Phylogenetic analyses elucidate the identity and distribution of two early-described species of Arctic Cirratulidae (Annelida, Sedentaria)

Recent expeditions to the North-East Atlantic and the Arctic Ocean yielded an abundance of specimens resembling two species of polychaetes of the family Cirratulidae Ryckholt, 1851 originally described in 1879 and tentatively placed within the genus Cirratulus Lamarck, 1818. In this study, we set out to confirm their identity, assess the potential presence of cryptic species, determine their correct generic placement, and gain insights into their distributions. Our approach involved comprehensive analyses using four molecular markers (COI, 16S, 28S, and 18S) through Bayesian inference, maximum likelihood analyses, and haplotype networks. Additionally, we conducted detailed morphological examinations of the specimens using both light and electron microscopy techniques. Notably, this study provides the first DNA sequences for a species within the genus Chaetocirratulus Blake, 2018. By integrating molecular phylogenetic analyses with morphological assessments, we establish the revised taxonomic placements of these two species as Chaetocirratulus abranchiatus (Hansen, 1879) comb. nov. and Aphelochaeta abyssorum (Hansen, 1879) comb. nov. Phylogenetic analyses also recover a close relationship between Chaetocirratulus, Cirratulus and Cirriformia. Furthermore, our research reveals the wide distribution of these species in the North-East Atlantic and Arctic waters, marking the first report of Chaetocirratulus in this region. We present re-descriptions of these species according to contemporary taxonomic standards, complemented by extensive illustrations. A lectotype is selected for Chaetocirratulus abranchiatus and a neotype is selected for Aphelochaeta abyssorum.