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Delineating biogeographical regions can provide important insights into the processes shaping large‐scale species distribution patterns. Here we aimed to 1) identify global marine benthic biogeographical regions for ophiuroids extending from shallow waters to the deep sea and 2) quantify the importance of contemporary environmental conditions and geological features in shaping the delineated biogeographical regions. We delineated marine benthic biogeographical regions using a bipartite network analysis applied to a historical dataset of brittle stars. We then examined the faunal exchanges between the regions, and applied random forest models to evaluate the relative role of contemporary environmental conditions and tectonic configuration underlying the proposed biogeographic scheme. We propose ten main large biogeographical regions across the benthos. The biogeographical regions with the highest species richness and endemicity rates were found in the Indo‐Pacific region, Tropical West Atlantic and Southern Ocean, and South America. The key transition regions fall within the subarctic areas of the Pacific and the waters surrounding Southern Australia and New Zealand. Tectonic configuration, surface temperature and salinity were found to be the most important predictors of the ten delineated biogeographical regions. Our biogeographic delineation, including the deep sea, is in partial agreement with those proposed in previous studies. Our results suggest that contemporary environmental conditions (sea water temperature and salinity) strongly influence the modern distribution of ophiuroids, but that plate tectonics left a marked imprint on regional species pools. Future work based on multiple benthic taxa, and with a better understanding of the environmental conditions in the deep sea, are needed to evaluate the robustness of our proposed division.

This study investigates the atypical formation of carbonaceous compressions in fossil echinoderms. The analyses focus on Devonian encrinasterids from the Ponta Grossa Lagerstätte and other fossil ophiuroids from laboratory-based diagenetic experiments. The results indicate that the kerogenization potential of organic remains is mainly limited by the skeletal structure and the type of burial sediment. Less protected viscera, such as those beneath the thin disk cover of ophiuroids, are more prone to compression and polymerization than other labile tissues and organs housed within robust calcareous modules. The antibacterial properties of purer kaolinitic clays may also contribute to this process, leading to thicker and more defined carbonaceous compressions compared to those formed in microbially complex mixed clays. Despite chemical and structural alterations during diagenesis, carbonaceous remains assist in mapping the anatomy of extinct groups, such as the disc-restricted viscera in the encrinasterids analyzed. Raman spectra of carbonaceous compressions in these fossils also proved excellent geothermometers, showing that the Ponta Grossa Lagerstätte reached a peak temperature of about 170 °C. Graphical abstract Examples of carbonaceous compressions preserved macro- and microscopically in natural and artificial fossils of ophiuroids

Severely understudied and poorly known ultra-abyssal (hadal) brittle-stars of the genus Abyssura were collected during a recent expedition to the Japan Trench at depths between 6183 and 6539 m and were examined for the first time for both their molecular and detailed morphological data. To date, family-level assignment of the genus Abyssura remains a complete enigma, despite a recent major reorganization of ophiuroid classification. In this study, we infer an all-family level phylogeny of the class Ophiuroidea and find phylogenetic placement for Abyssura, which turns out to be a sister taxon of another little-known ophiuroid genus, Ophiambix, found in hot-vent and cold-seep environments in association with sunken wood at depths between 146 and 5315 m. The sister relationship between the hadal genus Abyssura and the shallow-water-to-abyssal genus Ophiambix is robustly supported by our molecular data, and both external and micromorphological data for these genera are highly consistent. No similar taxa have been found in any of the currently recognized 34 ophiuroid families. Therefore, the genera Abyssura and Ophiambix are assigned to the new family, Abyssuridae fam. nov. This new family shows features of paedomorphic reduction and elucidates the linkage between fauna from both the shallower and the deepest parts of the world’s oceans and provides new insights into the global bathymetric, biogeographic, and diversity patterns of organisms.

