The Syllis gracilis species complex: A molecular approach to a difficult taxonomic problem (Annelida, Syllidae)

Syllis gracilis is an emblematic member of the subfamily Syllinae (Syllidae, Annelida), which inhabits shallow, temperate coastal waters and can be found on algae, coral rubble, and sponges. Their distinctive ypsiloid chaetae, usually found in specimens from populations all around the world, led to the consideration of the species as cosmopolitan, even though four other species have similar chaetae: Syllis magellanica, S. picta, S. mayeri and S. ypsiloides. The discovery of deeply divergent lineages in the Mediterranean Sea, that were morphologically similar, questioned the cosmopolitanism of S. gracilis and suggested the possibility of it being a species complex. In order to assess the speciation patterns within the putative S. gracilis complex, we undertook species delimitation and phylogenetic analyses on 61 specimens morphologically ascribed to Syllis gracilis and closely related species using a multilocus molecular dataset (two mitochondrial and two nuclear markers). Our results suggest high levels of genetic differentiation between the S. gracilis populations analyzed, some of which have morphologically distinctive features. Five to eight distinct lineages (depending on the analysis) were identified, all with geographically restricted distributions. Although the presence of ypsiloid chaetae has been traditionally considered the main character to identify S. gracilis, we conclude that this feature is homoplastic. Instead, we propose that characters such as the degree of fusion of blades and shafts in chaetae, the morphology of the posterior chaetae or the animal color pattern should be considered to differentiate lineages within the S. gracilis species complex. Our study does not support the cosmopolitanism of S. gracilis, and instead provides morphological and molecular evidence of the existence of a complex of pseudo-cryptic species.