Dynamics of symbiont populations in the facultativechemosymbioses of thyasirid bivalves
Within the family Thyasiridae, symbioses between hosts and sulphur-oxidizing bacteria are not present in all species. Bacteria that associate with thyasirids are extracellular, digested by host cells, and presumed to be facultatively symbiotic. Here, the dynamic and unstable nature of thyasirid-symbiont associations is characterized. We first describe a Thyasira cf. gouldi cryptic species complex in which some species possess symbionts, while others have lost them. Within symbiotic T. cf. gouldi species, both symbiont abundance and the nutritional importance of symbiont-derived nutrients fluctuate over yearly cycles. T. cf. gouldi symbionts are environmentally acquired, and association with host bivalves appears to result in the loss of both flagella and magnetosome chains. Finally, we show that hosts may control symbiont populations by mediating their access to reduced sulphur: exposing thyasirids to a constant source of reduced sulphur results in increased bacterial division which can result in host cell overgrowth and host mortality.