Spatial dietary variations in Laternula marilina (Bivalva) and Hediste spp. (Polychaeta) along environmental gradients in two brackish lagoons

Abstract

Spatial dietary variations in deposit feeding polychaetes Hediste spp. and the suspen- sion-feeding bivalve Laternula marilina were investigated at 6 stations along environmental gradi- ents in 2 brackish lagoons (Gamo and Idoura, Japan) using stable isotopic analyses. In the productive and marine-dominated Gamo lagoon, Hediste spp. (δ 13 C: -19.1 to -15.0 ‰) assimilated mainly ben- thic diatoms (-16.4 ‰) and autochthonous phytoplankton (-23.6 ‰). Dietary contribution of phyto- plankton clearly increased in the central lagoon with increasing phytoplanktonic biomass (chloro- phyll a: up to 110 µg l -1 ). In Gamo, L. marilina (δ 13 C: -18.7 to -17.8‰) mainly assimilated both marine particulate organic matter (POM) and resuspended benthic diatoms, while their low δ 15 N values sug- gested a contribution of 15 N-depleted diets (e.g. N2-fixing cyanobacteria). Conversely, in the less pro- ductive and river-dominated Idoura lagoon, benthic diatoms and marine POM were only minor diets. Low δ 13 C values for Hediste spp. (-26.1 to -23.2 ‰) and L. marilina (-27.6 to -25.0 ‰) indicated that terrestrial and riverine materials provided the primary sources of carbon. In each lagoon, the stable isotopic signatures of L. marilina differed distinctly from those of the sympatric suspension-feeding bivalves Ruditapes philippinarum and Crassostrea gigas. This suggests that L. marilina has physio- logical characteristics that allow utilization of refractory organic substances (e.g. digestive enzymes). The low δ 13 C values of Hediste spp. in Idoura also imply direct assimilation of plant detritus using cellulase. Dietary plasticity of the consumers may allow them to gain energy in brackish waters where indigestible terrestrial detritus is the major source of carbon.