Ultrastructure of the gill of the hydrothermal vent bivalveCalyptogena magnifica,with a discussion of its nutrition

Calyptogena magnifica Boss and Turner, 1980, a new Vesicomyidae found during the Galápagos expedition in hydrothermal vents of the East Pacific Rise, was collected in the same Rise at 21°N during the Oasis expedition (March 1982), and samples of the gill were fixed for ultrastructural observations. The large size and structure of the gill indicate that this is the organ mainly involved in the nutritional processes ofC. magnifica. Despite the classic structural appearance of the external cilia of its gill, and an obvious production of mucus,C. magnifica is not a filter-feeder, as it does not use filtering processes to provide its major source of nutrition. Negligible particulate transfer is evidenced by reduction of the ciliary groove, of the labial palps and of the digestive tube, as well as by the absence of mucous strings. Histological and ultrastructural observations endorse the hypothesis that endocellular chemoautotrophic bacteria play an important role in the nutrition of the clam. Except for a superficial zone of ciliated cells, most of the gill tissue is comprised of cells which appear to be bacteriocytes, and which are perfectly integrated into the gill tissue and contain abundant and normally reproducing bacteria. The differences observed in the structure of the bacteriocytes suggest a cyclic process of their colonization by bacteria, their possible resorption, and their replacement by new bacteria-infected cells. Energetic substrates (sulfides and organic molecules) are probably directly absorbed by the bacteriocytes through the microvilli of the epithelial cells. Abundant fingerprint-like mitochondria in ciliate cells attest to a particularly high metabolic activity, perhaps related to active biosynthesis.