Two new species of Thyasira Lamarck, 1818 (Bivalvia: Thyasiridae) from serpentinite seeps on the Mariana Forearc

ABSTRACT Members of the genus Thyasira host chemoautotrophic bacteria in their gills and are commonly found in hydrocarbon seeps and hydrothermal vents. Here, we report two new species of Thyasira from serpentinite seeps on Mariana Forearc mud volcanoes. One new species, Thyasira chamorroensis n. sp., is described from three articulated specimens recovered from the Summit Site of the South Chamorro Seamount (2915 m). The other new species, Thyasira tesoru n. sp., was collected from the Big Blue seep on the Asùt Tesoru Seamount (1240 m); it is described based on one articulated specimen and 15 disarticulated valves. The finding of two new Thyasira species from serpentinite seeps highlight the uniqueness of the biological community at this previously much-overlooked type of chemosynthetic habitat. These represent the first records of the genus Thyasira from the Mariana region, despite previous surveys of many hydrothermal vent systems there.