Bacterial symbiosis in the Nucinellidae (Bivalvia: Solemyida) with descriptions of two new species

Abstract

Nucinellidae are a family of small, monomyarian, nuculoid marine bivalves that live at depths from 6–3,500 m. Related to the Solemyidae, they are suspected of chemosymbiosis with sulphur-oxidizing bacteria, but hitherto without morphological or molecular confirmation. Two new species, Nucinella owenensis and Huxleyia habooba, were collected at depths of 3,400 and 84 m, respectively, during a survey of the oxygen-minimum zone associated with the seasonal upwelling area in the Arabian Sea, off southern Oman. Sections of the relatively large ctenidia revealed large thickened leaflets with abundant rod-shaped bacteria housed in bacteriocytes. This is the first morphological confirmation of likely chemosymbiosis in the family. Both species have a complete alimentary system and in N. owenensis the hindgut contained sediment, suggesting particulate feeding. Relationships of Nucinellidae and Solemyidae and the timing of acquisition of the chemosymbiosis are briefly discussed. Descriptions of the two new species are accompanied by a critical review of familial and generic characters, with rejection of previous reports of posterior adductor muscle scars in Huxleyia.