CHEMOAUTOTROPHIC SYMBIOSIS IN THE TROPICAL CLAM SOLEMYA-OCCIDENTALIS (BIVALVIA, PROTOBRANCHIA) - ULTRASTRUCTURAL AND PHYLOGENETIC ANALYSIS

A symbiosis between chemoautotrophic bacteria and the bivalve host Sol emya occidentalis Deshayes living in Caribbean coral reef sediments is described. Transmission electron microscopy revealed dense population s of cellular inclusions resembling Gram-negative bacteria in host gil l filaments. The autotrophic potential of these bacteria was determine d by DNA hybridization analyses, which indicated the Calvin-Benson cyc le gene for ribulose-1,5-bisphosphate carboxylase/oxygenase was presen t in DNA recovered from S. occidentalis gill but not in symbiont-free foot tissue. The bacterial gene coding for 16S ribosomal RNA was ampli fied from DNA isolated from clam gills, sequenced directly, and found to be unique to S. occidentalis. Phylogenetic analysis of this that th e S. occidentalis symbiont other known sulfur-oxidizing, chemoautotrop hic symbionts within the gamma-subdivision of the Proteobacteria. Its closest known relative is the Solemya velum symbiont. While bootstrap analyses consistently place the symbionts from the Atlantic species S. velum and S. occidentalis in a monophyletic group, their phylogenetic position relative to the symbiont of the Pacific host, S. Reidi, and other chemoautotrophic symbionts remains unresolved.