Species delimitation is particularly challenging in Mediterranean nudibranchs because morphological distinction between closely related species is subtle and several cryptic species have been discovered. In this study, we apply an integrative approach to investigate taxonomic distinction of Flabellina cavolini and F. gaditana and assess their genetic and morphologic distinction. We combined a comprehensive phylogeographic survey from across their distribution range and compared genetic variation at mitochondrial and nuclear markers with the variation in colour pattern of cerata that is used to discriminate these two species. Phylogenetic, phylogeographic and species delimitation analyses consistently identify two evolutionary and taxonomic units that do not match with the morphospecies, demonstrating that Flabellina cavolini and F. gaditana are indeed two cryptic, closely related, species and that colour pattern of cerata is not a valid diagnostic character. This study emphasizes how the comparison of range‐wide patterns of phylogenetic and morphological variation, with the inclusion of samples from the type localities, is essential for species delimitation of Mediterranean nudibranchs. Moreover, this study demonstrates the occurrence of Flabellina gaditana in the Mediterranean Sea, allowing to extend the known distribution range of the species, that was previously restricted to the eastern Atlantic Ocean, as well as increasing our inventory of Mediterranean nudibranch diversity.

The precise number of Okenia taxa inhabiting the Mediterranean Sea, as well as their general taxonomy, varies according to different specialists. So far, eight valid species have been reported from the area: Okenia aspersa (Alder & Hancock, 1845), Okenia cupella (Vogel & Schultz, 1970), Okenia elegans (Leuckart, 1828), Okenia hispanica Valdés & Ortea, 1995, Okenia impexa Er. Marcus, 1957, Okenia leachii (Alder & Hancock, 1854), Okenia mediterranea (Ihering, 1886), and Okenia zoobotryon (Smallwood, 1910). Of these, only three ( O . elegans , O . hispanica , and O . mediterranea ) have their type localities in the Mediterranean Sea, whereas the others were described from different biogeographic areas and later included in the Mediterranean biota. We carried out a review on Mediterranean Okenia species through an integrative approach, based on a wide literature search and a morphological and molecular analysis of available type material and samples collected recently. The present study confirmed the presence of O . aspersa , O . elegans , O . hispanica , and O . mediterranea in the Mediterranean Sea, although leaving remaining questions about some of those taxa. The distribution of O . cupella , O . impexa , and O . zoobotryon is limited to the western Atlantic, and of O . leachii to the eastern Atlantic. All specimens previously identified as O . cupella , O . impexa , and O . zoobotryon by different authors in the Mediterranean Sea were repeatedly misidentified. Thus, we describe Okenia problematica sp. nov. and Okenia longiductis sp. nov., from the “Mediterranean” Okenia cupella/impexa and O . zoobotryon . We also consider here Okenia pusilla Sordi, 1974 a nomen dubium and include a redescription of the holotype of O . cupella . A molecular phylogeny, including all the sequenced Okenia species, was performed in order to evaluate the evolutionary relationships of the newly described species with the other congeneric taxa.

A new sacoglossan species of the genus Elysia Risso, 1818 from southern Italy is described based on its external morphology, color pattern, radular teeth, and reproductive system. Specimens of this new species were previously attributed to Elysia viridis (Montagu, 1804) because of morphological similarity, but maximum-likelihood and Bayesian analyses based on four genes (mitochondrial cytochrome c oxidase subunit I, 16S rRNA, nuclear 28S rRNA, Histone 3), including 28 additional species of Elysia from the Atlantic, showed that they are distinct. In addition, to determine the identity of these specimens, different species delimitation analyses [Automatic Barcode Gap Discovery (ABGD), General Mixed Yule Coalescent (GMYC), Bayesian Poisson Tree Processes, (bPTP)] were carried out. The molecular results support the validity of this new species of Elysia from the Mediterranean Sea, named Elysia rubeni sp. nov. This new species is characterized by a bright light greenish surface color due to the abundance of kleptoplasts. It can be distinguished by its bright blue specks and minute sparkling yellow dots scattered along the body, as well as its smooth and greyish rhinophores.

The Salento peninsula is a portion of the Italian mainland separating two distinct Mediterranean basins, the Ionian and the Adriatic seas. Several authors have studied the marine Heterobranchia (Mollusca, Gastropoda) fauna composition living in the Ionian Sea, but to date further knowledge regarding this interesting group of mollusks is still needed. Recent studies have corroborated the peculiarity of the Mediterranean Sea showing high levels of endemism and cryptic diversity. On the other hand, marine sea slugs have been revealed to be important indicators of the marine ecosystem’s health, due to their species-specific diet that consist of a vast variety of sessile and benthic invertebrates. A baseline study of the marine Heterobranchia diversity is therefore a necessary step to reveal the hidden diversity and to monitor the possible presence of alien species. The present study shows results from approximately 600 scientific dives carried out during a nine-year period in all of the main submarine habitats of the studied area, while accounting for the marine Heterobranchia from both the Ionian and Adriatic Seas. With this contribution, the list of marine Heterobranchia inhabiting the Salento Peninsula rises to 160. Furthermore, it also reports, for the first time, the presence of one alien species and three new records for Italian waters. Ecological notes and geographical distribution for each added species are provided together with animal iconography, consisting mainly of in situ photographs, for species identification.

Runcinida is a small heterobranch order of sea slugs with 61 known species distributed worldwide across temperate and tropical latitudes with two established families (Ilbiidae and Runcinidae). Little is known about the phylogenetic relationships within Runcinida. Here, we present the first molecular phylogeny of the order with an emphasis on European species and we discuss the taxonomic status of the type species Runcina coronata. Molecular phylogenetics based on the mitochondrial genes cytochrome c oxidase subunit I and 16S rRNA and nuclear gene histone H3 is used to investigate relationships between species. Detailed morpho-anatomical worked was additionally employed to study Runcina coronata. Our results suggest the monophyly of Runcinida and showed that Runcina coronata is a complex of four species, namely: R. coronata proper, R. aurata and two new species here formally described (R. caletensis sp. nov. and R. tingensis sp. nov.).