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Vesicomyid bivalves are a consistent component of communities of sulphide-rich reducing environments distributed worldwide from 77° N to 70°S at depths from 100 to 9050 m. Up-to-now the taxonomy of the family has been uncertain. In this paper, the current state of vesicomyid taxonomy and distribution at the generic rank are considered. This survey is founded on a database including information both from literature sources and also unpublished data of the authors on all recent species of vesicomyids. We suggest that the Vesicomyidae is not a synonym of Kelliellidae, and is therefore a valid family name. We propose to divide the family Vesicomyidae into two subfamilies: Vesicomyinae and Pliocardiinae. The Vesicomyinae includes one genus, Vesicomya, which comprises small-sized bivalves characterized by non-reduced gut and the absence of subfilamental tissue in gills. Symbiosis with chemoautotrophic bacteria has, so far, not been proved for Vesicomya and the genus is not restricted to sulphide-rich reducing habitats. The subfamily Pliocardiinae currently contains about 15 genera with mostly medium or large body size, characterized by the presence of subfilamental tissue in the gills. The Pliocardiinae are highly specialized for sulphide-rich reducing environments, harbouring chemoautrophic bacteria in their gills. This is the first summary of the generic structure of the family Vesicomyidae that allow us to analyze the distribution of vesicomyids at the generic level. We recognize here five different distribution patterns that are related to the specific environmental demands. The general trends in the distribution patterns of the vesicomyids are an occurrence of the majority of genera in broad geographical ranges and the prevalence of near continental type of distribution.

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Large vesicomyid clams (Veneroida: Vesicomyidae: Pliocardiinae) are prominent members of the communities associated with sulphide-rich deep-sea habitats. Taxonomic uncertainties within the Pliocardiinae result from both plasticity in shell morphologies and the common occurrence of cryptic species. Molecular taxonomic studies have now clarified many species-level assignments and provided DNA-barcodes for more than 50 species worldwide. Nonetheless, genus-level assignments remain uncertain, because the existing COI barcode sequences are not sufficient for identifying higher-level groupings. To construct a robust phylogeny for this subfamily, we conducted a combined Bayesian analysis of the COI mitochondrial fragment and five additional independent nuclear gene segments. The phylogenetic results provide a better foundation for assessing genus-level assignments within the subfamily and reveal goals for future taxonomic research. Furthermore, morphological examinations helped to clarify and solidify generic classifications. Calibration of molecular clocks with recently verified fossil data permitted realistic estimates for the origins and evolutionary age of pliocardiins during the Cenozoic Era from a deep-dwelling ancestor.http://zoobank.org/urn:lsid:zoobank

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