Symbiont abundance in thyasirids (Bivalvia) is related to particulate food and sulphide availability

Abstract

Many bivalve species with chemoautotrophic symbionts have mixotrophic diets and some of their nutritional requirements are met by particulate feeding. The symbionts require reduced compounds (such as sulphide) for their autotrophic production. As the concentration of both particulate food and sulphide can vary in their habitat, it has been suggested that symbiont numbers may vary in response. To address whether symbiont abundance can be influenced by the external medium, we compared symbiont abundance (1) in specimens kept with or without particulate food, and (2) in specimens kept in sediments with low or high dissolved sulphide content. The relative surface area occupied by symbionts on semi-thin gill sections was determined for Thyasira flexuosa (and, in the sulfide experiment, for Parathyasira equalis and T. sarsi) kept for up to 3 wk in experimental microcosms, where the quantity of particulate food or dissolved sulphide was manipulated. Symbiont abundance declined with time; the decline was greater in particle-starved thyasirids and in specimens kept in low sulphide. In the latter conditions, T. flexuosa may have also had a greater symbiont digestion rate. The observed patterns may be due to a combination of increased symbiont uptake by the bivalve host, and reduced symbiont vigour under the imposed conditions. The flexible feeding mode of chemosymbiotic thyasirids may help them to survive in fluctuating environments. © Inter-Research 2006.