Food sources for three species of Nihonotrypaea (Decapoda: Thalassinidea: Callianassidae) from western Kyushu, Japan, as determined by carbon and nitrogen stable isotope analysis

Three species of the callianassid genus Nihonotrypaea occur intertidally in the Ariake Sound estuarine system, southern Japan; they consist of two sandflat species (N. japonica; N. harmandi) and one boulder-beach species (N. petalura). Nihonotrypaea harmandi and N. petalura are distributed along the coastline in the relatively oligotrophic sea area extending from the outermost part of the sound to the open sea, while N. japonica occurs in the more eutrophic area situated at the middle part of the sound. The trophic conditions of the two areas affect the abundance of phytoplankton in the water column relative to that of benthic microalgae in the sediment. The carbon and nitrogen stable isotope compositions of N. japonica and its potential food sources have been analysed in an earlier study, specifying phytoplankton as the exclusive food source. Potential food sources for N. harmandi and N. petalura were analysed to make interspecific comparisons of the assimilated diets in relation to the shrimp habitat characteristics. Food sources of the shrimps were assessed based on the diet-tissue isotopic fractionation of N. harmandi and N. japonica (δ13C = 2.0‰, δ15N = 4.0‰), which had been determined by an earlier laboratory feeding experiment. Of several potential food sources for N. harmandi and N. petalura, riverine organic matter, sewage effluents, live/detrital terrestrial plants, and seagrass were not food sources. For N. petalura, live seaweed, in particular Sargassum spp., growing on boulders and cobbles during the seaweed high-growth season, and seaweed-derived detritus buried in the sediment and live Enteromorpha compressa during the seaweed low-growth season were the most likely food sources. For N. harmandi, phytoplankton (or fresh phytoplankton-derived detritus) and benthic microalgae constituted the most likely food sources. The δ13C value of the estimated diet for N. harmandi was higher than that for N. japonica by 0.6‰, while the δ15N value for N. harmandi was lower by 2.6‰. The food sources for the three species of Nihonotrypaea were species-specific, depending on each habitat characteristics.