Digging in the sand: new species and records of mesopsammic sea slugs in the Western Mediterranean (Mollusca: Gastropoda)

Marine interstitial habitats (mesopsammon) have been understudied due to complications of exploring its millimetric and highly adapted organisms. The restrictive ecological conditions of this neglected habitat include faint light and a limited amount of space. These have forced its fauna, with gastropod molluscs as one of the most common groups, to adapt by reducing body size to minute (often shell-less) vermiform organisms. These usually simplified heterobranch species are hard to collect and problematic to determine, often including cases of cryptic and pseudo-cryptic speciation. Here, we explore the interstitial heterobranch diversity alongside the Spanish and Catalan coast (Western Mediterranean) to shed light on the taxonomy of this group. Among the 15 localities, we have collected 315 specimens directly from coarse samples and associated habitats, belonging to ca. 39 different species, providing live photographs, species identification based on the current taxonomic knowledge, and species delimitations based on newly generated sequences of the cytochrome <i>c</i> oxidase I (<i>COI</i>). Results show the first records for Spain and the Catalan Coast for some of these neglected taxa, thereby reinforcing or expanding its distribution with molecular data. Moreover, two species are described, the lower heterobranch <i>Rhodope salviniplaweni</i> <b>sp. n</b>. and the nudibranch <i>Embletonia mediterranea</i> <b>sp. n</b>. and the cephalaspidean <i>Laona vestita</i> comb. nov. is reassigned to a different family. Overall, this study represents a significant advancement in the taxonomic and biogeographic knowledge of interstitial fauna, highlighting the value of continued research in these underexplored habitats. http://www.zoobank.org/urn:lsid:zoobank.org:pub:10B5A669-99B1-46E7-BBE3-42C36D4B3663