Crabs assimilating cellulose materials drive the detritus food chain in a mangrove estuary

Abstract

The low digestibility of cellulose-rich detritus in primary consumers has been considered to limit its incorporation by higher level consumers. A recent study revealed, however, that the sesarmid crab Parasesarma bidens, which is a dominant benthic consumer in East Asian mangrove forests, possesses high cellulase enzyme activity enabling the species to efficiently digest and assimilate refractory detritus. To elucidate whether P. bidens acts as a trophic intermediate facilitating the incorporation of mangrove detritus into the food chain, the food habits of benthivorous fishes within a subtropical mangrove estuary were investigated. Stomach content analyses indicated that crabs, especially P. bidens, made a large contribution to the diets of the fishes examined, including the snapper Lutjanus argentimaculatus and the seabream Acanthopagrus sivicolus, which were both common at the study site. Furthermore, carbon and nitrogen stable isotope analyses revealed that mangrove detritus contributed significantly to all fish species examined, due to their consumption of P. bidens. These findings suggested that the detrital trophic pathway, via crabs with greater ability to digest/assimilate cellulose materials, plays an integral part in maintaining food chain structures in mangrove ecosystems.