteros This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8832905/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Among the limited number of species currently included in the nudibranch genus Madrella Alder & Hancock, 1864 , only one has been documented from European waters. Nevertheless, our knowledge of the distribution and biology of this species remains limited, largely due to the paucity of observations, which are confined to a small number of records from the Mediterranean and the north-east Atlantic. During a series of sampling dives conducted in Galicia to catalogue benthic fauna, a single specimen of Madrella aurantiaca Vayssière, 1902 was discovered. Given the scarcity of records for this species, the specimen was photographed and collected for further study. Additional specimens captured in the Balearic Islands (western Mediterranean) during the FAUNA III oceanographic campaign were also examined in order to compare and update the available information on the species. As a result, new data on morphology and biology are presented, including a description of the egg masses. Historical records of the species are also reviewed, providing an overview of its current status and an updated distribution. The addition of new records for such a rarely reported species is of considerable value for improving our understanding of its distribution and biology, and ultimately for refining our knowledge of global diversity. Diversity Atlantic Mediterranean Reproduction Systematics Sea slug Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction Sea slugs comprise a diverse assemblage of marine gastropods, typically characterised by the absence of an external shell in the adult stage and, in many taxa, by the presence of externally exposed gills. They exhibit a wide range of body forms and colour patterns, which has contributed to their popularity among underwater photographers. Sea slugs occur worldwide and inhabit a broad spectrum of marine environments, often showing close ecological associations with their primary food sources (Robison et al. 2024). Strictly speaking, and following the recent systematic revision by Korshunova et al. ( 2025 ), former nudibranchs are now assigned to two orders within the subclass Heterobranchia (Mollusca, Gastropoda): Doridida and Nudibranchia, a classification currently adopted by WoRMS. Prior to this revision, both groups were included within the taxon Nudibranchia Cuvier, 1817, which is now restricted to the former Cladobranchia, characterised by the presence of a branched digestive gland. The total diversity of sea slugs has been estimated at approximately 2,700 species (Wägele 2004 ; WoRMS 2025), although this figure varies among sources as a result of ongoing systematic revisions and the continual description of new taxa (Schrödl et al. 2011 ; Johnson and Gosliner 2012 ). Research interest in sea slugs has evolved considerably over time, progressing from early studies primarily concerned with documenting species diversity to a more integrative approach encompassing molecular phylogenetics and ecological adaptations (Wägele and Willan 2000; McDonald 2009 ; Martynov 2011 ). Although the description of new species remains frequent within this modern framework, a substantial proportion of currently recognised species originates from these pioneering studies. Such works were often broad in scope and commonly based on material obtained during scientific expeditions, regional surveys, or examinations of museum collections. While the shift in research focus has resulted in a more detailed understanding of their evolutionary history and diversification patterns, some species remain poorly known, particularly those inhabiting deep-sea or remote environments, as well as taxa that are genuinely rare or difficult to detect (Valdés et al. 2012 ; Almón et al. 2019 ; Robison et al. 2024). The genus Madrella Alder & Hancock, 1864 represents a poorly known group within Nudibranchia, comprising several morphologically similar species and characterised by a long history of taxonomic uncertainty (Pola et al. 2019 ). The genus was originally erected to accommodate a new species described from several specimens collected in southern India. This species, Madrella ferruginea Alder & Hancock, 1864 , exhibited several distinctive characters in comparison with other members of the group, including the lateral position of the anus and the absence of oral tentacles, which are replaced by a veil-like expansion of the head (Alder and Hancock 1864 ). Although Madrella was initially assigned to the family Proctonotidae Gray, 1853, subsequent studies recognised that the genus Madrella was sufficiently distinct to justify placement in a separate family. Consequently, the family Madrellidae was erected to include this genus alone (Preston 1911 ), and was later expanded by the inclusion of a second genus, Eliotia Vayssière, 1909. The genus Madrella currently comprises six valid species that share a broadly similar external appearance and the diagnostic characters of the genus: Madrella amphora Pola & Gosliner, 2019, from Papua New Guinea; M. aurantiac a Vayssière, 1903 , from the Mediterranean Sea and north-east Atlantic; M. ferruginosa Alder & Hancock, 1864 , from the Indian coasts, the Philippines, and the Marshall Islands, and more recently recorded from South Africa, where it is considered non-native; M. gloriosa Baba, 1949 and M. granularis Baba, 1949, from Japan; and M. sanguinea (Angas, 1864), from Australian waters and the seas of China and Japan (WoRMS 2025). Records and biological information for M. ferruginosa and M. sanguinea are relatively well documented by Rudman ( 2001a , b ). Owing to the general paucity of information available worldwide, species-level differentiation within the genus has proven difficult and has been subject to differing interpretations. Some authors have suggested that M. ferruginosa , M. aurantiaca and M. granularis might be junior synonyms of M. sanguinea (Thompson et al. 1990 ), based primarily on their reddish or orange coloration and on the variability of certain characters during ontogenetic development. Moreover, M. sanguinea was originally described within the genus Janus in 1864 and was later transferred to Madrella , which would confer priority to the former name in the event of synonymization. At present, however, all six species are regarded as distinct and valid lineages (WORMS, 2025). This consensus is based largely on the interpretation of the observed morphological differences as evolutionary significant. Nevertheless, future studies incorporating comparative genetic analysis across the different Madrella species will be essential to rigorously test this hypothesis. To date, only M. ferruginosa and M. amphora have been genetically characterised, with sequences available in public databases (Pola et al. 2019 ). In European waters, Madrella aurantiaca is the only species of the genus recorded to date. Records of this inconspicuous species are scarce and largely restricted to the Mediterranean Sea. In the Atlantic, confirmed records are limited to southern Portugal (Calado and Silva 2012 ), the Atlantic coast of France, with a report from Arcachon bay (Bouchet and Tardy 1976 ), and another from the Roscoff region in France (Cornet and Marche-Marchad 1951 ; Bouchet and Tardy 1976 ), as well as a single specimen captured on 12 July 1932 at Mellersta Kattegatt, Lilla Middelgrunds, Sweden. This specimen is preserved in the Invertebrates Collection of the Gothenburg Natural History Museum (GNM) (Bohm and Johnson 2023). The recent discovery of a specimen of M. aurantiaca in Galicia has prompted a reassessment of the species’ status and distribution, together with the examination of previously unpublished records from the Balearic Islands (Mediterranean Sea). In the present study, the literature pertaining to the species was reviewed and updated in order to clarify uncertainties associated with historical records and the resulting distribution in European waters. Data obtained from the examination of freshly collected specimens allow the original description to be complemented and contribute to an improved understanding of the species’ ecological status. Material and Methods The material examined comprises four individuals collected from different locations in Spanish waters (Fig. 1 a). The Galician specimen was observed and hand-collected during a SCUBA dive conducted by the Grupo de Estudo do Medio Mariño (GEMM) as part of its programme to catalogue the marine fauna of the Galician coast. During the FAUNA III research campaign aboard the O/V García del Cid in the Balearic Islands, three additional specimens were collected: one by trawl dredge ( bou de varas ) and two by SCUBA diving. The specimens were studied and photographed in situ to document their external morphology, colouration, and habitat. Following a preliminary examination, they were preserved in 70% ethanol for subsequent laboratory study. Measurements refer to the fully extended living specimen from Galicia, whereas the lengths of the Balearic specimens correspond to slightly contracted individuals. Illustrations were prepared from the largest Balearic specimen, and the radula and jaws of another individual were examined by dissection through the dorsal region. The description of the spawning mass is based on an egg ribbon laid in the maintenance tank in which the Galician specimen was kept alive. Consequently, it cannot be stated with certainty that the clutch produced under captive conditions fully reflects that of wild individuals in terms of size or extent, although it is considered representative of the general shape and colouration. Given the rarity of records for this species, the specimens have been deposited in the invertebrate collection of the Museo de Historia Natural of Santiago de Compostela (MNH-USC) under MHN USC-25235 reference code, and the Centre de Recursos de Biodiversitat Animal (CRBA) under the codes CRBA-114209 and CRBA-114210. In addition, a map illustrating the confirmed worldwide distribution of the different Madrella species is provided for comparative purposes (Fig. 1 b). Distributional data were retrieved from GBIF and OPK-Opisthobranquis databases (GBIF 2025; Ballesteros et al. 2025 ). All digital artwork was prepared using Adobe Photoshop CS6, and the map compositions were generated with QGIS ( https://qgis.org/ ). Line drawings correspond to digitised versions of the original illustrations produced in the field by M. Ballesteros. Results Class Gastropoda Cuvier, 1795 Subclass Heterobranchia Burmeister, 1837 Superorder Nudipleura Wägele & Willan, 2000 Order Nudibranchia Cuvier, 1817 Suborder Janolina Minichev & Starobogatov, 1979 Superfamily Proctonotoidea J. E. Gray, 1853 Family Madrellidae Preston, 1911 Madrella aurantiaca Vayssière, 1902 Material examined One fresh specimen, 40 mm (measured out of water), (CRBA-114209), 39°35'25.0"N 2°18'46.1"E, north of Dragonera Island (Balearic Islands), collected on the seabed among the irregular sea urchin Spatangus purpureus and other echinoderms, 86 m depth, 24 June 1994; two slightly contracted specimens, 9 mm, (CRBA-114210), 39°21'39.3"N 2°47'12.6"E, Cap Blanc, south-eastern Bay of Palma de Mallorca, among algae and marine invertebrates, 20 m depth, 2 July 1994; one live individual, 30 mm (MHN USC-25235), 42°30'33.0"N 8°56'25.8"W, Os Esqueiros shelf, under a stone, 24 m depth, 26 May 2025. Description The examined specimens exhibited an overall reddish colouration. The Galician specimen was translucent orange, with reddish apices on the papillae and mantle (Fig. 2 ), whereas the Balearic specimens displayed a more or less uniform, intense red colour over the entire body. The dorsal surface of the mantle was covered by small conical tubercles of varying size, many of which possessed white apices (Figs. 2 b, c; 3 a-b, d). The lateral margins were ornamented with at least five or six rows of long digitiform to fusiform papillae (Figs. 2 a-c; 3 a-c, e). When fully extended, these papillae curved towards the central dorsal region, partially obscuring it. The anterior margin bore papillae similar in shape and arrangement to those on the lateral sides, although smaller in size; these papillae were occasionally tipped in white. The rhinophores (Figs. 2 a-c; 3 b, f) were long, broad at the base and tapering towards the apex. The upper half bore numerous contractile digitiform tubercles arranged along the lateral and posterior surfaces, leaving the apex free. The apex was short and rounded, bearing several small white spots. The frontal veil was well developed and