Photoreception and vision are fundamental aspects of animal sensory biology and ecology, but important gaps remain in our understanding of these processes in many species. The colour-changing brittle star Ophiocoma wendtii is iconic in vision research, speculatively possessing a unique whole-body visual system that incorporates information from nerve bundles underlying thousands of crystalline ‘microlenses’. The hypothesis that these might form a sophisticated compound eye-like system regulated by chromatophores has been extensively reiterated, with investigations into biomimetic optics and similar supposedly ‘visual’ structures in living and fossil taxa. However, no photoreceptors or visual behaviours have ever been identified. We present the first evidence of photoreceptor networks in three Ophiocoma species, both with and without microlenses and colour-changing behaviour. High-resolution microscopy, immunohistochemistry and synchrotron tomography demonstrate that putative photoreceptors cover the animals' oral, lateral and aboral surfaces, but are absent at the hypothesized focal points of the microlenses. The structural optics of these crystal ‘lenses’ are an exaptation and do not fulfil any apparent visual role. This contradicts previous studies, yet the photoreceptor network in Ophiocoma appears even more widespread than previously anticipated, both taxonomically and anatomically.

Biogenic habitats, such as coral reefs, facilitate diverse communities. In aquatic systems, aggregations of mobile benthic species may play a similar ecological role to that of typically sessile biogenic habitats; however, this has rarely been considered. We quantified the abundance of sessile horse mussels Modiolus modiolus and aggregating brittle stars Ophiothrix fragilis and tested for correlations between the density of mussels (live and dead) and brittle stars each with (1) abundance, biomass, diversity, and assemblage structure of associated benthic macrofauna; and (2) percent organic matter of the sediment. We found that the abundance of live M. modiolus was positively associated with the abundance and biomass of macrofauna. The positive association between M. modiolus and macrofaunal abundance was further amplified with an increase in brittle stars and a decrease in dead mussel shells. Macrofaunal biomass was lower with a higher percentage of dead mussel shells, and macrofaunal diversity increased with greater abundances of live M. modiolus and brittle stars. Sediment organic matter was positively related to brittle star density, but not to the abundance of live or dead mussels. The positive relationship between brittle stars and sediment organic matter suggests that brittle stars could enhance rates of benthic–pelagic coupling. Given the importance of understanding the functional role of threatened habitats, it is essential that the underlying community patterns be understood through robust observational studies to then derive testable hypotheses to determine drivers. These findings provide novel insight into the ecological role of aggregations of mobile species, which is essential to prioritize conservation and restoration strategies.

Five specimens of Ophiuroidea from deep-sea seamounts in the West Pacific were collected and identified as two species, Astrodia duospina sp. nov. and Astrodia abyssicola . The new species, Astrodia duospina sp. nov. , can be distinguished from its congeners by having indistinct or underdeveloped oral papillae, relatively short genital slits, crescent-shaped lateral arm plates, and plate-shaped external ossicles on the aboral surface of the disc. One specimen was identified as Astrodia abyssicola , which has been reported in the north-western Pacific and the north-eastern coast of Japan. The most recent tabular key of Astrodia was revised with two more key characteristics added, the shape and presence of oral papillae and the number of arm spines. The phylogenetic relationship of Astrodia and Asteronyx was analyzed based on 16S and COI sequences. The discovery of the two species further expanded the geographical distribution of the genus Astrodia .

Ocean warming poses a significant threat to the growth and survival of marine cold-water species. This study focused on Ophiopholis mirabilis , a representative cold-water Ophiuroid species, prevalent in the Yellow Sea Cold Water Mass (YSCWM), which has been at risk of warming and frequent heat waves in recent decades. Despite the risks, the molecular mechanisms underlying the response of O. mirabilis to warming stress remain poorly understood. In this study, we sampled O. mirabilis specimens from the YSCWM (121.82°E, 38.28°N, 52 m depth), and conducted transcriptome analysis on their arm tissues under three temperature conditions to investigate their molecular response to elevated temperatures. The results showed that 1104 differentially expressed genes (DEGs), four significant profiles, 23 transcription factor genes from nine different families, and one functional protein interaction network were identified. Under one week of heat exposure, upregulated DEGs primarily participated in immune defense systems, tissue remodeling and repair, and energy metabolism, suggesting the potential activation of inflammatory responses under increased temperature. Notably, heat shock proteins and co-chaperones were downregulated, and the protein folding and binding were negatively regulated under high temperature, indicating unique molecular responses to heat stress in cold-water species. In addition, O. mirabilis exhibited regulation of the apoptotic system by reducing the expression of key genes after one-week increased temperature. This study enhances our understanding of the acclimatization strategies employed by O. mirabilis in response to climate change, and provides underlying data for studying the molecular mechanisms of marine cold-water species to ocean warming.

