Food spectrum of the Chinese mitten crab (Eriocheir sinensis): insights from the Lower River Rhine comparing stable isotope mixing models and genetic gut content analyses

Originating from South East Asia the catadromous Chinese mitten crab (Eriocheir sinensis H. Milne Edwards, 1853) is listed as one of the 100 world's worst invasive species. In 1912 E. sinensis was first recorded in Central Europe. Nowadays the species is common in various European rivers and widespread in large rivers of the northern hemisphere. High densities of the crab can cause serious economic damage in water management and to the fishing industry. During their natural migration from the estuary upstream, subadult individuals are able to reach river sections several hundred kilometers away from the delta. The ecological impacts on community and food web structures of the Chinese mitten crab (CMC) are less often considered in scientific studies than their economic effects. The diet of CMC under natural conditions is rarely studied, but macrophytes are known to be an important food source of the crab. However, in the main stretch of the Lower Rhine macrophytes are broadly missing. Initial stable isotope analyses showed that CMC occupies a trophic level comparable to carnivorous fish in this system. Thus, a strong predatory impact of CMC on the fauna is likely. Here we use qualitative and quantitative approaches to investigate the diet of Chinese mitten crab to estimate their impact on the food web. Bulk stable isotope analyses of 813C and 815N of E. sinensis and potential food resources were conducted and genetic gut content analyses (GGCA) via group-specific primers for common macroinvertebrates of the River Rhine were used to determine prey organisms. While GGCA only rarely detected invertebrate prey and many plant fibres were visible in the stomachs and guts of the CMCs, stable isotope mixing model (simmr) analyses revealed a high contribution of some easily accessible macroinvertebrate species to the CMCs diet. This contradiction between the gut content findings and the simmr mixing model results indicate not only that animal material is more easily assimilated, but also that the CMC may have a strong impact on the benthic fauna of the Lower Rhine, which would have been underestimated if solely gut content analyses were used.