slightly lobed. The mouth was narrow and flanked by two elongated red lobes extending laterally along the head, as well as by a posterior orange lobe. The foot was very broad, projecting beyond the margins of the mantle, slightly whitish in colour, and exhibited a faint dark central spot corresponding to the internal viscera (Fig. 3 c). Anatomy One of the two 9 mm specimens, which was immature, was dissected. The jaws are well developed, extending to almost half the length of the animal; they are yellowish, strongly cornified, and enclose the entire radular bulb (Fig. 4 a). In lateral view (Fig. 4 b), the anterior portion of the jaw resembles the fruiting body of a mushroom. The radula is small and narrow, comprising only 16 rows of teeth (Fig. 4 c). It is triseriate, with three teeth per row. The median tooth has a broad base and bears a fine, sharp central cusp flanked by delicate denticulations. On each side of the median tooth, the lateral teeth have broad bases with serrated denticulations, terminating in an elongated, sharp marginal denticulation curved inwards (Fig. 4 d). All of these observations agree with Vayssière's original description (1920). Egg mass The egg mass consists of irregular clusters of eggs that are loosely connected and separated by small interspaces, producing a mosaic-like pattern arranged in a spiral of slightly more than two complete whorls (Fig. 2 d). The overall diameter of the mass is approximately 25 mm. Individual eggs are irregular in shape and measure about 200–300 µm in diameter. The colour is bright reddish orange, closely matching that of the adult specimen. Habitat Based on historical records, the depth range of Madrella aurantiaca was previously considered to lie between 40 and 80 m. Recent observations, however, document the occurrence of the species in considerably shallower waters (26 m for the Galician specimen, 20 m for the Cap Blanc specimen, and approximately 18 m for the specimen from Portimão, Portugal). The species has consistently been reported in association with its presumed prey, primarily bryozoans of the genera Pentapora and Smittina , a pattern that is in agreement with our own observations. The specimen from Dragonera Island was collected at a depth of 86 m on a seabed characterised by a high abundance of irregular sea urchins ( Spatangus purpureus ) and other echinoderms, including the holothurian Parastichopus regalis , the starfish Luidia ciliaris , the brittle star Ophiura texturata , and the regular sea urchin Centrostephanus longispinus . Remarks The species was originally described by Vayssière ( 1903 ) on the basis of a small specimen (12 mm) collected in the Gulf of Marseille at a depth of 60–80 m, near the port of Carry-le-Rouet. The specimen was obtained by dredging and was associated with remains of Smittina cervicornis and Pentapora fascialis. In his original account, Vayssière described the external morphology and attempted to preserve the specimen in 4% formalin in order to retain its colouration; however, it became completely white and gelatinous, although the radula could still be examined and described. Subsequent searches and dredging efforts conducted by Vayssière in the same area failed to yield additional material. In 1916, a second specimen was discovered, slightly smaller than the original and externally much degraded, but still suitable for examination of the internal anatomy. This specimen is presumed to have originated from the same locality in the Gulf of Marseille, although no depth information was reported. The study of this second individual allowed Vayssière to complete the species description (Vayssiere 1920). Cornet and Marche-Marchad ( 1951 ) reported the species from Roscoff, citing only the following note: "A specimen on a colony of Hypodiplosia foliacea (= Pentapora foliacea ), dredged at Château du Taureau (Cornet 1951 )", without further comment or explanation. The reference to Cornet ( 1951 ) is absent from the bibliography, but is assumed to correspond to unpublished data collected by the author and incorporated into the work published by Cornet and Marche-Marchad ( 1951 ). In 1960, Haefelfinger identified Madrella aurantiaca from two dredging stations in the roadstead of Villefranche-sur-Mer (French Mediterranean), on “coral bottoms at a depth of 40–60 m”, without specifying the number or size of the specimens examined. Barletta ( 1976 ) subsequently recorded a specimen measuring 10–12 mm from Portofino (northern Italy), collected by dredging at a depth of 45 m. A few years later, Bouchet and Tardy ( 1976 ) listed the species in the Atlantic from two localities: Roscoff, based on the record of Cornet and Marche-Marchad 1951 , and Arcachon. Information regarding the latter is limited; the authors stated only that all records for this area were derived from Bebbington and Thompson ( 1968 ) and Cuénot ( 1927 ), and were “complétés par les récoltes de Mademoiselle H. Gantès et Madame F. Salvat, qui nous ont aimablement communiqué leurs résultats”. A review of these sources reveals no published references to M. aurantiaca , and it therefore remains unclear whether the Arcachon record was based solely on personal communications. In 1987 Cattaneo and Chemello included this species in a list of sea slug taxa reported from several localities in Sicily, again without information on specimen size, condition, or collection data, and citing only "a personal communication from Angelo Giudice". Subsequently, Sabelli et al. ( 1990 ) listed Madrella aurantiaca and Madrella sanguinea , without further detail, in a systematic catalogue of Mediterranean molluscs. In the same year, Thompson et al. ( 1990 ) reviewed the family Madrellidae in the Mediterranean, focusing primarily on M. sanguinea and proposing that M. ferruginosa , M. aurantiaca and M. granularis should be regarded as synonyms. During the FAUNA III scientific expedition around the Balearic Islands in 1994, Ballesteros and Templado collected three specimens among material obtained by beam trawl and SCUBA diving. Information associated with these findings was presented at the XI National Congress of Malacology (Ballesteros and Templado 1996 ) and was later incorporated as a valid record by Cervera et al. ( 2004 ) in their checklist of sea slugs from Spain and Portugal. However, this record and its associated data have never been fully published in a peer-reviewed format. Finally, Calado and Silva ( 2012 ) documented a specimen observed by Pedro Caleja at a popular dive site near Portimão (Portugal), at an approximate depth of 18 m. The addition of these three new records for such a rarely reported species is particularly noteworthy and substantially improves current knowledge of its distribution and habitat preferences. Discussion Historical documentation on the presence of Madrella aurantiaca is sparse and often limited to simple occurrence records lacking biological or ecological detail (see Remarks). As a result, the original descriptions by Vayssière's (1903, 1920) remain the primary sources of detailed information on the species' diagnostic characters. Although these descriptions were comprehensive by the standards of their time, examination of live specimens larger than those studied by Vayssière has revealed additional characters that refine current understanding of the species. In particular, the presence of dorsal tubercles and the pigmentation of the apical region of the rhinophores have not been previously reported. Furthermore, both the morphology and arrangement of the papillae differ significantly from those described in the original account. In living specimens, the papillae are digitiform, with rounded tips and an almost constant diameter along their length; when removed from the water, however, they retract to varying degrees and assume a more fusiform appearance. The number of papillae also differs substantially, with a greater number of lateral rows than previously reported. In live specimens, papillae are arranged in at least five well-defined series covering the entire lateral and frontal surfaces, although those on the anterior region are smaller. When fully extended, the lateral papillae curve inwards, partially obscuring the dorsal surface. By contrast, earlier descriptions referred to only two series of papillae on the lateral margins and a single series on the anterior region (Vayssière 1903 ). These discrepancies are most likely attributable to capture methods and the resulting condition of the specimens. Partial loss of papillae may occur during trawling operations, as may contraction of the body and associated appendages. Among the material examined, the specimens from the Balearic Islands most closely match previous descriptions, as they were collected using similar methods and were therefore slightly damaged upon retrieval. In contrast, the specimen reported by Calado and Silva ( 2012 ), which was observed and photographed in situ , closely resembles the Galician specimen in both the number and arrangement of papillae, as well as in their shape. Internal characters, such as the radula and jaws, were consistent among all specimens and are in agreement with the original description (Vayssière 1920 ). These features can therefore be regarded as more reliable diagnostic characters for taxonomic differentiation within the genus, particularly when dealing with damaged material. The reddish-orange colouration of both the specimens and their egg masses, together with their frequent occurrence on bryozoans of the genera Pentapora and Smittina , suggest a clear ecological dependence on these substrates (McDonald 1997 ; Lidgard 2008 ). Previous studies have explored this close association in detail, proposing several factors that contribute to its persistence. Highly specialized marine slugs such as Madrella typically engage in sublethal predation, consuming individual zooids. This feeding strategy allows the bryozoan colonies to regenerate lost structures, thereby maintaining a renewable food resource (Ligard, 2008). However, because bryozoans have a relatively low energetic value and are heavily calcified, associated sea slugs tend to be small-bodied and characterised by low metabolic rates, ensuring that the energetic costs of locating and processing their prey remain favourable. Mimicry strategies in sea slugs - whereby body texture and pigmentation closely resemble those of their prey- render these animals particularly difficult to detect during visual surveys. Such strategies form part of a broad repertoire of defensive mechanisms known in sea slugs (Camps-Castellà et al. 2020 ; Winters et al. 2021), and include a wide variety of structural and chromatic imitations, such as those of sponges (Rudman and Avern 1989 ; Almón et al. 2019 ), cnidarians (Rudman 1981 ), algae (Krug et al. 2018 ), and bryozoans (MacFarland 1966 ), among others. Since Thompson ( 1989 ) proposed the “escape and radiation” hypothesis -linking the evolution of aposematism with adaptive radiation -this framework has become central to interpretations of the role of chemical defences in nudibranch evolution. The development of aposematic patterns may facilitate broader access to resources by alleviating the constraints imposed by strict reliance on cryptic habitats to avoid predation (van den Berg et al. 2024). Under this hypothesis, reduced dependence on crypsis may promote increased rates of speciation and diversification. Consequently, camouflage may represent the ancestral condition in sea slugs, a view that is consistent with recent phylogenetic analyses of the Madrellidae (Korshunova et al. 2025 ). The characteristics of this species suggest that its apparent historical rarity is likely to reflect underreporting rather than true scarcity, arising from its cryptic ecology and from the historical concentration of sampling efforts at greater depths (Pola et al. 2019 ). Its mimetic colouration renders it difficult to detect when resting on its typical prey, and recent records have extended the documented bathymetric range into shallower waters, down to 20–24 m (present study) and 18 m (Calado and Silva 2012 ). These findings indicate that the species is probably more widespread than previously assumed. This information may assist in the design of targeted surveys in areas that fulfil the species' primary ecological requirements, particularly the presence of suitable food sources within the relevant depth range. Ideally, such surveys should combine in situ observation (photography, video and diving) with controlled manual collection. By contrast, sampling methods that damage both specimens and substrate, such as