The Mediterranean Sea diversity is still far from being fully disclosed. Marine Heterobranchia are one of the most paradigmatic species-rich groups, with many recent systematic studies revealing the high density of new, cryptic, and endemic species occurring in the Mediterranean basin. In this study, sea slug Doriopsilla areolata, which was considered until today one of the most widespread nudibranchs worldwide, was investigated using a molecular approach to compare Mediterranean and Atlantic populations for the first time. The molecular analyses involved three different molecular markers, the two mitochondrial COI and 16S, and the nuclear H3 gene. The results revealed a complex of species within D. areolata that indeed consists of three potentially species, two of which are endemic to the Mediterranean Sea: Doriopsilla areolata, which is distributed in the Adriatic Sea (the type locality of the former species), D. rarispinosa, which occurs in the Western Mediterranean basin and along the Tunisian coast, and one additional Atlantic species here provisionally defined as Doriopsilla sp. 1. This study helps to unveil another case of cryptic diversity within Mediterranean Heterobranchia and to increase the knowledge on Doriopsilla genus diversity.

The Mediterranean Sea hosts a great Nudibranchia diversity and has proved to be particularly intriguing in the case of the family Onchidorididae, a group of dorid nudibranchs that lately increased its diversity with the addition of one recently described Mediterranean species. The Onchidorididae family has a troubled systematic history to date, characterized by uncertainties and genera that are considered valid or not, according to the different authors. This confused taxonomy reflects the lack of a broad and comprehensive view on the phylogenetic relationships occurring between Onchidorididae members, an incorrect interpretation of the diagnostic morphological characters, and a poor knowledge of important biological aspects characterizing the different genera included in the family. To shed some light on the systematics of Onchidorididae, an integrative taxonomic revision was carried out involving morphological, ecological, and molecular analyses on an updated dataset. Mediterranean specimens and species were added to the dataset of the already known Onchidorididae and a new species from the Adriatic Sea (Central Mediterranean Sea) is described here. Furthermore, historical controversies are clarified due to the discovery of new important synapomorphies useful to define genera belonging to the Onchidorididae family and to describe a new genus. Finally, the taxonomic status of all the known Onchidorididae species is investigated and discussed, filling the gap of knowledge on neglected species.

The Mediterranean Sea faces escalating environmental pressures from the unsustainable exploitation of resources and insufficient management. Marine protected areas (MPAs), such as the Parc Natural del Montgrí, les Illes Medes i el Baix Ter (PNMMBT) on Spain’s Costa Brava, serve as critical refuges for biodiversity conservation. Between 2010 and 2019, we conducted systematic visual surveys of sea slugs (Mollusca: Heterobranchia) within the PNMMBT, documenting 8289 individuals representing 98 species: 76 Nudibranchia, 10 Sacoglossa, 5 Aplysiida, 3 Pleurobranchida, 2 Umbraculida, 1 Runcinida, and 1 Cephalaspidea. Temporal, spatial, and bathymetric distribution analyses revealed a steady increase in species richness over time, suggesting the MPA’s ecosystems appear to remain in relatively good condition. To further refine our assessment, we incorporated citizen science records from the MINKA platform, expanding the documented diversity to 141 species including 99 Nudibranchia, 12 Sacoglossa, 5 Aplysiida, 6 Pleurobranchida, 2 Umbraculida, 4 Runcinida, 10 Cephalaspidea, 1 Acteonimorpha, 1 Pteropoda, and 1 Ringiculimorpha. This integrated approach highlights the value and complementarity of structured scientific surveys and community-contributed data in biodiversity monitoring. Citizen science fosters public engagement in research and provides scalable, cost-effective data that would otherwise be logistically unfeasible to obtain. By bridging these methodologies, we demonstrate how participatory science enhances the detection of species distributions, tracks ecological shifts, and bolsters conservation strategies in the current era of global biodiversity decline.

Basic and applied research reveals the importance of sea slugs as a source of new bioactive molecules or of still little-known intra/intercellular processes, mainly linked to the highly specialised defensive strategies typical of this group of shell-less molluscs. In this context, the nudibranch Doris verrucosa (Gastropoda, Mollusca), commonly known as ‘warty dorid’, is particularly interesting due to its ability to produce de novo biochemical compounds with pharmacological properties and being the type species of the genus Doris, one of the oldest and richest in species, currently characterised by a troubled systematics. Despite its wide distribution across the Eastern Atlantic Ocean and the Mediterranean Sea, this species has not yet been characterised from a genetic point of view. Considering the importance of assessing species identity to correctly investigate the systematics and to properly unravel potentially useful applications, results from a molecular assessment of such interesting species are provided. Genetic analysis involved species delimitation, phylogeny and haplotype network methods carried out on specimens of D. verrucosa collected from highly anthropised areas of Southern Italy (central Mediterranean Sea). Furthermore, in situ observations allowed us to fill some gaps in knowledge on the ecology and the morphological variability of this species that could be useful for future comparisons.