Feeding strategies in symbiotic and asymbiotic thyasirid bivalves

Abstract

Many bivalves living in symbiosis with chemoautotrophic bacteria are mixotrophs, deriving nutrients both from their symbionts and from external sources. The bivalve family Thyasiridae contains symbiotic and asymbiotic species that live infaunally and presumably suspension-feed. Symbiotic thyasirids use their foot to construct numerous elongate and ramified burrows, or pedal tracts, in surrounding sediments; pedal tracts were interpreted as conduits for accessing reduced sulfur required by gill-associated symbionts. Recently, some asymbiotic thyasirids were found to form similar burrows, and this behaviour, accompanied by the irrigation of pedal tracts, was thought to be associated with deposit feeding. We used stable isotope analyses to examine the food sources of three taxa of co-occurring symbiotic and asymbiotic thyasirids sampled from a fjord in Newfoundland, Canada at different dates and sampling sites. We considered suspended particulate organic matter, sedimentary organic matter, and sulfur-oxidizing bacteria as potential food sources, and found that sediment organic matter characteristics varied significantly according to site but not sampling date, and that suspended organic matter near the seafloor was of relatively low quality and abundance at all dates and sites. The non-gill tissues of symbiotic and asymbiotic thyasirids had similar carbon isotopic compositions, but nitrogen isotope signatures were heavier and less variable in asymbiotic taxa than in the symbiotic Thyasira cf. gouldi. The two asymbiotic thyasirid species studied herein appeared to vary in their feeding strategies, with Parathyasira appearing to mostly farm sulfur-oxidizing bacteria along burrow linings and then collect them using their foot (a mode of bivalve deposit-feeding known as pedal feeding). This work expands our understanding of the breadth of feeding strategies in the widespread and often abundant thyasirid bivalves, and provides support for microbial farming along burrow linings in some thyasirids.