Recent molecular studies revealed high level of endemism and numerous cryptic species within opisthobranchs, with Mediterranean taxa clearly understudied. Here we used genetic data from both mitochondrial and nuclear gene fragments as well as morphological data from taxonomically relevant characters to investigate the phylogenetic relationships and systematics of Mediterranean taxa of the Flabellinidae and Piseinotecidae families. Phylogenetic analyses based on Bayesian and Maximum-Likelihood methods indicate that Flabellinidae and Pisenotecidae taxa and species within the genera Flabellina, Calmella and Piseinotecus do not form monophyletic clades. These results are supported by our morphological analyses which allowed the re-evaluation of the triseriate radula condition in Pisenotecidae and Calmella taxa and their inclusion in the genus Flabellina as Flabellina gaditanacomb. nov. (synonym of F. confusa), Flabellina gabiniereicomb. nov. and Flabellina cavolinicomb. nov. Species delimitation and barcoding gap analyses allowed uncovering cryptic species within Flabellina gracilis (Alder and Hancock, 1844), F. trophina (Bergh, 1890), F. verrucosa (M. Sars, 1829) and F. ischitana Hirano and Thompson, 1990, the latter with an Atlantic form which is under description. This study corroborates the relevance of combining molecular and morphological data from multiple populations and species in the assessment of nudibranch diversity and classification.
Keywords: Cryptic species; Gastropoda; Mediterranean Sea; Mollusca; Multilocus study; Sibling species; Species delimitation.
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