The bioluminescent European brittle star Amphiura filiformis produces blue light at the arm-spine level thanks to a biochemical reaction involving coelenterazine as substrate and a Renilla -like luciferase as an enzyme. This echinoderm light production depends on a trophic acquisition of the coelenterazine substrate. Without an exogenous supply of coelenterazine, this species loses its luminous capabilities. Moreover, this species was recently shown not to produce coelenterazine storage forms. As an infaunal suspensive feeder, A. filiformis is assumed to find enough substrate to maintain its bioluminescence capabilities efficiently. To date, no studies have investigated the putative source of coelenterazine in the brittle star diet. A combined analysis using listing based on visual observations and metabarcoding on the planktonic communities highlights planktonic species known as light emitters using coelenterazine. Besides, the A. filiformis stomach content was analyzed seasonally via metabarcoding technique, and coelenterazine-related preys were underlined. Results provide evidence of the presence of preys containing coelenterazine in the fjord environment and within the stomach content of the ophiuroid throughout the year. The results are consistent with the demonstration of the trophic acquisition of luminous capabilities in A. filiformis and give a new step by underlying the constant presence of coelenterazine suppliers throughout the year for the luminescence reaction occurring within this species.

Since 2012, when Ophiothela was first described in the Atlantic, there has been no consensus regarding its identification. It has been described as O . mirabilis , O . cf. mirabilis , O . danae , or only Ophiothela sp. In order to fill these gaps, our aim was to test if specimens from Brazil are Ophiothela mirabilis and/or Ophiothela danae . Syntypes from the Museum of Comparative Zoology and United States National Museum, Smithsonian Institution, were used. We examined species boundaries of the small six-rayed brittle star Ophiothela using independent character sets utilizing morphology (external morphology and morphometry) and molecular data (16S and COI). Concordance was found between the analyses indicating that Ophiothela sp. from Brazil (BR), Ophiothela mirabilis and Ophiothela danae are closely related. We suggest that O . danae should be considered as a junior synonym of O . mirabilis . A detailed description of O . mirabilis BR is presented using external morphology and microstructural ossicles (arm plates, vertebra, dental and oral plates). This description includes new diagnostic features, particularly regarding its microstructures: (i) transspondylous articulation (first record in Ophiotrichidae); (ii) eight smooth knobs on the dorsal surface of the vertebrae (to date only in Ophiothela ); (iii) vertebrae with distal keel divided into two separate end processes matching the two large dorsal grooves proximally (first time in the literature); and (iv) an opening on both sides of the oral plate (as seen in other fissiparous species Ophiactis savignyi and Ophiocomella ophiactoides ).

Gess and Reddy describes a discovery of brittle star fossils (ophiuroids) in South Africa. These fossils are the oldest recorded brittle stars from the southern hemisphere and provide new insights into the origin of the Malvinoxhosan Realm, a biogeographic region with a unique marine fauna. The fossils were found in the Baviaanskloof Formation, part of the Table Mountain Group in South Africa. The rock layers date back to the Pragian Age, approximately 410 million years old. Two distinct species of brittle stars were identified, one previously known and one entirely new to science. This discovery suggests that the Malvinoxhosan Realm fauna existed earlier than previously thought. The scarcity of fossils from this period makes this finding particularly significant.