A new species of chemosymbiotic clam (Bivalvia: Vesicomyidae) from the Nankai Trough and Mariana Forearc.

Chemosymbiotic vesicomyid clams in the subfamily Pliocardiinae are a major component of deep-sea chemosynthetic ecosystems, yet their diversity remains incompletely documented. Here, we report a new species of pliocardiine clam collected from two cold seeps, the 2,526 m deep Oomine Ridge site in the Nankai Trough, Japan and the 2,915 m deep Summit Site of South Chamorro Seamount, Mariana Forearc. This ovate species (maximum shell length 34.4 mm) is included in the genus Waisiuconcha Beets, 1943. It is most similar to Waisiuconcha helios Krylova & Janssen, 2006 but differs by having a larger shell, a subumbonal pit, a steeply inclined anterior cardinal tooth of the right valve (3a), a long and straight anterior cardinal tooth of the left valve (2a), as well as a thick and straight posterior cardinal tooth of the left valve (4b). Anatomical examination revealed a single prominent papilla-like projection on the inner vulva of the inhalant siphon previously only known from Calyptogena Dall, 1891. As such, we describe our species as Waisiuconcha takashiisp. nov. We also present a phylogenetic reconstruction using the mitochondrial cytochrome c oxidase subunit I (COI) gene, including the first molecular data available for Waisiuconcha. The finding of a single pliocardiine species at two seeps ~2,400 km apart adds evidence to members of the subfamily having high dispersal capabilities, expanding our knowledge of seep fauna in the northwestern Pacific Ocean.