Ambusher in sponge: a new species of Eunice (Annelida, Eunicidae) commensal within deep-sea Farreidae (Porifera, Hexactinellida) on northwest Pacific seamounts

Deep-sea sponges create complex biogenic structures and attract a wide array of deep-sea organisms, including symbionts. In this study, we describe <i>Eunicesiphoninsidiator</i> <b>sp. nov.</b>, a new eunicid species living in the central cavity of deep-sea farreid glass sponges found on northwest Pacific seamounts. The new species closely resembles the Atlantic <i>Eunicenorvegica</i> both morphologically and molecularly, but it differs in the relative length of palp compared to peristomium, starting points of subacicular hooks, and shape of pectinate chaetae. A 13% <i>COI</i> genetic distance between the two species further supports the establishment of <i>E.siphoninsidiator</i> as a distinct species. Gut content analyses reveals fragments of barnacles and brittle stars, suggesting a carnivorous diet and a sit-and-wait predatory strategy. The eunicid gains protection from living inside the sponge, which consistently harbored the polychaete in all specimens examined, while the sponge benefits from the cleaning of epibionts, pointing to a potentially mutualistic relationship.