Resolving the status of the families Vesicomyidae and Kelliellidae (Bivalvia: Venerida), with notes on their ecology

The bivalve family Vesicomyidae comprises two subfamilies: Pliocardiinae and Vesicomyinae. The Pliocardiinae include conspicuous mostly large- and medium- sized clams, which live in symbiosis with chemoautotrophic bacteria and often dominate chemosynthetic communities in the deep sea. The Vesicomyinae comprise poorly- studied tiny bivalves, which at first sight differ considerably from their better known, large- sized relatives. Vesicomyines have a worldwide distribution from mid- slope to hadal depths and reach abundances of thousands of specimens in one trawl catch in deep- sea trenches. The genus Vesicomya is morphologically very similar to the genus Kelliella, the type genus of family Kelliellidae. This fact has long caused taxonomic ambiguities and it has been suggested that Vesicomya and Kelliella, as well as their respective nominate families, should be synonymized. In this paper we study the relationship between Vesicomya and Kelliella, for the first time using not only morphological characters but also genetic divergence based on the mitochondrial cytochrome c oxidase subunit I ( COI) gene. Our molecular data indicate that the genera Vesicomya and Kelliella are clearly distinct and that Kelliella is distantly related to the Vesicomyidae. Results of morphological analysis support the molecular inferences. We provide further evidence for the relationship of vesicomyines and pliocardiines and propose an evolutionary scenario according to which Vesicomyinae is the less derived group and Pliocardiinae the more derived. We provide the first information on stable carbon and nitrogen isotopic composition of soft tissues of Vesicomya and Kelliella. These data do not suggest involvement of chemosynthetically fixed carbon in the nutrition of either Vesicomya or Kelliella. We revise current usage of these two genera and provide lists of the species of each, with details of their distribution. A new species, Vesicomya alleni, is described on the basis of morphological and molecular characters.