intensive dredging, increase the risk of losing diagnostic characters and may further contribute to underreporting (Pontes et al. 2025 ). Furthermore, the description of the spawning mass provides a practical reference for detecting the species in the field and suggests a reproductive strategy potentially linked to the microecology of its feeding substrate. Although no specific threats to the species have been documented to date, its strong association with bryozoans -a group known to be vulnerable to environmental change and habitat degradation- may justify its inclusion in regional inventories as a species of interest for monitoring benthic communities (Smith 2014 ; Fortunato 2015 ). In this context, improving ecological knowledge and ensuring accurate species delimitation are of particular importance. Nevertheless, the current lack of molecular data for most Madrella species precludes a robust phylogenetic assessment of genus-level composition and a more precise delimitation of lineages (Pola et al. 2019 ). As a result, several taxonomical uncertainties remain unresolved and will require further investigation as additional material becomes available. Building on the updated overview of M. aurantiaca and its congeners presented here, future research priorities should include the acquisition of mitochondrial and nuclear sequences from both Atlantic and Mediterranean specimens in order to assess population structure and phylogenetic relationships. The development of standardised sampling protocols based on ecological criteria would improve estimates of relative abundance and clarify actual distributional patterns. In parallel, dietary studies and direct observations of interactions between specimens and bryozoans of the genera Pentapora and Smittina will be essential for quantifying the degree of trophic specialisation. In this study, we document new records of Madrella aurantiaca in Spanish waters, expand the morphological knowledge of the species based on observations of living individuals, and provide the first detailed description of a freshly laid egg mass. Our data extend the known bathymetric range into shallower waters (approximately 18–24 m) and confirm a close trophic association with bryozoans of the genera Pentapora and Smittina . Together, these new records and observations substantially improve current understanding of this poorly known species, revealing greater morphological plasticity and a broader depth range than previously recognized. They also underscore the need to integrate sampling strategies adapted to its cryptic lifestyle with molecular approaches in order to resolve outstanding taxonomic issues. Future studies employing targeted sampling and genetic analyses will be essential for clarifying the species’ status, abundance, and distribution across European waters. Statements and Declarations Competing Interests The authors have no competing interests to declare that are relevant to the content of this article. Acknowledgements We gratefully acknowledge the assistance of the members of Grupo de Estudo do Medio Mariño (GEMM) during the sampling dives. M. Ballesteros thanks his colleagues from the FAUNA III campaign and the crew of the Garcia del Cid for their invaluable help in collecting specimens from the Balearic Islands. He also thanks the Marine Biodiversity and Evolution Research Group at the University of Barcelona (SGR 01271), to which he belongs, for providing facilities during the preparation of this manuscript. Funding No funding was received for the preparation of this manuscript. Conflict of Interest The authors declare that they have no conflicts of interest. Ethical approval No animal testing was performed during this study. Sampling and field studies All necessary permits for sampling and observational field studies have been obtained by the authors from the competent authorities and are mentioned in the acknowledgements, if applicable. The study is compliant with CBD and Nagoya protocols. Data availability Authors confirms that all data generated or analysed during this study are included in this published article. Author Contribution Statement BA and JP conceived and designed the study. BA, JP, MP, and MB conducted the sampling and collected the material. BA and MB analysed the data. BA drafted the manuscript, and MP, MB, JT, and JP reviewed and contributed to successive versions of the text. MP and JP compiled historical records. MP and BA prepared the distribution maps. Photographs are original works by JP (Galicia) and MB (Balearic Islands). Figures were produced by BA and line drawings are original works by MB. All authors read and approved the final manuscript. References Alder J, Hancock A (1864) Notice of a collection of nudibranchiate Mollusca made in India by Walter Elliot Esq. with descriptions of several new genera and species. Trans Zool Soc Lond 5:113–147. https://doi.org/10.1111/j.1096-3642.1864.tb00643.x Almón B, Pérez J, Trigo JE, Ferreras D (2019) New shallow-water nudibranch records (Mollusca: Gastropoda: Heterobranchia) from North West Atlantic coast of Iberian Peninsula. Cah Biol Mar 60:21–29. https://dx.doi.org/10.21411/CBM.A.F13F82ED Ballesteros M, Madrenas E, Pontes M (2025) Madrella aurantiaca in OPK-Opistobranquis. Published: 28/08/2012. Accessed: 11/12/2025. Available at ( https://opistobranquis.info/en/?p=2214) Ballesteros M, Templado J (1996) Opistobranquios de las islas Baleares y de las Columbretes. In: Moreno D (ed) Libro de Résumenes XI Congresso Nacional de Malacología, Sociedad Española de Malacología. Almería, Spain, pp 40–41 Barletta G (1976) Considerazioni sulla bionomia dei Nudibranchi e sulla loro alimentazione (nota preliminare). Conchiglie (Notiziario Mensile della Unione Malacologica Italiana. 12(5–6):117–128 Bebbington A, Thompson TE (1968) Note sur les Opisthobranches du bassin d’Arcachon. Actes Soc Linn Bourdeaux 105(3A):1–35 Bohm C, Jonsson C (2023) Invertebrates of the Gothenburg Natural History Museum (GNM). Version 6.5. Gothenburg Natural History Museum (GNM). Occurrence dataset https://doi.org/10.15468/v0gwot accessed via GBIF.org on 2025-11-08. https://www.gbif.org/occurrence/683643806 Bouchet P, Tardy J (1976) Faunistique et biogéographie des Nudibranches des côtes françaises de l'Atlantique et de la Manche. Ann lnst Océanogr 52(2):205–213 Calado G, Silva JP (2012) Lesmas do Mar do Algalve. Edições Subnauta. 164 pp. ISBN: 978-989-96406-2-7 Camps-Castellà J, Ballesteros M, Trobajo R, Pontes M, Prado P (2020) Not all nudibranchs are carnivorous: trophic ecology of Polycerella emertoni in the Ebro Delta. Mar Ecol Prog Ser 645:67–82. https://doi.org/10.3354/meps13379 Cattaneo R, Chemello R (1987) Alcune considerazioni sui molluschi opistobranchi della Sicilia. Bollettino Malacologico (Pubblicazione Mensile Edita dalla Societa Italiana di. Malacologia) 23(5–8):207–222 Cervera JL, Calado G, Gavaia C, Malaquías MAE, Templado J, Ballesteros M, García-Gómez JC, Megina C (2004) An annotated and updated checklist of the opisthobranchs (Mollusca: Gastropoda) from Spain and Portugal (including islands and archipelagos). Bol Inst Esp Oceanogr 20(1–4):1–111 Cornet R, Marche-Marchad I (1951) Inventaire de la faune marine de Roscoff Mollusques. Travaux de la Station Biologique de Roscoff, Suppl 5:1–80. Available at https://web.archive.org/web/20160610162926/http://www.sb-roscoff.fr/sites/www.sb-roscoff.fr/files/documents/station-biologique-roscoff-mollusques-1951.pdf Cuénot L (1927) Contribution á la faune du bassin d’Arcachon. IX: Revue générale de la faune et bibliographie. Bull Stat Biol Arcachon 24:229–308 Fortunato H (2015) Bryozoans in climate and ocean acidification research: a reappraisal of an under-used tool. Reg Stud Mar Sci 2:32–44. https://doi.org/10.1016/j.rsma.2015.08.010 GBIF.org (2025) GBIF Occurrence Download https://doi.org/10.15468/dl.z5hn8p Haefelfinger HR (1960) Catalogue des opisthobranches de la Rade de Villefranche-sur-Mer et ses environs (Alpes Maritimes). Rev suisse zool 67(3):323–351 Johnson RF, Gosliner TM (2012) Traditional Taxonomic Groupings Mask Evolutionary History: A Molecular Phylogeny and New Classification of the Chromodorid Nudibranchs. PLoS ONE 7(4):e33479. https://doi.org/10.1371/journal.pone.0033479 Korshunova T, Fletcher K, Martynov A (2025) The endless forms are the most differentiated—how taxonomic pseudo-optimization masked natural diversity and evolution: the nudibranch case. ZJLS 204(4):zlaf057. https://doi.org/10.1093/zoolinnean/zlaf057 Krug PJ, Wong NLWS, Medina MR, Gosliner TM, Valdés ÁA (2018) Cryptic speciation yields remarkable mimics: A new genus of sea slugs that masquerade as toxic algae ( Caulerpa spp). Zool Scr 47:699–713. https://doi.org/10.1111/zsc.12310 Lidgard S (2008) Predation on marine bryozoan colonies: taxa, traits and trophic groups. Mar Ecol Prog Ser 359:117–131. https://doi.org/10.3354/meps07322 Martynov AV (2011) From tree-thinking to cycle-thinking: ontogenetic systematics of nudibranch molluscs. Thalassas 27(2):193–224 McDonald GR, Nybakken JW (1997) List of the worldwide food habits of nudibranchs. Veliger, 40(2) McDonald GR (2009) Bibliographia Nudibranchia, second edition. UC Santa Cruz: Institute of Marine Sciences. Retrieved from https://escholarship.org/uc/item/8115h0wzecz MacFarland FM (1966) Studies of opisthobranchiate mollusks of the Pacific coast of North America. Memoirs of the California Academy of Sciences VI: xvi + 546 pp ISBN: 978-0940228108 Pola M, Hallas JM, Gosliner TM (2019) Welcome back Janolidae and Antiopella : Improving the understanding of Janolidae and Madrellidae (Cladobranchia, Heterobranchia) with description of four new species. J Zool Syst Evol Res 57:345–368. https://doi.org/10.1111/jzs.12257 Pontes M, Madrenas E, Garcia-Tort A, Espada O, Ballesteros M (2025) Sea Slugs (Mollusca, Gastropoda, Heterobranchia) from the Medes Islands (Costa Brava, NE Spain): Biodiversity and Ecological Study over a Decade. Diversity 17(9):606. https://doi.org/10.3390/d17090606 Preston HB (1911) Records of zoological literature. VIII Mollusca Zool Rec 46(1):1–92 Robinson BH, Haddock HD (2024) Discovery and description of a remarkable bathypelagic nudibranch, Bathydevius caudactylus , gen. et. sp. nov. Deep-Sea Res I: Oceanogr Res Pap 214:104414. https://doi.org/10.1016/j.dsr.2024.104414 Rudman WB (1981) Polyp mimicry in a new species of aeolid nudibranch mollusc. J Zool 193:421–427. https://doi.org/10.1111/j.1469-7998.1981.tb03454.x Rudman WB (2001a) Madrella ferruginosa Alder & Hancock, 1864. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/madrferr Accessed: 8 Nov. 2025 Rudman WB (2001b) Madrella sanguinea (Angas, 1864). [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/madrsang Accessed: 8 Nov. 2025 Rudman WB, Avern G (1989) The genus Rostanga (Nudibranchia: Dorididae) in the Indo-West Pacific. ZJLS 96(3):281–338. https://doi.org/10.1111/j.1096-3642.1989.tb01832.x Sabelli B, Gianuzzi-Savelli R, Bedulli D (1990) Catalogo annotato dei Molluschi marini del Mediterraneo. Libreria Naturalistica Bolognese. Bologna, Italy: 348 pp Schrödl M, Jörger KM, Klussmann-Kolb A, Wilson NG (2011) Bye bye Opisthobranchia! A review on the contribution of mesopsammic sea slugs to euthyneuran systematics. Thalassas 27(2):101–112 Smith AM (2014) Growth and calcification of marine bryozoans in a changing ocean. Biol Bull 226(3):203–210. https://doi.org/10.1086/BBLv226n3p203 Thompson JN (1989) Concepts of coevolution. Trends Ecol Evol 4(6):179–183. https://doi.org/10.1016/0169-5347(89)90125-0 Thompson TE, Cattaneo R, Wong YM (1990) Eastern Mediterranean Opisthobranchia: Dotidae (Dendronotoidea), Arminidae and Madrellidae (Arminoidea). J Molluscan Stud 56(3):393–413. https://doi.org/10.1093/mollus/56.3.393 Valdés Á, Moran AL, Woods HA (2012) Revision of several poorly known Antarctic aeolid nudibranch species (Mollusca: Gastropoda), with the description of a new species. JMBA UK 92(5):1161–1174. https://doi.org/10.1017/S0025315411000348 Vayssière A (1903) Recherches Zoologiques et Anatomiques sur les Mollusques Opistobranches du Golfe de Marseille. Supplément. Ann Mus hist nat Marseille, tome VIII, section de Zoologie. Mémoire nº 3:73–108 pl. III Vayssière A (1920) Nouvelle étude sur le Madrella aurantiaca . Ann Sci Nat (Paris) Ser 3(10):161–171 1 pl van den Berg CP (2022) Highly defended nudibranchs ‘escape’ to visually distinct background habitats. Behav Ecol 35(5):arae053. https://doi.org/10.1093/beheco/arae053 Wagele H, Willan R (2000) Phylogeny of the Nudibranchia. ZJLS 130(1):83–181. https://doi.org/10.1111/j.1096-3642.2000.tb02196.x Wägele H (2004) Potential key characters in Opisthobranchia (Gastropoda, Mollusca) enhancing adaptive radiation. Org Divers Evol 4:175–188. https://doi.org/10.1016/j.ode.2004.03.002 Winters AE, Chan W, White AM, van den Berg CP, Garson MJ, Cheney KL (2022) Weapons or deterrents? Nudibranch molluscs use distinct ecological modes of chemical defence against predators. J Anim Ecol 91(4):831–844. https://doi.org/10.1111/1365-2656.13643 WoRMS Editorial Board (2025) World Register of Marine Species. Available from https://www.marinespecies.org at VLIZ. Accessed 2025-07-15. https://doi.org/10.14284/170 Cite Share Download PDF Status: Posted Version 1 posted 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. