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.

Effects of Long-Term Starvation on a Host Bivalve (Codakia orbicularis, Lucinidae) and Its Symbiont Population

“…There was also a similar symbiont decline in thyasirid species and bathymodiolines after starvation in seawater devoid of sulfide or while kept under controlled conditions along with particle feeding (20,40). The conditions in this study were more drastic for the host and its symbionts because no particle feeding was allowed for the host and no sulfide was available as an energy source for the symbionts.…”

“…Apart from the decrease in symbiont abundance suggested by transmission electron microscopy (TEM) analysis and a decrease in sulfur and protein content in the gill tissue of thyasirids (20,38,40), little is known about the physiological status of these symbionts and the changes undergone by the symbiotic population of starved bivalves. A previous study of the population was carried out under natural conditions with Codakia orbicularis, a chemoautotrophic bivalve.…”

Nutritional and reproductive strategies in a chemosymbiotic bivalve living in a tropical intertidal seagrass bed

“…Consequently, analysis of δ 13 C signatures of host tissues revealed that the relative importance of particulatederived nutrition in chemosymbiotic bivalves can vary be tween 2 and 74% (Cary et al 1989, Conway et al 1989, Dando & Spiro 1993, Rossi et al 2013. This flexible feeding mode may help chemosymbiotic bivalves survive in environments where the concentration of particulate food, sulphide and oxygen varies in space and/or in time (Dufour & Felbeck 2006, van Gils et al 2012, Rossi et al 2013.…”

An overview of chemosynthetic symbioses in bivalves from the North Atlantic and Mediterranean Sea

“…This has been demonstrated in vent and seep Mytilidae, in which relative amounts of sulfur-versus methane-oxidizing bacteria reflect the availability of their respective substrates, and can also display age-or time-related variations (Le Fiala-Médioni et al, 2002;Halary et al, 2008;Riou et al, 2008). Although they only have sulfur-oxidizing bacteria, Thyasiridae can also display inter-habitat variability in symbiont densities, with higher densities when sulfide is more abundant in the environment (Dufour and Felbeck, 2006). Using carbon stable isotopes ratios, Dando and Spiro (1993) have shown that the contribution of chemoautotrophic bacteria could vary inter-annually in Thyasira sarsi and T. equalis, in relation to environmental change in the habitat.…”

“…Symbiont chemoautotrophy is supported by the occurrence of APS reductase-encoding genes in symbionts of some species , and by the carbon stable isotope signatures of animal tissue that are in the range of values measured in chemosymbiotic metazoans harboring sulfur-oxidizing symbionts. Variability in symbiont abundances and the host nutritional strategy depends upon environmental conditions (presence of sulfide and particles), as shown in Thyasira flexuosa, T. sarsi and Parathyasira equalis, confirming their ability to withstand fluctuating environments (Dando and Spiro, 1993;Dufour and Felbeck, 2006). Symbiosis has been investigated in 9 identified species from coastal and deep sediment, including cold seeps, but molecular data on symbionts remain very scarce (Table 1).…”