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.

Procaryotes on the tentacles of deep‐sea holothurians: A novel form of dietary supplementation

“…Internally the bacteria are characterized by DNA threads, membrane-bound vesicles, and small electron-dense bodies (Fig. 2D), similar to polyphosphate granules reported elsewhere (Southward 1986).…”

Magnetosome-containing bacteria living as symbionts of bivalves

“…gouldi indicate that host cells can engulf and digest symbionts, a likely means of nutrient uptake in thyasirids (Southward, 1986;Dufour, 2005). Through this process, magnetic particles appear resistant to host degradation, concentrating in whorls and in the cytoplasm of epithelial cells.…”

“…The sulfur-oxidizing symbionts of thyasirid bivalves are among the few examples of chemosymbionts that are likely facultative (that is, capable of living both in hosts and in the outside environment), given that they are located outside rather than inside host gill cells (Southward, 1986;Dufour, 2005) and that, in at least one species (Thyasira n. sp. Guiness), host individuals can associate with different symbiont phylotypes, suggesting acquisition from surrounding sediments (Duperron et al, 2012a).…”

“…gouldi from Bonne Bay were recently described as forming a cryptic complex in which there are two symbiotic operational taxonomic units (OTUs 1 and 2) and an asymbiotic OTU (3) of bivalves, which vary slightly in shell shape (Batstone et al, 2014). Similar to other thyasirids (Southward, 1986;Dufour, 2005), the symbionts of T. cf. gouldi OTUs 1 and 2 are maintained at the surface of gill epithelial cells within extracellular 'pockets' bounded by cytoplasmic extensions of host cells that often bear microvilli (Batstone et al, 2014).…”

“…gouldi OTUs 1 and 2 are maintained at the surface of gill epithelial cells within extracellular 'pockets' bounded by cytoplasmic extensions of host cells that often bear microvilli (Batstone et al, 2014). Symbiotic thyasirids meet part of their nutritional requirements by assimilating carbon fixed by symbionts (Dando and Spiro, 1993), and electron micrographs show that symbionts are periodically engulfed within gill epithelial cells, a process that results in whorls of lysed bacterial products in host cells (Southward, 1986;Dufour, 2005). As thyasirid symbionts are held among gill epithelial cells, they likely benefit from host bioirrigation and sulfurmining behaviors, which give them access to oxygen and reduced sulfur (Dufour and Felbeck, 2003;Dando et al, 2004).…”