Allometry of gill weights, gill surface areas, and foot biomass δ13C values of the chemoautotroph–bivalve symbiosis Solemya velum

Abstract

The protobranch bivalve Solemya velum Say, 1822 has large gills, which harbor chemolithoautotrophic bacteria that supply the majority of the clam’s organic carbon. A substantial portion of the CO2, O2, H2S, and other nutrients necessary for symbiont autotrophy and host heterotrophy are acquired from the environment through the gills, whose large size may be necessary to facilitate the acquisition of sufficient O2 from S. velum’s habitat to meet the combined demands of the host and symbionts. Large gills may also result in an oversupply of CO2, which may in turn be responsible for the isotopically depleted δ13C values observed in S. velum biomass (−31 to −34‰). Alternatively, gill hypertrophy may simply be an adaptation to house a large population of symbionts adjacent to their environmental source of dissolved gases and other nutrients. To better understand gill function in this symbiosis, gill weights, gill surface areas, and foot δ13C values were measured as a function of total weights. S. velum gill weights were found to be a substantial portion of total clam weight, averaging 38% of wet weight, compared to nonsymbiotic protobranch bivalves Yoldia limatula Say, 1831 (5%) and Nucula proxima Say, 1822 (11%). Gill weights are a smaller percentage of total weight in larger individuals; the allometric equation for gill weight (G) as a function of total weight (M) is G=0.26M0.85. Dry weights scale similarly. Gill surface areas are immense; the average gill surface area measured was 107 cm2 g−1 total soft tissue wet weight, the highest value for any marine invertebrate. Gill surface area (SA) also scales with size (SA=69.8M0.85). When gill surface areas were calculated with respect to gill wet weights, they did not scale with size. The δ13C values do not scale with size either, consistent with high rates of CO2 supply at all sizes. Extraordinarily high rates of CO2 supply relative to demand are supported by a model for CO2 delivery based on Fick’s law and the allometric relationship between surface areas and total weight, consistent with a role for large gill surface areas in the generation of isotopically depleted tissue δ13C values.