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 c oxidase I (COI). 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 Rhodope salviniplaweni sp. n. and the nudibranch Embletonia mediterranea sp. n. and the cephalaspidean Laona vestita 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

Funding for this project was generously provided by the association Artenvielfalt erforschen und retten UG and the Malacological Society of London, to JF-S’s research. Additional support was received from the World Congress of Malacology 2022, facilitating an oral communication and participation in the Symposium in honour of Philippe Bouchet. JM acknowledges support from the Spanish Government through the HETGEN1000 project (PID2021-127037NA-I00/MCIN/AEI/10.13039/501100011033/ and by FEDER una manera dehacer Europa)

With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S)

44 pages, 20 figures, supplementary material https://doi.org/10.1080/00222933.2024.2439056

Peer reviewed