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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 philippinarum 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 consisting 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 juveniles 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.KEY WORDS: Stable isotopes · Estuary · Intertidal sandflat · Trophic structure · Phytoplankton · Benthic microalgae Resale or republication not permitted without written consent of the publisher

Three species of the callianassid genus Nihonotrypaea occur in the Ariake Sound estuarine system, southern Japan; they consist of two tidal-flat species (N. harmandi; N. japonica) and one boulder-beach species (N. petalura), with maximum population densities of 1440, 343, and 12 ind m−2, respectively. Nihonotrypaea harmandi and N. petalura are distributed along the coastline from the outermost part of the sound to the open sea, while N. japonica occurs in the middle part of the sound. Nihonotrypaea japonica has an extended reproductive period from late winter to autumn, while those of the other species are from late spring or summer to autumn. Interspecific comparisons were made for recently laid egg size (as volume) and clutch size (as number of eggs per female). Only in N. japonica was a seasonal egg size variation observed, being significantly larger in winter to spring (mean=0.106 mm3) than in summer (0.080 mm3). By contrast, clutch size was significantly smaller in winter to spring, resulting in nearly the same clutch volume per female (product of the mean egg volume and clutch size) between the seasons. Among the three species, the egg size was ordered as N. japonica (overall mean volume through the seasons=0.092 mm3)>>N. petalura (0.057 mm3)>N. harmandi (0.054 mm3). The clutch size was ordered as N. harmandi>N. petalura≈N. japonica. The clutch volume was ordered as N. japonica≈N. harmandi>N. petalura. The smallest clutch volume value for N. petalura female showed an opposite trend to the relative size of the major cheliped found in a previous study.

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