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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-8832905","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":593994389,"identity":"f9cbf5d4-cff6-45e2-983b-d2be9f983230","order_by":0,"name":"Bruno Almón","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA4klEQVRIiWNgGAWjYBACAwhlAcTMBx8wMBwgWosEELMlG5CqhcdMgigt5tJnzB58qJGI5pduMKvmqbkjx8/AfPgDPi2WfTnmhjOOSeTOnHMg7TbPsWfGkg1saRJ4HXaGx0yah00id8ONhGO3edgOJ244wGOG3y9gLf9AWhLbinn+gbTwf8brMLAW3jaQlmQ2Zt42sC0MeB1m2cNWJjmzD+iXGWnMknP7DhtLNrOZ4dVizsO8TeLDN5vcfon8jx/efDssx8/e/Bivw1AAEw+IZCZaPRAw/iBF9SgYBaNgFIwYAADSd0hJchXqJwAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0001-7350-6035","institution":"Consejo Superior de Investigaciones Cientificas","correspondingAuthor":true,"prefix":"","firstName":"Bruno","middleName":"","lastName":"Almón","suffix":""},{"id":593994390,"identity":"3fbd5765-8306-44a1-9c8d-a448d9c0156c","order_by":1,"name":"Jacinto Pérez","email":"","orcid":"","institution":"Grupo de Estudo do Medio Mariño","correspondingAuthor":false,"prefix":"","firstName":"Jacinto","middleName":"","lastName":"Pérez","suffix":""},{"id":593994391,"identity":"b09f55da-f41f-44bc-b147-2d883e311524","order_by":2,"name":"Juan Trigo","email":"","orcid":"","institution":"Sociedade Galega de Historia Natural","correspondingAuthor":false,"prefix":"","firstName":"Juan","middleName":"","lastName":"Trigo","suffix":""},{"id":593994392,"identity":"52b3656a-ac14-4c2e-bf8e-da7424e71a3e","order_by":3,"name":"Miquel Pontes","email":"","orcid":"","institution":"Self employed at VIMAR","correspondingAuthor":false,"prefix":"","firstName":"Miquel","middleName":"","lastName":"Pontes","suffix":""},{"id":593994393,"identity":"87905d72-b0aa-44f7-9e1f-ee1ea3e3bedc","order_by":4,"name":"Manuel Ballesteros","email":"","orcid":"","institution":"University of Barcelona: Universitat de Barcelona","correspondingAuthor":false,"prefix":"","firstName":"Manuel","middleName":"","lastName":"Ballesteros","suffix":""}],"badges":[],"createdAt":"2026-02-09 16:55:58","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8832905/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8832905/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":103346425,"identity":"e350a7b9-023f-4b94-80ce-36235f1c5cd9","added_by":"auto","created_at":"2026-02-24 16:21:45","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":345805,"visible":true,"origin":"","legend":"\u003cp\u003eOverview of the distribution of \u003cem\u003eMadrella \u003c/em\u003especies\u003cem\u003e. \u003c/em\u003e\u003cstrong\u003ea\u003c/strong\u003e Occurrences of \u003cem\u003eM. aurantiaca\u003c/em\u003e around the Iberian Peninsula, including the new occurrences reported in this study. \u003cstrong\u003eb\u003c/strong\u003e Worldwide distribution of \u003cem\u003eMadrella\u003c/em\u003e species. An interactive online version of this map provides access to the background information associated with each record: https://www.google.es/maps/d/edit?hl=en\u0026amp;mid=1PPieMXgEeGcZn_p9umcooCqybhsknec\u0026amp;usp=sharing\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8832905/v1/5cb7f1f116dfece26a1eb2e6.png"},{"id":103346429,"identity":"60e962a6-df0b-4edd-b7bd-12f13b091c00","added_by":"auto","created_at":"2026-02-24 16:21:45","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1438332,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eMadrella aurantiaca\u003c/em\u003e from Galicia. \u003cstrong\u003ea–c\u003c/strong\u003eDetailed photographs of the living specimen. \u003cstrong\u003ed\u003c/strong\u003e Egg mass deposited by the same individual. Scale bars: \u003cstrong\u003ea–c\u003c/strong\u003e = 1 cm; \u003cstrong\u003ed\u003c/strong\u003e = 100 µm. The white arrows mark the conical tubercles of the mantle.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8832905/v1/874f1d193b60ca9206f62706.png"},{"id":103346426,"identity":"348e3679-3105-4a35-9f01-e28d382533db","added_by":"auto","created_at":"2026-02-24 16:21:45","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":642607,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eMadrella aurantiaca\u003c/em\u003e from Dragonera Island (Balearic Islands). \u003cstrong\u003ea\u003c/strong\u003e Freshly collected, slightly contracted living animal (photo: D. Moreno). \u003cstrong\u003eb\u003c/strong\u003e Schematic dorsal view of the animal. \u003cstrong\u003ec\u003c/strong\u003e Ventral view of the anterior region. \u003cstrong\u003ed\u003c/strong\u003eDetail of the dorsal tubercles.; \u003cstrong\u003ee\u003c/strong\u003e Detail of a fusiform papilla on the body margin. \u003cstrong\u003ef\u003c/strong\u003e Detail of a rhinophore, with an enlarged view of the tuberculate apical region shown on the right\u003cem\u003e.\u003c/em\u003e Scale bars: \u003cstrong\u003ea–c\u003c/strong\u003e = 20 mm; \u003cstrong\u003ed-f\u003c/strong\u003e = 5 mm.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8832905/v1/86aa14b8a5b9d532a616ab97.png"},{"id":103505932,"identity":"02d245ea-e246-4ea9-8ba0-31ffda6400b5","added_by":"auto","created_at":"2026-02-26 13:33:33","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":168620,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eMadrella aurantiaca\u003c/em\u003e. Jaws and radula of the Cap Blanc specimen (Bay of Palma, Mallorca, Balearic Islands). \u003cstrong\u003ea\u003c/strong\u003e Dorsal view of the jaws and radula in their natural position. \u003cstrong\u003eb\u003c/strong\u003e Jaw in lateral view. \u003cstrong\u003ec\u003c/strong\u003e Radula and radular membrane. \u003cstrong\u003ed\u003c/strong\u003e Schematic drawing of the teeth in a central radular row. Scale bars: \u003cstrong\u003ea–c\u003c/strong\u003e = 1 mm; \u003cstrong\u003ed\u003c/strong\u003e = 100 µm.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8832905/v1/e650645ef0f3098a49ebf00b.png"},{"id":105565965,"identity":"2d260a3e-9839-47ab-9513-b58e653ae50a","added_by":"auto","created_at":"2026-03-27 12:54:54","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3678217,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8832905/v1/ab78cdb8-7e14-4d16-ba95-d4105f608bd0.pdf"}],"financialInterests":"","formattedTitle":"Assessment of the status and distribution of the poorly known nudibranch Madrella aurantiaca (Gastropoda: Nudibranchia: Madrellidae)","fulltext":[{"header":"Introduction","content":"\u003cp\u003eSea slugs comprise a diverse assemblage of marine gastropods, typically characterised by the absence of an external shell in the adult stage and, in many taxa, by the presence of externally exposed gills. They exhibit a wide range of body forms and colour patterns, which has contributed to their popularity among underwater photographers. Sea slugs occur worldwide and inhabit a broad spectrum of marine environments, often showing close ecological associations with their primary food sources (Robison et al. 2024).\u003c/p\u003e \u003cp\u003eStrictly speaking, and following the recent systematic revision by Korshunova et al. (\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2025\u003c/span\u003e), former nudibranchs are now assigned to two orders within the subclass Heterobranchia (Mollusca, Gastropoda): Doridida and Nudibranchia, a classification currently adopted by WoRMS. Prior to this revision, both groups were included within the taxon Nudibranchia Cuvier, 1817, which is now restricted to the former Cladobranchia, characterised by the presence of a branched digestive gland.\u003c/p\u003e \u003cp\u003eThe total diversity of sea slugs has been estimated at approximately 2,700 species (W\u0026auml;gele \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; WoRMS 2025), although this figure varies among sources as a result of ongoing systematic revisions and the continual description of new taxa (Schr\u0026ouml;dl et al. \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Johnson and Gosliner \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2012\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eResearch interest in sea slugs has evolved considerably over time, progressing from early studies primarily concerned with documenting species diversity to a more integrative approach encompassing molecular phylogenetics and ecological adaptations (W\u0026auml;gele and Willan 2000; McDonald \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Martynov \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). Although the description of new species remains frequent within this modern framework, a substantial proportion of currently recognised species originates from these pioneering studies. Such works were often broad in scope and commonly based on material obtained during scientific expeditions, regional surveys, or examinations of museum collections.\u003c/p\u003e \u003cp\u003eWhile the shift in research focus has resulted in a more detailed understanding of their evolutionary history and diversification patterns, some species remain poorly known, particularly those inhabiting deep-sea or remote environments, as well as taxa that are genuinely rare or difficult to detect (Vald\u0026eacute;s et al. \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Alm\u0026oacute;n et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Robison et al. 2024).\u003c/p\u003e \u003cp\u003eThe genus \u003cem\u003eMadrella\u003c/em\u003e Alder \u0026amp; Hancock, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1864\u003c/span\u003e represents a poorly known group within Nudibranchia, comprising several morphologically similar species and characterised by a long history of taxonomic uncertainty (Pola et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). The genus was originally erected to accommodate a new species described from several specimens collected in southern India. This species, \u003cem\u003eMadrella ferruginea\u003c/em\u003e Alder \u0026amp; Hancock, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1864\u003c/span\u003e, exhibited several distinctive characters in comparison with other members of the group, including the lateral position of the anus and the absence of oral tentacles, which are replaced by a veil-like expansion of the head (Alder and Hancock \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1864\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eAlthough \u003cem\u003eMadrella\u003c/em\u003e was initially assigned to the family Proctonotidae Gray, 1853, subsequent studies recognised that the genus \u003cem\u003eMadrella\u003c/em\u003e was sufficiently distinct to justify placement in a separate family. Consequently, the family Madrellidae was erected to include this genus alone (Preston \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e1911\u003c/span\u003e), and was later expanded by the inclusion of a second genus, \u003cem\u003eEliotia\u003c/em\u003e Vayssi\u0026egrave;re, 1909.