Two Vesicomyid Species from Cold Seeps in the Nankai Trough and Sagami Bay, Including a New Species of Isorropodon

Abstract

Vesicomyid clams are commonly found in deep-sea chemosynthetic ecosystems and species in the subfamily Pliocardiinae harbour endosymbiotic bacteria in their gills. We collected two pliocardiine species from three cold seeps off Japan, including two in the Nankai Trough (Off Cape Muroto 4,600 m site and the Ryuyo Canyon) and one in the Sagami Bay (Off Hatsushima). Isorropodon fragilis n. sp. is newly described from the Off Cape Muroto 4,600 m site in the Nankai Trough. This new species can be distinguished from other related species by having a large shell (up to 45.2 mm) with the beak located anteriorly at one-third of shell length, a bluntly pointed posterior margin, an only slightly inflated umbo, an inner ridge from the beak to the posterior-ventral corner and a lunule not demarcated by any incised line. "Vesicomya" kaikoae (Okutani, Fujikura & Kojima, 2000) was recovered from the Ryuyo Canyon in the Nankai Trough at 1,099 m and the Off Hatsushima seep in Sagami Bay at 856 m. The hinge of this species is similar to that of Isorropodon. Eleven specimens of "Vesicomya" kaikoae from the Ryuyo Canyon and one specimen of "V." kaikoae from the Off Hatsushima seep were drilled by naticids and muricids. Among these, the Ryuyo Canyon record represents the deepest occurrence of predatory drilling on vesicomyids.