An updated inventory of marine opisthobranch (Mollusca, Gastropoda) from the territorial waters of the Republic of Mauritius

Island ecosystems support diverse aquatic invertebrate communities comprising endemic taxa. Documentation of existing species is important for conservation. In this study, a checklist of marine opisthobranch from the Republic of Mauritius is presented. A combination of benthic surveys (50 m×5 m in triplicates), rover diving techniques and photo documentation were used over two years (2018–2020) within 35 sheltered and unsheltered lagoons. Morphological and molecular analysis were used for identification. Species composition within sheltered and unsheltered areas in Mauritius was estimated using the Bray-Curtis similarity. The checklist featured 117 species belonging to 61 genera and 28 families, of which 13 are new records. The findings increased the knowledge of opisthobranch diversity from the Mauritius by 15.4%. Among the listed species, the distribution range of Cyerce nigra, Actinocyclus papillatus , and Phyllidia picta extended from the Western Pacific to the South Western Indian Ocean. Molecular analysis of the undescribed Gymnodoris sp. showed it resembled Gymnodoris sp. from Hawaii and were different by a genetic distance value of 10.6%. The species richness and evenness were higher within the sheltered regions of Mauritius which harboured the food resource of opisthobranch. These areas as compared to unsheltered regions were heavily populated, suggesting the probable influence of wave actions on opisthobranch diversity and abundance. The order Nudibranchia was reported as most speciose, with 86 species. The Sacoglossa and Nudibranchia were observed only on macroalgae and sponges respectively. High abundance was also recorded on shipwrecks which are the most common form of artificial reefs. With the inclusion of observations from previous studies, 201 species belonging to 94 genera and 36 families are now known from the Mauritius.