A new species of Axinus (Bivalvia: Thyasiroidea) from the Baby Bare Seamount, Cascadia Basin, NE Pacific with a description of the anatomy

A second and new species of living thyasirid is added to the genus Axinus. A. cascadiensis n. sp. was collected from the hydrothermal spring site at Baby Bare Seamount, Cascadia Basin, NE Pacific Ocean at a depth of 2592m. The anatomy is described in detail and compared with the Atlantic species, A. grandis. A. cascadiensis is shown to have highly modifled gills that support dense aggregations of bacteriocytes suggesting that this species is highly dependant on the bacterial symbiosis. The mantle is highly modifled with a series of partitions and folds that are proposed to function to direct and control the ciliary currents entering and traversing the mantle cavity. The ecological setting is described and thought not to support typical hot vent or cold seep communities due to low levels of sulphides and hydrocarbons. The highly sporadic distribution of both species of Axinus suggests that the genus is localised to small-scale environments, the details of which are not understood. Despite the appearance of fossil Axinus in the Eocene, modern species display the most highly modifled anatomy of any extant thyasirid, suggesting that the Thyasiroidea may be much earlier in origin.