| Summary: | A new species of Thyasira (T. methanophila n. sp.) is described from a methane seepage area off Concepción, Chile (~36ºS) at bathyal depths. This species shows affinity with other southern ocean species placed in the subgenus Maorithyas. The gross anatomy of the type species of Maorithyas, M. marama Fleming, 1950, is described for the first time. There are no gross anatomical differences in the ctendia, foot and gut, between the above taxa, which suggest that there are no differences anatomically between species living at seeps and species living in other environments. Furthermore, wider comparison of the shells suggests that the subgenus Maorithyas cannot be supported, as the differential characters grade into those of Thyasira. Scanning electron microscopy of the ctenidium, of T. methanophila, revealed filaments with closely packed bacteriocytes each with dense aggregations of small rod shaped bacteria each about 0.5 mm in length. The bivalve is assumed to be highly dependent on the symbiotic relationship. A second thyasirid was recorded from worn valves only and is assigned to the genus Conchocele. The material is too fragmentary to describe but was compared with C. bisecta and C. novaeguinensis.
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