Trophic analysis of two subtropical South American freshwater crabs using stable isotope ratios

Crustaceans with crayfish- and crab-morphologies do not co-occur often. However, the crab families Aeglidae (crayfish morphology) and Trichodactylidae (crab morphology) are sympatric in many subtropical South American streams. We investigated the trophic status of Aegla uruguayana (Aeglidae) and Trichodactylus panoplus (Trichodactylidae) in a South American subtropical piedmont river (Cuareim River, Uruguay) using delta C-13 and delta N-15 ratios. We estimated the relative importance of prey items using a five-source mixing model. Stable isotope analysis revealed that the two crabs have different trophic niches. Three fractionation rates (-1, 0, and +1 aEuro degrees) influenced the estimated assimilation (%) of prey items to consumers. However, the relative importance of prey items was unaffected. A. uruguayana showed an ontogenetic shift from herbivore-detritivore to omnivore. Shared morphology between crayfishes and aeglids likely facilitates similar trophic roles; however, A. uruguayana occupies a much lower trophic position than is typical for crayfishes. T. panoplus is a strict herbivore-detritivore. In contrast to tropical crabs, they do not engage in carnivory or exploit terrestrial subsidies. In subtropical South American streams, aeglids may be the functional equivalent of crayfishes, whereas trichodactylids may fill a trophic role atypical for freshwater crabs.