Gill Symbionts in Thyasirids and Other Bivalve Molluscs

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.