Gill Symbionts in Thyasirids and Other Bivalve Molluscs

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Abstract
Prokaryote organisms have been found in the gills of six species of Thyasiridae, collected in the north-east Atlantic region, from depths of 15 to 1250 m. The fine structure of the gill epithelia and the symbiotic Gram-negative bacteria is described. The bacteria occur extracellularly, between a thin cuticle and the apical membrane of the host cell. This differs from the intracellular position of bacterial symbionts in two lucinid species, Lucinoma borealis and Myrtea spinifera, and many other bivalves already known to harbour symbionts in their gills. Four species of Thyasiridae and Diplodonta rotundata, a member of the closely related family Ungulinidae, were found to lack gill symbionts.