Molecular phylogenetic and morphological analyses of the 'monospecific' Hesiolyra (Annelida: Hesionidae) reveal two new species.

Although many deep-sea species are considered to have transoceanic distribution, this assumption has rarely been critically tested. Using Hesiolyra (Hesionidae) as a case study, we show that careful molecular and morphological analyses are required for refuting/accepting claims about such wide distribution. Hesiolyra is a genus of polychaetes commonly found in hydrothermal vents of the Eastern Pacific, Atlantic and Indian Ocean. Nevertheless, H. bergi Blake, 1985, a species originally described from the Eastern Pacific Rise (EPR), is the only recognized species in this genus. Phylogenetic analyses based on the COI and 16 S rRNA genes revealed two monophyletic clades that were distinct from a clade consisting of H. bergi sequences. We erected two new species of Hesiolyra , herein named as H. longqiensis n. sp. from the Southwest Indian Ridge (SWIR) and H. heteropoda n. sp. from the Central Indian Ridge (CIR) and the Carlsberg Ridge (CR). Among them, H. longqiensis n. sp. is more closely related to H. bergi than to H. heteropoda n. sp.. Morphologically, the two newly described species can be easily distinguished from H. bergi. The notopodia and neuropodia of H. bergi and H. longqiensis n. sp. , are of similar sizes, but the morphology of their lyrate chaetae and their dorsal cirri alternation patterns are different. In H. heteropoda n. sp. , the notopodia are remarkably slimmer than the neuropodia, and it lacks lyrate chaetae. Overall, in this report two new species of Hesiolyra from the Indian Ocean ridges and show that the distribution of H. bergi is restricted to the Eastern Pacific Rise. Future exploration of other mid-ocean ridges is required to discover the true diversity of this genus of polychaetes inhabiting hydrothermal vent fields. • Hesiolyra are common polychaetes living on chimney walls of hydrothermal vents. • The genus was considered to be monospecific. • Hesiolyra bergi from East Pacific Rise was the only recognized species. • We described two new species of Hesiolyra from Indian Ocean ridges. [ABSTRACT FROM AUTHOR]