An ecological study of two species of chemosymbiotrophic bivalve molluscs (Bivalvia: Vesicomyidae: Pliocardiinae) from the Deryugin Basin of the Sea of Okhotsk using analyses of the stable isotope ratios and fatty acid compositions

Pliocardiines (Bivalvia: Vesicomyidae: Pliocardiinae) are symbiotrophic vesicomyid molluscs harbouring symbiotic sulphide-oxidizing chemoautotrophic bacteria in their gills. The pliocardiines, Phreagena soyoae and Archivesica gigas, are a common component of numerous deep-sea chemosynthesis-based communities in the Eastern and Western North Pacific. We examined the stable isotope composition of carbon, nitrogen and sulphur as well as the fatty acid composition of P. soyoae and A. gigas from two methane seep sites, Clam Hill (1427 – 1443 m) and Barite Ridge (1455 – 1548 m) in the Deryugin Basin in the Sea of Okhotsk. For identification of bacterial endosymbiont lineages, we determined the nucleotide sequences of COI genes of symbionts of P. soyoae and A. gigas and confirmed that both species have the same bacterial lineages as their Eastern Pacific counterpart populations. Stable isotope analysis showed that site-specific differences between pliocardiines from the two seeps of the Deryugin Basin far exceeded interspecies variations. There were obvious differences in the sulphur isotopic compositions among pliocardiines from Clam Hill and from Barite Ridge suggesting that the intensity of anaerobic methane oxidation and methane flow varied in different cold seeps from the Deryugin Basin. The δ13C values recorded for P. soyoae and A. gigas samples were in the narrow range typical for studied pliocardiines. Both species showed an unusually broad range of δ15N values (∼18‰) that overlapped the known range of this signature for pliocardiines. The main polyunsaturated fatty acids in P. soyoae and A. gigas were nonmethylene-interrupted 20:2(5,13), 20:3(5,13,16) and 20:4(5,13,16,19), which were synthesized by the molluscs themselves and not by their symbionts.