A sea of worms: the striking cases of the European Perinereis cultrifera and P. rullieri (Annelida: Nereididae) species complexes, with description of 13 new species

Molecular data have been suggesting the existence of a complex of cryptic species within the taxon Perinereis cultrifera, which has not been fully explored yet. In this study, we performed a morphological and molecular analysis (mtCOI-5P, 16S rRNA and 28SD2 rRNA) of Perinereis specimens from intertidal marine and brackish European localities, mostly focusing on the Mediterranean Sea and the Canary Islands. Two major phylogenetic clades with at least 18 divergent (COI, 19.8; 6.4-28.5%) and completely sorted lineages were uncovered based on original data, 13 of which occurred exclusively in the Mediterranean Sea, a further 5 of which are unique to Italian brackish waters. An additional morphologically similar lineage, corresponding to P. oliveirae, coexisting with the single NE Atlantic lineage of the complex, was also retrieved as an ingroup. Careful morphological inspection, combined with the deep divergence between the two major molecular clades and the perfect match of each clade to the specific paragnath and chaetal types, highlighted the existence of two distinct groups of European Perinereis species: Clade A, which shows features matching historical descriptions of P. cultrifera, and Clade B corresponding to an overlooked morphotype described as P. rullieri. Although paragnaths show a similar pattern in the two clades, their sizes are considerably smaller in P. rullieri and the chaetae are characterised by coarse serration at the base of the spiniger blades and long falciger blades, as opposed to the lightly serrated blades and short falcigers in P. cultrifera. Further overlooked morphological features mainly based on thickness, direction and length of paragnaths, as well as the expansion of posteriormost dorsal ligules were also revealed within each major clade, which together with geographic and environmental boundaries allowed for the differentiation of most of these lineages without molecular data. Thirteen new species are here formally described, eight belonging to Clade A: P. caesarea sp. nov., P. faulwetterae sp. nov., P. houbinae sp. nov., P. maleniae sp. nov., P. miquellai sp. nov., P. muscoi sp. nov., P. nieri sp. nov. and P. twobae sp. nov.; and five belonging to Clade B: P. castellii sp. nov., P. juno sp. nov., P. jupiter sp. nov., P. minerva sp. nov. and P. tibicena sp. nov. The new combination P. beaucoudrayi is also proposed for Nereis beaucoudrayi, previously considered synonymous with P. cultrifera, for the only lineage occurring in the NE Atlantic. Lastly, Perinereis cultrifera s.s., P. rullieri s.s and the ingroup P. oliveirae are redescribed using topotypical material, with available syntypes and lectotypes assigned to the former two.