\u003c/p\u003e \u003cp\u003eThe genus \u003cem\u003eMadrella\u003c/em\u003e currently comprises six valid species that share a broadly similar external appearance and the diagnostic characters of the genus: \u003cem\u003eMadrella amphora\u003c/em\u003e Pola \u0026amp; Gosliner, 2019, from Papua New Guinea; \u003cem\u003eM. aurantiac\u003c/em\u003ea Vayssi\u0026egrave;re, \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e1903\u003c/span\u003e, from the Mediterranean Sea and north-east Atlantic; \u003cem\u003eM. ferruginosa\u003c/em\u003e Alder \u0026amp; Hancock, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1864\u003c/span\u003e, from the Indian coasts, the Philippines, and the Marshall Islands, and more recently recorded from South Africa, where it is considered non-native; \u003cem\u003eM. gloriosa\u003c/em\u003e Baba, 1949 and \u003cem\u003eM. granularis\u003c/em\u003e Baba, 1949, from Japan; and \u003cem\u003eM. sanguinea\u003c/em\u003e (Angas, 1864), from Australian waters and the seas of China and Japan (WoRMS 2025). Records and biological information for \u003cem\u003eM. ferruginosa\u003c/em\u003e and \u003cem\u003eM. sanguinea\u003c/em\u003e are relatively well documented by Rudman (\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2001a\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003eb\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eOwing to the general paucity of information available worldwide, species-level differentiation within the genus has proven difficult and has been subject to differing interpretations. Some authors have suggested that \u003cem\u003eM. ferruginosa\u003c/em\u003e, \u003cem\u003eM. aurantiaca\u003c/em\u003e and \u003cem\u003eM. granularis\u003c/em\u003e might be junior synonyms of \u003cem\u003eM. sanguinea\u003c/em\u003e (Thompson et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e1990\u003c/span\u003e), based primarily on their reddish or orange coloration and on the variability of certain characters during ontogenetic development. Moreover, \u003cem\u003eM. sanguinea\u003c/em\u003e was originally described within the genus \u003cem\u003eJanus\u003c/em\u003e in 1864 and was later transferred to \u003cem\u003eMadrella\u003c/em\u003e, which would confer priority to the former name in the event of synonymization.\u003c/p\u003e \u003cp\u003eAt present, however, all six species are regarded as distinct and valid lineages (WORMS, 2025). This consensus is based largely on the interpretation of the observed morphological differences as evolutionary significant. Nevertheless, future studies incorporating comparative genetic analysis across the different \u003cem\u003eMadrella\u003c/em\u003e species will be essential to rigorously test this hypothesis. To date, only \u003cem\u003eM. ferruginosa\u003c/em\u003e and \u003cem\u003eM. amphora\u003c/em\u003e have been genetically characterised, with sequences available in public databases (Pola et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2019\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn European waters, \u003cem\u003eMadrella aurantiaca\u003c/em\u003e is the only species of the genus recorded to date. Records of this inconspicuous species are scarce and largely restricted to the Mediterranean Sea. In the Atlantic, confirmed records are limited to southern Portugal (Calado and Silva \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2012\u003c/span\u003e), the Atlantic coast of France, with a report from Arcachon bay (Bouchet and Tardy \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e1976\u003c/span\u003e), and another from the Roscoff region in France (Cornet and Marche-Marchad \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e1951\u003c/span\u003e; Bouchet and Tardy \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e1976\u003c/span\u003e), as well as a single specimen captured on 12 July 1932 at Mellersta Kattegatt, Lilla Middelgrunds, Sweden. This specimen is preserved in the Invertebrates Collection of the Gothenburg Natural History Museum (GNM) (Bohm and Johnson 2023).\u003c/p\u003e \u003cp\u003eThe recent discovery of a specimen of \u003cem\u003eM. aurantiaca\u003c/em\u003e in Galicia has prompted a reassessment of the species\u0026rsquo; status and distribution, together with the examination of previously unpublished records from the Balearic Islands (Mediterranean Sea). In the present study, the literature pertaining to the species was reviewed and updated in order to clarify uncertainties associated with historical records and the resulting distribution in European waters. Data obtained from the examination of freshly collected specimens allow the original description to be complemented and contribute to an improved understanding of the species\u0026rsquo; ecological status.\u003c/p\u003e"},{"header":"Material and Methods","content":"\u003cp\u003eThe material examined comprises four individuals collected from different locations in Spanish waters (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). The Galician specimen was observed and hand-collected during a SCUBA dive conducted by the Grupo de Estudo do Medio Mari\u0026ntilde;o (GEMM) as part of its programme to catalogue the marine fauna of the Galician coast. During the FAUNA III research campaign aboard the O/V \u003cem\u003eGarc\u0026iacute;a del Cid\u003c/em\u003e in the Balearic Islands, three additional specimens were collected: one by trawl dredge (\u003cem\u003ebou de varas\u003c/em\u003e) and two by SCUBA diving.\u003c/p\u003e \u003cp\u003eThe specimens were studied and photographed \u003cem\u003ein situ\u003c/em\u003e to document their external morphology, colouration, and habitat. Following a preliminary examination, they were preserved in 70% ethanol for subsequent laboratory study. Measurements refer to the fully extended living specimen from Galicia, whereas the lengths of the Balearic specimens correspond to slightly contracted individuals. Illustrations were prepared from the largest Balearic specimen, and the radula and jaws of another individual were examined by dissection through the dorsal region.\u003c/p\u003e \u003cp\u003eThe description of the spawning mass is based on an egg ribbon laid in the maintenance tank in which the Galician specimen was kept alive. Consequently, it cannot be stated with certainty that the clutch produced under captive conditions fully reflects that of wild individuals in terms of size or extent, although it is considered representative of the general shape and colouration.\u003c/p\u003e \u003cp\u003eGiven the rarity of records for this species, the specimens have been deposited in the invertebrate collection of the Museo de Historia Natural of Santiago de Compostela (MNH-USC) under MHN USC-25235 reference code, and the Centre de Recursos de Biodiversitat Animal (CRBA) under the codes CRBA-114209 and CRBA-114210. In addition, a map illustrating the confirmed worldwide distribution of the different \u003cem\u003eMadrella\u003c/em\u003e species is provided for comparative purposes (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). Distributional data were retrieved from GBIF and OPK-Opisthobranquis databases (GBIF 2025; Ballesteros et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2025\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eAll digital artwork was prepared using Adobe Photoshop CS6, and the map compositions were generated with QGIS (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://qgis.org/\u003c/span\u003e\u003cspan address=\"https://qgis.org/\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e). Line drawings correspond to digitised versions of the original illustrations produced in the field by M. Ballesteros.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003eClass Gastropoda Cuvier, 1795\u003c/p\u003e \u003cp\u003eSubclass Heterobranchia Burmeister, 1837\u003c/p\u003e \u003cp\u003eSuperorder Nudipleura W\u0026auml;gele \u0026amp; Willan, 2000\u003c/p\u003e \u003cp\u003eOrder Nudibranchia Cuvier, 1817\u003c/p\u003e \u003cp\u003eSuborder Janolina Minichev \u0026amp; Starobogatov, 1979\u003c/p\u003e \u003cp\u003eSuperfamily Proctonotoidea J. E. Gray, 1853\u003c/p\u003e \u003cp\u003eFamily Madrellidae Preston, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e1911\u003c/span\u003e\u003c/p\u003e \u003cp\u003e \u003cb\u003eMadrella aurantiaca\u003c/b\u003e \u003cb\u003eVayssi\u0026egrave;re, 1902\u003c/b\u003e\u003c/p\u003e \u003cp\u003e \u003cstrong\u003eMaterial examined\u003c/strong\u003e \u003cp\u003eOne fresh specimen, 40 mm (measured out of water), (CRBA-114209), 39\u0026deg;35'25.0\"N 2\u0026deg;18'46.1\"E, north of Dragonera Island (Balearic Islands), collected on the seabed among the irregular sea urchin \u003cem\u003eSpatangus purpureus\u003c/em\u003e and other echinoderms, 86 m depth, 24 June 1994; two slightly contracted specimens, 9 mm, (CRBA-114210), 39\u0026deg;21'39.3\"N 2\u0026deg;47'12.6\"E, Cap Blanc, south-eastern Bay of Palma de Mallorca, among algae and marine invertebrates, 20 m depth, 2 July 1994; one live individual, 30 mm (MHN USC-25235), 42\u0026deg;30'33.0\"N 8\u0026deg;56'25.8\"W, Os Esqueiros shelf, under a stone, 24 m depth, 26 May 2025.\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003cstrong\u003eDescription\u003c/strong\u003e \u003cp\u003eThe examined specimens exhibited an overall reddish colouration. The Galician specimen was translucent orange, with reddish apices on the papillae and mantle (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), whereas the Balearic specimens displayed a more or less uniform, intense red colour over the entire body.\u003c/p\u003e \u003c/p\u003e \u003cp\u003eThe dorsal surface of the mantle was covered by small conical tubercles of varying size, many of which possessed white apices (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb, c; \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea-b, d). The lateral margins were ornamented with at least five or six rows of long digitiform to fusiform papillae (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea-c; \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea-c, e). When fully extended, these papillae curved towards the central dorsal region, partially obscuring it. The anterior margin bore papillae similar in shape and arrangement to those on the lateral sides, although smaller in size; these papillae were occasionally tipped in white.\u003c/p\u003e \u003cp\u003eThe rhinophores (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea-c; \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb, f) were long, broad at the base and tapering towards the apex. The upper half bore numerous contractile digitiform tubercles arranged along the lateral and posterior surfaces, leaving the apex free. The apex was short and rounded, bearing several small white spots.\u003c/p\u003e \u003cp\u003eThe frontal veil was well developed and slightly lobed. The mouth was narrow and flanked by two elongated red lobes extending laterally along the head, as well as by a posterior orange lobe. The foot was very broad, projecting beyond the margins of the mantle, slightly whitish in colour, and exhibited a faint dark central spot corresponding to the internal viscera (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec).\u003c/p\u003e \u003cp\u003e \u003cstrong\u003eAnatomy\u003c/strong\u003e \u003cp\u003eOne of the two 9 mm specimens, which was immature, was dissected. The jaws are well developed, extending to almost half the length of the animal; they are yellowish, strongly cornified, and enclose the entire radular bulb (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). In lateral view (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb), the anterior portion of the jaw resembles the fruiting body of a mushroom.\u003c/p\u003e \u003c/p\u003e \u003cp\u003eThe radula is small and narrow, comprising only 16 rows of teeth (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec). It is triseriate, with three teeth per row. The median tooth has a broad base and bears a fine, sharp central cusp flanked by delicate denticulations. On each side of the median tooth, the lateral teeth have broad bases with serrated denticulations, terminating in an elongated, sharp marginal denticulation curved inwards (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed).\u003c/p\u003e \u003cp\u003eAll of these observations agree with Vayssi\u0026egrave;re's original description (1920).\u003c/p\u003e \u003cp\u003e \u003cstrong\u003eEgg mass\u003c/strong\u003e \u003cp\u003eThe egg mass consists of irregular clusters of eggs that are loosely connected and separated by small interspaces, producing a mosaic-like pattern arranged in a spiral of slightly more than two complete whorls (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed). The overall diameter of the mass is approximately 25 mm. Individual eggs are irregular in shape and measure about 200\u0026ndash;300 \u0026micro;m in diameter. The colour is bright reddish orange, closely matching that of the adult specimen.\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003cstrong\u003eHabitat\u003c/strong\u003e \u003cp\u003eBased on historical records, the depth range of \u003cem\u003eMadrella aurantiaca\u003c/em\u003e was previously considered to lie between 40 and 80 m. Recent observations, however, document the occurrence of the species in considerably shallower waters (26 m for the Galician specimen, 20 m for the Cap Blanc specimen, and approximately 18 m for the specimen from Portim\u0026atilde;o, Portugal). The species has consistently been reported in association with its presumed prey, primarily bryozoans of the genera \u003cem\u003ePentapora\u003c/em\u003e and \u003cem\u003eSmittina\u003c/em\u003e, a pattern that is in agreement with our own observations.