A New Species of Natsushima (Annelida: Chrysopetalidae) Living in the Mantle Cavity of a Deep-Sea Solemyid Clam

Natsushima is a genus of deep-sea Chrysopetalidae (Annelida) characterized by numerous bifurcate chaetae. It is poorly known, with three species living in the mantle cavity of bivalves in chemosynthetic habitats. Here we describe Natsushima nanhaiensis n. sp. based on an integrative morphological and molecular phylogenetic analysis of specimens collected from the Haima cold seep in the South China Sea. Morphologically, the new species can be distinguished from its congeneric species by the shape and number of the neuropodial hooks and bifurcate chaetae, the shape of the parapodia, and the long dorsal cirri. Sequence comparison and phylogenetic analysis based on the mitochondrial COI and 16S rRNA gene sequences supported the placement of Natsushima nanhaiensis n. sp. in Natsushima and its status as a distinct species. We also present a key to species of Natsushima and discuss their biogeography.