Living with a giant parchment tube worm: a description of a new nudibranch species (Gastropoda: Heterobranchia) associated with the annelid Chaetopterus

Small fionid nudibranch specimens collected within a Chaetopterus sp. tube worm are described here as a new species Tenellia chaetopterana sp. nov. This case is the first example of symbiotic association between a mollusk and an annelid host amongst cladobranch sea slugs. The external morphology of this species suggests it is adapted for living inside the worm’s tube: flattened body, laterally directed cerata and rhinophores, and wide foot. Molecular data, including partial sequences of mitochondrial COI and 16S and nuclear H3 genes, indicates that this species is distinct from other members of the genus Tenellia as well as other fionids. The species is close to the coral-feeding fionids of the genus Tenellia (formerly in genus Phestilla ) in several morphological characters such as general radula morphology, absence of cnidosacs, and flattened body shape, as well as by molecular data. Tenellia chaetopterana sp. nov. has unique biology, implicating possible diversity previously overlooked by nudibranch faunistic studies.