\u003c/p\u003e \u003c/p\u003e \u003cp\u003eThe specimen from Dragonera Island was collected at a depth of 86 m on a seabed characterised by a high abundance of irregular sea urchins (\u003cem\u003eSpatangus purpureus\u003c/em\u003e) and other echinoderms, including the holothurian \u003cem\u003eParastichopus regalis\u003c/em\u003e, the starfish \u003cem\u003eLuidia ciliaris\u003c/em\u003e, the brittle star \u003cem\u003eOphiura texturata\u003c/em\u003e, and the regular sea urchin \u003cem\u003eCentrostephanus longispinus\u003c/em\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\n\u003ch3\u003eRemarks\u003c/h3\u003e\n\u003cp\u003eThe species was originally described by Vayssi\u0026egrave;re (\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e1903\u003c/span\u003e) on the basis of a small specimen (12 mm) collected in the Gulf of Marseille at a depth of 60\u0026ndash;80 m, near the port of Carry-le-Rouet. The specimen was obtained by dredging and was associated with remains of \u003cem\u003eSmittina cervicornis\u003c/em\u003e and \u003cem\u003ePentapora fascialis.\u003c/em\u003e In his original account, Vayssi\u0026egrave;re described the external morphology and attempted to preserve the specimen in 4% formalin in order to retain its colouration; however, it became completely white and gelatinous, although the radula could still be examined and described. Subsequent searches and dredging efforts conducted by Vayssi\u0026egrave;re in the same area failed to yield additional material.\u003c/p\u003e \u003cp\u003eIn 1916, a second specimen was discovered, slightly smaller than the original and externally much degraded, but still suitable for examination of the internal anatomy. This specimen is presumed to have originated from the same locality in the Gulf of Marseille, although no depth information was reported. The study of this second individual allowed Vayssi\u0026egrave;re to complete the species description (Vayssiere 1920).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eCornet and Marche-Marchad (\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e1951\u003c/span\u003e) reported the species from Roscoff, citing only the following note: \"A specimen on a colony of \u003cem\u003eHypodiplosia foliacea\u003c/em\u003e (=\u0026thinsp;\u003cem\u003ePentapora foliacea\u003c/em\u003e), dredged at Ch\u0026acirc;teau du Taureau (Cornet \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e1951\u003c/span\u003e)\", without further comment or explanation. The reference to Cornet (\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e1951\u003c/span\u003e) is absent from the bibliography, but is assumed to correspond to unpublished data collected by the author and incorporated into the work published by Cornet and Marche-Marchad (\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e1951\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn 1960, Haefelfinger identified \u003cem\u003eMadrella aurantiaca\u003c/em\u003e from two dredging stations in the roadstead of Villefranche-sur-Mer (French Mediterranean), on \u0026ldquo;coral bottoms at a depth of 40\u0026ndash;60 m\u0026rdquo;, without specifying the number or size of the specimens examined.\u003c/p\u003e \u003cp\u003eBarletta (\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e1976\u003c/span\u003e) subsequently recorded a specimen measuring 10\u0026ndash;12 mm from Portofino (northern Italy), collected by dredging at a depth of 45 m. A few years later, Bouchet and Tardy (\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e1976\u003c/span\u003e) listed the species in the Atlantic from two localities: Roscoff, based on the record of Cornet and Marche-Marchad \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e1951\u003c/span\u003e, and Arcachon. Information regarding the latter is limited; the authors stated only that all records for this area were derived from Bebbington and Thompson (\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e1968\u003c/span\u003e) and Cu\u0026eacute;not (\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e1927\u003c/span\u003e), and were \u0026ldquo;compl\u0026eacute;t\u0026eacute;s par les r\u0026eacute;coltes de Mademoiselle H. Gant\u0026egrave;s et Madame F. Salvat, qui nous ont aimablement communiqu\u0026eacute; leurs r\u0026eacute;sultats\u0026rdquo;. A review of these sources reveals no published references to \u003cem\u003eM. aurantiaca\u003c/em\u003e, and it therefore remains unclear whether the Arcachon record was based solely on personal communications.\u003c/p\u003e \u003cp\u003eIn 1987 Cattaneo and Chemello included this species in a list of sea slug taxa reported from several localities in Sicily, again without information on specimen size, condition, or collection data, and citing only \"a personal communication from Angelo Giudice\". Subsequently, Sabelli et al. (\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e1990\u003c/span\u003e) listed \u003cem\u003eMadrella aurantiaca\u003c/em\u003e and \u003cem\u003eMadrella sanguinea\u003c/em\u003e, without further detail, in a systematic catalogue of Mediterranean molluscs. In the same year, Thompson et al. (\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e1990\u003c/span\u003e) reviewed the family Madrellidae in the Mediterranean, focusing primarily on \u003cem\u003eM. sanguinea\u003c/em\u003e and proposing that \u003cem\u003eM. ferruginosa\u003c/em\u003e, \u003cem\u003eM. aurantiaca\u003c/em\u003e and \u003cem\u003eM. granularis\u003c/em\u003e should be regarded as synonyms.\u003c/p\u003e \u003cp\u003eDuring the FAUNA III scientific expedition around the Balearic Islands in 1994, Ballesteros and Templado collected three specimens among material obtained by beam trawl and SCUBA diving. Information associated with these findings was presented at the XI National Congress of Malacology (Ballesteros and Templado \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e1996\u003c/span\u003e) and was later incorporated as a valid record by Cervera et al. (\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2004\u003c/span\u003e) in their checklist of sea slugs from Spain and Portugal. However, this record and its associated data have never been fully published in a peer-reviewed format.\u003c/p\u003e \u003cp\u003eFinally, Calado and Silva (\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2012\u003c/span\u003e) documented a specimen observed by Pedro Caleja at a popular dive site near Portim\u0026atilde;o (Portugal), at an approximate depth of 18 m.\u003c/p\u003e \u003cp\u003eThe addition of these three new records for such a rarely reported species is particularly noteworthy and substantially improves current knowledge of its distribution and habitat preferences.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eHistorical documentation on the presence of \u003cem\u003eMadrella aurantiaca\u003c/em\u003e is sparse and often limited to simple occurrence records lacking biological or ecological detail (see Remarks). As a result, the original descriptions by Vayssi\u0026egrave;re's (1903, 1920) remain the primary sources of detailed information on the species' diagnostic characters. Although these descriptions were comprehensive by the standards of their time, examination of live specimens larger than those studied by Vayssi\u0026egrave;re has revealed additional characters that refine current understanding of the species.\u003c/p\u003e \u003cp\u003eIn particular, the presence of dorsal tubercles and the pigmentation of the apical region of the rhinophores have not been previously reported. Furthermore, both the morphology and arrangement of the papillae differ significantly from those described in the original account. In living specimens, the papillae are digitiform, with rounded tips and an almost constant diameter along their length; when removed from the water, however, they retract to varying degrees and assume a more fusiform appearance. The number of papillae also differs substantially, with a greater number of lateral rows than previously reported. In live specimens, papillae are arranged in at least five well-defined series covering the entire lateral and frontal surfaces, although those on the anterior region are smaller. When fully extended, the lateral papillae curve inwards, partially obscuring the dorsal surface. By contrast, earlier descriptions referred to only two series of papillae on the lateral margins and a single series on the anterior region (Vayssi\u0026egrave;re \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e1903\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThese discrepancies are most likely attributable to capture methods and the resulting condition of the specimens. Partial loss of papillae may occur during trawling operations, as may contraction of the body and associated appendages. Among the material examined, the specimens from the Balearic Islands most closely match previous descriptions, as they were collected using similar methods and were therefore slightly damaged upon retrieval. In contrast, the specimen reported by Calado and Silva (\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2012\u003c/span\u003e), which was observed and photographed \u003cem\u003ein situ\u003c/em\u003e, closely resembles the Galician specimen in both the number and arrangement of papillae, as well as in their shape. Internal characters, such as the radula and jaws, were consistent among all specimens and are in agreement with the original description (Vayssi\u0026egrave;re \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e1920\u003c/span\u003e). These features can therefore be regarded as more reliable diagnostic characters for taxonomic differentiation within the genus, particularly when dealing with damaged material.\u003c/p\u003e \u003cp\u003eThe reddish-orange colouration of both the specimens and their egg masses, together with their frequent occurrence on bryozoans of the genera \u003cem\u003ePentapora\u003c/em\u003e and \u003cem\u003eSmittina\u003c/em\u003e, suggest a clear ecological dependence on these substrates (McDonald \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e1997\u003c/span\u003e; Lidgard \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). Previous studies have explored this close association in detail, proposing several factors that contribute to its persistence. Highly specialized marine slugs such as \u003cem\u003eMadrella\u003c/em\u003e typically engage in sublethal predation, consuming individual zooids. This feeding strategy allows the bryozoan colonies to regenerate lost structures, thereby maintaining a renewable food resource (Ligard, 2008). However, because bryozoans have a relatively low energetic value and are heavily calcified, associated sea slugs tend to be small-bodied and characterised by low metabolic rates, ensuring that the energetic costs of locating and processing their prey remain favourable.