Stable isotope signature and pigment biomarker evidence of the diet sources of Gaetice depressus (Crustacea: Eubrachyura: Varunidae) in a boulder shore ecosystem

Gaetice depressus, a varunid crab common on intertidal boulder shores, is a potential key organism for monitoring organic matter flow through the food web. In order to elucidate its biogeochemical role, the diet source and trophic position of this crab on the boulder shore of an island off the Izu Peninsula, Japan, were estimated using three approaches: foregut content examination, stable isotope signature (S13C and S15N) and pigment biomarkers. The results suggest that G. depressus utilizes green macroalgae (Ulva sp., Ulvaria sp.) as its main diet source together with red macroalgae (Gloiopeltis complanata, Gl. furcata). This crab also utilizes periphytic and planktonic microau-totrophs (occasionally tissues of heterotrophs) when macroalgae prove insufficient due to seasonal change. Therefore, G. depressus can be considered to be an omnivore since it consumes both autotrophs and heterotrophs, although it obtains organic matter mostly from autotrophs.