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Juveniles of bivalves Mactra veneriformis and Ruditapes philippinarum, and ghost shrimps Nihonotrypaea japonica and N. harmandi were reared on a microalga of a constant isotopic value to quantify their diet-tissue isotopic fractionation. The weights of the animals increased by >7-fold, resulting in isotopic equilibria with their diet. Fractionation for bivalve soft tissues was 0.6 to 0.9‰ for carbon and 3.4 to 3.6‰ for nitrogen, which fell within the range of the currently accepted fractionation values (0 to 1 and 3 to 4‰). Examinations of acid-treated or untreated whole body, mus- cle and exoskeleton of the ghost shrimps showed (1) large variations in δ 13 C for untreated exoskele- tons, (2) reduced δ 13 C for acid-treated exoskeletons by 3.5 to 6.2‰, (3) confined ranges in 13 C and 15 N fractionations for muscles (2.0 to 2.2 and 3.6 to 4.0‰), (4) only slight effects of acid treatment on 13 C and 15 N fractionation for muscles (≤0.3‰ differences), (5) a significant difference in 13 C fractionation for acid-treated whole bodies between N. japonica (-0.3‰) and N. harmandi (-1.7‰), and (6) 2.3 to 3.0‰ of 15 N fractionation for whole bodies, which were smaller than for muscles due to negative frac- tionation for exoskeletons (-3.0 to -1.9‰). These findings suggest that carbonates in exoskeletons should be removed by acid and that muscle is the most appropriate tissue for isotopic analysis. Although 15 N fractionation for ghost-shrimp muscle was within the above-mentioned accepted range, 13 C fractionation was outside this range. The present study highlights that fractionation is species- and tissue-specific, and that the accepted fractionation values may not be universally applicable.

To delineate a soft-shore trophic structure, macrobenthos was collected from an intertidal sandflat at the mouth of the Shirakawa River in the central part of Ariake Sound, Kyushu, Japan, on 3 occasions in 2003 (January, March, and June to July during the rainy season), and the isotopic compositions of animals and their potential food sources were analyzed. Food sources of animals were assessed based on the diet - tissue isotopic fractionation (δ 13 C = 0.6 to 2.0‰, δ 15 N = 3.4 to 3.9‰) of 3 main constituent species (the bivalves Mactra veneriformis and Ruditapes philippi- narum and the ghost shrimp Nihonotrypaea japonica), which had been determined by an earlier laboratory experiment. The results revealed that (1) consumers were classified into 3 groups; a group consisting of 51 species including 4 main species (the above-mentioned species and the mud shrimp Upogebia major) that derived nutrition mainly from coastal phytoplankton, one consisting of 7 species that incorporated more 13 C-enriched material (e.g. benthic microalgae) and a group con- sisting of 4 species that incorporated more 13 C-reduced material (e.g. riverine organic matter); (2) the phytoplankton-based trophic structure was composed of 3 trophic levels; (3) M. veneriformis juve- niles collected during the rainy season appeared to incorporate riverine organic matter; and (4) reeds from the flood plain, detrital terrestrial plant material, sewage effluents and seaweeds were not food sources for most consumers. We concluded that the high phytoplankton biomass and scarcity of benthic microalgae on this sandflat account for its phytoplankton-based trophic structure.