\u003c/p\u003e \u003cp\u003eMimicry strategies in sea slugs - whereby body texture and pigmentation closely resemble those of their prey- render these animals particularly difficult to detect during visual surveys. Such strategies form part of a broad repertoire of defensive mechanisms known in sea slugs (Camps-Castell\u0026agrave; et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Winters et al. 2021), and include a wide variety of structural and chromatic imitations, such as those of sponges (Rudman and Avern \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e1989\u003c/span\u003e; Alm\u0026oacute;n et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2019\u003c/span\u003e), cnidarians (Rudman \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e1981\u003c/span\u003e), algae (Krug et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2018\u003c/span\u003e), and bryozoans (MacFarland \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1966\u003c/span\u003e), among others. Since Thompson (\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e1989\u003c/span\u003e) proposed the \u0026ldquo;escape and radiation\u0026rdquo; hypothesis -linking the evolution of aposematism with adaptive radiation -this framework has become central to interpretations of the role of chemical defences in nudibranch evolution. The development of aposematic patterns may facilitate broader access to resources by alleviating the constraints imposed by strict reliance on cryptic habitats to avoid predation (van den Berg et al. 2024). Under this hypothesis, reduced dependence on crypsis may promote increased rates of speciation and diversification. Consequently, camouflage may represent the ancestral condition in sea slugs, a view that is consistent with recent phylogenetic analyses of the Madrellidae (Korshunova et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2025\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe characteristics of this species suggest that its apparent historical rarity is likely to reflect underreporting rather than true scarcity, arising from its cryptic ecology and from the historical concentration of sampling efforts at greater depths (Pola et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Its mimetic colouration renders it difficult to detect when resting on its typical prey, and recent records have extended the documented bathymetric range into shallower waters, down to 20\u0026ndash;24 m (present study) and 18 m (Calado and Silva \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). These findings indicate that the species is probably more widespread than previously assumed. This information may assist in the design of targeted surveys in areas that fulfil the species' primary ecological requirements, particularly the presence of suitable food sources within the relevant depth range. Ideally, such surveys should combine \u003cem\u003ein situ\u003c/em\u003e observation (photography, video and diving) with controlled manual collection. By contrast, sampling methods that damage both specimens and substrate, such as intensive dredging, increase the risk of losing diagnostic characters and may further contribute to underreporting (Pontes et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2025\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eFurthermore, the description of the spawning mass provides a practical reference for detecting the species in the field and suggests a reproductive strategy potentially linked to the microecology of its feeding substrate. Although no specific threats to the species have been documented to date, its strong association with bryozoans -a group known to be vulnerable to environmental change and habitat degradation- may justify its inclusion in regional inventories as a species of interest for monitoring benthic communities (Smith \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Fortunato \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). In this context, improving ecological knowledge and ensuring accurate species delimitation are of particular importance. Nevertheless, the current lack of molecular data for most \u003cem\u003eMadrella\u003c/em\u003e species precludes a robust phylogenetic assessment of genus-level composition and a more precise delimitation of lineages (Pola et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). As a result, several taxonomical uncertainties remain unresolved and will require further investigation as additional material becomes available.\u003c/p\u003e \u003cp\u003eBuilding on the updated overview of \u003cem\u003eM. aurantiaca\u003c/em\u003e and its congeners presented here, future research priorities should include the acquisition of mitochondrial and nuclear sequences from both Atlantic and Mediterranean specimens in order to assess population structure and phylogenetic relationships. The development of standardised sampling protocols based on ecological criteria would improve estimates of relative abundance and clarify actual distributional patterns. In parallel, dietary studies and direct observations of interactions between specimens and bryozoans of the genera \u003cem\u003ePentapora\u003c/em\u003e and \u003cem\u003eSmittina\u003c/em\u003e will be essential for quantifying the degree of trophic specialisation.\u003c/p\u003e \u003cp\u003eIn this study, we document new records of \u003cem\u003eMadrella aurantiaca\u003c/em\u003e in Spanish waters, expand the morphological knowledge of the species based on observations of living individuals, and provide the first detailed description of a freshly laid egg mass. Our data extend the known bathymetric range into shallower waters (approximately 18\u0026ndash;24 m) and confirm a close trophic association with bryozoans of the genera \u003cem\u003ePentapora\u003c/em\u003e and \u003cem\u003eSmittina\u003c/em\u003e. Together, these new records and observations substantially improve current understanding of this poorly known species, revealing greater morphological plasticity and a broader depth range than previously recognized. They also underscore the need to integrate sampling strategies adapted to its cryptic lifestyle with molecular approaches in order to resolve outstanding taxonomic issues. Future studies employing targeted sampling and genetic analyses will be essential for clarifying the species\u0026rsquo; status, abundance, and distribution across European waters.\u003c/p\u003e"},{"header":"Statements and Declarations","content":"\u003cp\u003e\u003cstrong\u003eCompeting Interests\u003c/strong\u003e The authors have no competing interests to declare that are relevant to the content of this article.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe gratefully acknowledge the assistance of the members of Grupo de Estudo do Medio Mariño (GEMM) during the sampling dives. M. Ballesteros thanks his colleagues from the FAUNA III campaign and the crew of the \u003cem\u003eGarcia del Cid\u003c/em\u003e for their invaluable help in collecting specimens from the Balearic Islands. He\u0026nbsp;also thanks the \u003cem\u003eMarine Biodiversity and Evolution Research Group\u003c/em\u003e at the University of Barcelona (SGR 01271), to which he belongs, for providing facilities during the preparation of this manuscript.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e No funding was received for the preparation of this manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflict of Interest\u003c/strong\u003e The authors declare that they have no conflicts of interest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthical approval\u003c/strong\u003e No animal testing was performed during this study.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSampling and field studies\u003c/strong\u003e All necessary permits for sampling and observational field studies have been obtained by the authors from the competent authorities and are mentioned in the acknowledgements, if applicable. The study is compliant with CBD and Nagoya protocols.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e Authors confirms that all data generated or analysed during this study are included in this published article.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contribution Statement\u003c/strong\u003e \u003cem\u003eBA and JP conceived and designed the study. BA, JP, MP, and MB conducted the sampling and collected the material. BA and MB analysed the data. BA drafted the manuscript, and MP, MB, JT, and JP reviewed and contributed to successive versions of the text. MP and JP compiled historical records. MP and BA prepared the distribution maps. Photographs are original works by JP (Galicia) and MB (Balearic Islands). Figures were produced by BA and line drawings are original works by MB. All authors read and approved the final manuscript.\u003c/em\u003e\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAlder J, Hancock A (1864) Notice of a collection of nudibranchiate Mollusca made in India by Walter Elliot Esq. with descriptions of several new genera and species. Trans Zool Soc Lond 5:113\u0026ndash;147. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1111/j.1096-3642.1864.tb00643.x\u003c/span\u003e\u003cspan address=\"10.1111/j.1096-3642.1864.tb00643.x\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAlm\u0026oacute;n B, P\u0026eacute;rez J, Trigo JE, Ferreras D (2019) New shallow-water nudibranch records (Mollusca: Gastropoda: Heterobranchia) from North West Atlantic coast of Iberian Peninsula. Cah Biol Mar 60:21\u0026ndash;29. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://dx.doi.org/10.21411/CBM.A.F13F82ED\u003c/span\u003e\u003cspan address=\"10.21411/CBM.A.F13F82ED\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBallesteros M, Madrenas E, Pontes M (2025) \u003cem\u003eMadrella aurantiaca\u003c/em\u003e in OPK-Opistobranquis. Published: 28/08/2012. Accessed: 11/12/2025. Available at (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://opistobranquis.info/en/?p=2214)\u003c/span\u003e\u003cspan address=\"https://opistobranquis.info/en/?p=2214)\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBallesteros M, Templado J (1996) Opistobranquios de las islas Baleares y de las Columbretes. In: Moreno D (ed) Libro de R\u0026eacute;sumenes XI Congresso Nacional de Malacolog\u0026iacute;a, Sociedad Espa\u0026ntilde;ola de Malacolog\u0026iacute;a. Almer\u0026iacute;a, Spain, pp 40\u0026ndash;41\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBarletta G (1976) Considerazioni sulla bionomia dei Nudibranchi e sulla loro alimentazione (nota preliminare). Conchiglie (Notiziario Mensile della Unione Malacologica Italiana. 12(5\u0026ndash;6):117\u0026ndash;128\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBebbington A, Thompson TE (1968) Note sur les Opisthobranches du bassin d\u0026rsquo;Arcachon. Actes Soc Linn Bourdeaux 105(3A):1\u0026ndash;35\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBohm C, Jonsson C (2023) Invertebrates of the Gothenburg Natural History Museum (GNM). Version 6.5. Gothenburg Natural History Museum (GNM). Occurrence dataset https://doi.org/10.15468/v0gwot accessed via GBIF.org on 2025-11-08. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.gbif.org/occurrence/683643806\u003c/span\u003e\u003cspan address=\"https://www.gbif.org/occurrence/683643806\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBouchet P, Tardy J (1976) Faunistique et biog\u0026eacute;ographie des Nudibranches des c\u0026ocirc;tes fran\u0026ccedil;aises de l'Atlantique et de la Manche. Ann lnst Oc\u0026eacute;anogr 52(2):205\u0026ndash;213\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eCalado G, Silva JP (2012) Lesmas do Mar do Algalve. Edi\u0026ccedil;\u0026otilde;es Subnauta. 164 pp. ISBN: 978-989-96406-2-7\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eCamps-Castell\u0026agrave; J, Ballesteros M, Trobajo R, Pontes M, Prado P (2020) Not all nudibranchs are carnivorous: trophic ecology of \u003cem\u003ePolycerella emertoni\u003c/em\u003e in the Ebro Delta. Mar Ecol Prog Ser 645:67\u0026ndash;82. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3354/meps13379\u003c/span\u003e\u003cspan address=\"10.3354/meps13379\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eCattaneo R, Chemello R (1987) Alcune considerazioni sui molluschi opistobranchi della Sicilia. Bollettino Malacologico (Pubblicazione Mensile Edita dalla Societa Italiana di. Malacologia) 23(5\u0026ndash;8):207\u0026ndash;222\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eCervera JL, Calado G, Gavaia C, Malaqu\u0026iacute;as MAE, Templado J, Ballesteros M, Garc\u0026iacute;a-G\u0026oacute;mez JC, Megina C (2004) An annotated and updated checklist of the opisthobranchs (Mollusca: Gastropoda) from Spain and Portugal (including islands and archipelagos). Bol Inst Esp Oceanogr 20(1\u0026ndash;4):1\u0026ndash;111\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eCornet R, Marche-Marchad I (1951) Inventaire de la faune marine de Roscoff Mollusques. Travaux de la Station Biologique de Roscoff, Suppl 5:1\u0026ndash;80. Available at \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://web.archive.org/web/20160610162926/http://www.sb-roscoff.fr/sites/www.sb-roscoff.fr/files/documents/station-biologique-roscoff-mollusques-1951.pdf\u003c/span\u003e\u003cspan address=\"https://web.archive.org/web/20160610162926/http://www.sb-roscoff.fr/sites/www.sb-roscoff.fr/files/documents/station-biologique-roscoff-mollusques-1951.pdf\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eCu\u0026eacute;not L (1927) Contribution \u0026aacute; la faune du bassin d\u0026rsquo;Arcachon. IX: Revue g\u0026eacute;n\u0026eacute;rale de la faune et bibliographie. Bull Stat Biol Arcachon 24:229\u0026ndash;308\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eFortunato H (2015) Bryozoans in climate and ocean acidification research: a reappraisal of an under-used tool. Reg Stud Mar Sci 2:32\u0026ndash;44. