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 suspension-feeding bivalve Laternula marilina were investigated at 6 stations along environmental gradients in 2 brackish lagoons (Gamo and Idoura, Japan) using stable isotopic analyses. In the productive and marine-dominated Gamo lagoon, Hediste spp. (δ13C: -19.1 to -15.0‰) assimilated mainly benthic diatoms (-16.4‰) and autochthonous phytoplankton (-23.6‰). Dietary contribution of phytoplankton clearly increased in the central lagoon with increasing phytoplanktonic biomass (chlorophyll a: up to 110 μg l-1). In Gamo, L. marilina (δ13C: -18.7 to -17.8‰) mainly assimilated both marine particulate organic matter (POM) and resuspended benthic diatoms, while their low δ15N values suggested a contribution of 15N-depleted diets (e.g. N2-fixing cyanobacteria). Conversely, in the less productive and river-dominated Idoura lagoon, benthic diatoms and marine POM were only minor diets. Low δ13C 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 physiological characteristics that allow utilization of refractory organic substances (e.g. digestive enzymes). The low δ13C 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. © Inter-Research 2008.