Temporal variation in chemoautotrophic symbiont abundance in the thyasirid bivalve Thyasira cf. gouldi

Within the bivalve family Thyasiridae, symbioses with chemoautotrophic, sulphur-oxidizing bacteria do not occur in all lineages; variation in symbiont presence and in the degree of abfrontal expansion of gill filaments occurs on fine phylogenetic scales within the family. Thyasirid symbionts are harboured extracellularly and are periodically engulfed and digested by host gill epithelial cells. Symbiotic thyasirids are mixotrophic, retaining the capacity to feed on particulate matter; the relative importance of particulate feeding and symbiont-derived nutrition to host metabolism may vary on temporal and spatial scales depending on the abundance of particulate organic matter and sediment sulphide availability. Here, we demonstrate the existence of a temporal trend in symbiont abundance in Thyasira cf. gouldi from Bonne Bay, Newfoundland, Canada, over 18 months (2011 and 2012): symbiont abundance is highest during the months of autumn and is lowest in spring. The density of membrane whorls, lysosomal microbodies associated with the digestion of bacterial symbionts, does not display a similar temporal trend, indicating that relationships between the contribution of symbiont-derived nutrition and seasonal fluctuations in environmental parameters are less clear. Along with the extracellular status of thyasirid symbioses, their highly dynamic nature may explain the possible evolutionary loss or gain of chemosymbioses among some members of this group.