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.rsma.2015.08.010\u003c/span\u003e\u003cspan address=\"10.1016/j.rsma.2015.08.010\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGBIF.org (2025) GBIF Occurrence Download \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.15468/dl.z5hn8p\u003c/span\u003e\u003cspan address=\"10.15468/dl.z5hn8p\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHaefelfinger HR (1960) Catalogue des opisthobranches de la Rade de Villefranche-sur-Mer et ses environs (Alpes Maritimes). Rev suisse zool 67(3):323\u0026ndash;351\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJohnson RF, Gosliner TM (2012) Traditional Taxonomic Groupings Mask Evolutionary History: A Molecular Phylogeny and New Classification of the Chromodorid Nudibranchs. PLoS ONE 7(4):e33479. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1371/journal.pone.0033479\u003c/span\u003e\u003cspan address=\"10.1371/journal.pone.0033479\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKorshunova T, Fletcher K, Martynov A (2025) The endless forms are the most differentiated\u0026mdash;how taxonomic pseudo-optimization masked natural diversity and evolution: the nudibranch case. ZJLS 204(4):zlaf057. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1093/zoolinnean/zlaf057\u003c/span\u003e\u003cspan address=\"10.1093/zoolinnean/zlaf057\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKrug PJ, Wong NLWS, Medina MR, Gosliner TM, Vald\u0026eacute;s \u0026Aacute;A (2018) Cryptic speciation yields remarkable mimics: A new genus of sea slugs that masquerade as toxic algae (\u003cem\u003eCaulerpa\u003c/em\u003e spp). Zool Scr 47:699\u0026ndash;713. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1111/zsc.12310\u003c/span\u003e\u003cspan address=\"10.1111/zsc.12310\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLidgard S (2008) Predation on marine bryozoan colonies: taxa, traits and trophic groups. Mar Ecol Prog Ser 359:117\u0026ndash;131. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3354/meps07322\u003c/span\u003e\u003cspan address=\"10.3354/meps07322\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMartynov AV (2011) From tree-thinking to cycle-thinking: ontogenetic systematics of nudibranch molluscs. Thalassas 27(2):193\u0026ndash;224\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMcDonald GR, Nybakken JW (1997) List of the worldwide food habits of nudibranchs. Veliger, 40(2)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMcDonald GR (2009) Bibliographia Nudibranchia, second edition. UC Santa Cruz: Institute of Marine Sciences. Retrieved from \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://escholarship.org/uc/item/8115h0wzecz\u003c/span\u003e\u003cspan address=\"https://escholarship.org/uc/item/8115h0wzecz\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMacFarland FM (1966) Studies of opisthobranchiate mollusks of the Pacific coast of North America. Memoirs of the California Academy of Sciences VI: xvi\u0026thinsp;+\u0026thinsp;546 pp ISBN: 978-0940228108\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePola M, Hallas JM, Gosliner TM (2019) Welcome back Janolidae and \u003cem\u003eAntiopella\u003c/em\u003e: Improving the understanding of Janolidae and Madrellidae (Cladobranchia, Heterobranchia) with description of four new species. J Zool Syst Evol Res 57:345\u0026ndash;368. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1111/jzs.12257\u003c/span\u003e\u003cspan address=\"10.1111/jzs.12257\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePontes M, Madrenas E, Garcia-Tort A, Espada O, Ballesteros M (2025) Sea Slugs (Mollusca, Gastropoda, Heterobranchia) from the Medes Islands (Costa Brava, NE Spain): Biodiversity and Ecological Study over a Decade. Diversity 17(9):606. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3390/d17090606\u003c/span\u003e\u003cspan address=\"10.3390/d17090606\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePreston HB (1911) Records of zoological literature. VIII Mollusca Zool Rec 46(1):1\u0026ndash;92\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRobinson BH, Haddock HD (2024) Discovery and description of a remarkable bathypelagic nudibranch, \u003cem\u003eBathydevius caudactylus\u003c/em\u003e, gen. et. sp. nov. Deep-Sea Res I: Oceanogr Res Pap 214:104414. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.dsr.2024.104414\u003c/span\u003e\u003cspan address=\"10.1016/j.dsr.2024.104414\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRudman WB (1981) Polyp mimicry in a new species of aeolid nudibranch mollusc. J Zool 193:421\u0026ndash;427. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1111/j.1469-7998.1981.tb03454.x\u003c/span\u003e\u003cspan address=\"10.1111/j.1469-7998.1981.tb03454.x\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRudman WB (2001a) \u003cem\u003eMadrella ferruginosa\u003c/em\u003e Alder \u0026amp; Hancock, 1864. \u003cem\u003e[In] Sea Slug Forum.\u003c/em\u003e Australian Museum, Sydney. Available from \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttp://www.seaslugforum.net/find/madrferr\u003c/span\u003e\u003cspan address=\"http://www.seaslugforum.net/find/madrferr\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e Accessed: 8 Nov. 2025\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRudman WB (2001b) \u003cem\u003eMadrella sanguinea\u003c/em\u003e (Angas, 1864). [In] Sea Slug Forum. Australian Museum, Sydney. Available from \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttp://www.seaslugforum.net/factsheet/madrsang\u003c/span\u003e\u003cspan address=\"http://www.seaslugforum.net/factsheet/madrsang\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e Accessed: 8 Nov. 2025\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRudman WB, Avern G (1989) The genus \u003cem\u003eRostanga\u003c/em\u003e (Nudibranchia: Dorididae) in the Indo-West Pacific. ZJLS 96(3):281\u0026ndash;338. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1111/j.1096-3642.1989.tb01832.x\u003c/span\u003e\u003cspan address=\"10.1111/j.1096-3642.1989.tb01832.x\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSabelli B, Gianuzzi-Savelli R, Bedulli D (1990) Catalogo annotato dei Molluschi marini del Mediterraneo. Libreria Naturalistica Bolognese. Bologna, Italy: 348 pp\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSchr\u0026ouml;dl M, J\u0026ouml;rger KM, Klussmann-Kolb A, Wilson NG (2011) Bye bye Opisthobranchia! A review on the contribution of mesopsammic sea slugs to euthyneuran systematics. Thalassas 27(2):101\u0026ndash;112\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSmith AM (2014) Growth and calcification of marine bryozoans in a changing ocean. Biol Bull 226(3):203\u0026ndash;210. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1086/BBLv226n3p203\u003c/span\u003e\u003cspan address=\"10.1086/BBLv226n3p203\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eThompson JN (1989) Concepts of coevolution. Trends Ecol Evol 4(6):179\u0026ndash;183. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/0169-5347(89)90125-0\u003c/span\u003e\u003cspan address=\"10.1016/0169-5347(89)90125-0\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eThompson TE, Cattaneo R, Wong YM (1990) Eastern Mediterranean Opisthobranchia: Dotidae (Dendronotoidea), Arminidae and Madrellidae (Arminoidea). J Molluscan Stud 56(3):393\u0026ndash;413. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1093/mollus/56.3.393\u003c/span\u003e\u003cspan address=\"10.1093/mollus/56.3.393\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eVald\u0026eacute;s \u0026Aacute;, Moran AL, Woods HA (2012) Revision of several poorly known Antarctic aeolid nudibranch species (Mollusca: Gastropoda), with the description of a new species. JMBA UK 92(5):1161\u0026ndash;1174. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1017/S0025315411000348\u003c/span\u003e\u003cspan address=\"10.1017/S0025315411000348\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eVayssi\u0026egrave;re A (1903) Recherches Zoologiques et Anatomiques sur les Mollusques Opistobranches du Golfe de Marseille. Suppl\u0026eacute;ment. Ann Mus hist nat Marseille, tome VIII, section de Zoologie. M\u0026eacute;moire n\u0026ordm; 3:73\u0026ndash;108 pl. III\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eVayssi\u0026egrave;re A (1920) Nouvelle \u0026eacute;tude sur le \u003cem\u003eMadrella aurantiaca\u003c/em\u003e. Ann Sci Nat (Paris) Ser 3(10):161\u0026ndash;171 1 pl\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003evan den Berg CP (2022) Highly defended nudibranchs \u0026lsquo;escape\u0026rsquo; to visually distinct background habitats. Behav Ecol 35(5):arae053. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1093/beheco/arae053\u003c/span\u003e\u003cspan address=\"10.1093/beheco/arae053\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWagele H, Willan R (2000) Phylogeny of the Nudibranchia. ZJLS 130(1):83\u0026ndash;181. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1111/j.1096-3642.2000.tb02196.x\u003c/span\u003e\u003cspan address=\"10.1111/j.1096-3642.2000.tb02196.x\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eW\u0026auml;gele H (2004) Potential key characters in Opisthobranchia (Gastropoda, Mollusca) enhancing adaptive radiation. Org Divers Evol 4:175\u0026ndash;188. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.ode.2004.03.002\u003c/span\u003e\u003cspan address=\"10.1016/j.ode.2004.03.002\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWinters AE, Chan W, White AM, van den Berg CP, Garson MJ, Cheney KL (2022) Weapons or deterrents? Nudibranch molluscs use distinct ecological modes of chemical defence against predators. J Anim Ecol 91(4):831\u0026ndash;844. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1111/1365-2656.13643\u003c/span\u003e\u003cspan address=\"10.1111/1365-2656.13643\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWoRMS Editorial Board (2025) World Register of Marine Species. Available from \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.marinespecies.org\u003c/span\u003e\u003cspan address=\"https://www.marinespecies.org\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e at VLIZ. Accessed 2025-07-15. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.14284/170\u003c/span\u003e\u003cspan address=\"10.14284/170\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"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":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Diversity, Atlantic, Mediterranean, Reproduction, Systematics, Sea slug","lastPublishedDoi":"10.21203/rs.3.rs-8832905/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8832905/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eAmong the limited number of species currently included in the nudibranch genus \u003cem\u003eMadrella\u003c/em\u003e Alder \u0026amp; Hancock, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1864\u003c/span\u003e, only one has been documented from European waters. Nevertheless, our knowledge of the distribution and biology of this species remains limited, largely due to the paucity of observations, which are confined to a small number of records from the Mediterranean and the north-east Atlantic. During a series of sampling dives conducted in Galicia to catalogue benthic fauna, a single specimen of \u003cem\u003eMadrella aurantiaca\u003c/em\u003e Vayssi\u0026egrave;re, 1902 was discovered. Given the scarcity of records for this species, the specimen was photographed and collected for further study. Additional specimens captured in the Balearic Islands (western Mediterranean) during the FAUNA III oceanographic campaign were also examined in order to compare and update the available information on the species. As a result, new data on morphology and biology are presented, including a description of the egg masses. Historical records of the species are also reviewed, providing an overview of its current status and an updated distribution. The addition of new records for such a rarely reported species is of considerable value for improving our understanding of its distribution and biology, and ultimately for refining our knowledge of global diversity.\u003c/p\u003e","manuscriptTitle":"Assessment of the status and distribution of the poorly known nudibranch Madrella aurantiaca (Gastropoda: Nudibranchia: Madrellidae)","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-02-24 16:21:36","doi":"10.21203/rs.3.rs-8832905/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"2c6f830d-7f93-4e1e-8e3c-d83b52ace44d","owner":[],"postedDate":"February 24th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-03-26T17:02:01+00:00","versionOfRecord":[],"versionCreatedAt":"2026-02-24 16:21:36","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8832905","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8832905","identity":"rs-8832905","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}