There is more than one 'killer shrimp': trophic positions and predatory abilities of invasive amphipods of Ponto-Caspian origin

1. Biological invasions are regarded as one of the greatest threats to biological diversity. One of the macroinvertebrate groups with the largest number of invasive species in fresh water are gammarid amphipods. Their omnivorous (including predatory) feeding behaviour may facilitate their spread and establishment in new areas. 2. Dikerogammarus villosus, the killer shrimp', is a well-known example of a Ponto-Caspian gammarid that is a very effective predator and successful coloniser in Europe. There are, however, other invasive Ponto-Caspian amphipods, which have spread successfully in Northern, Central and Western Europe. Our aim here was to test whether two of such invaders (Pontogammarus robustoides and Dikerogammarus haemobaphes) are also more predacious than a native species (Gammarus fossarum). 3. Stable isotope analysis (15N and 13C) of Ponto-Caspian amphipods coexisting in a reservoir demonstrated that the trophic positions of P.robustoides and D.haemobaphes were similar to that of D.villosus. Echinogammarus ischnus and Chelicorophium curvispinum occupied the lowest position in the food web, while the native Gammarus fossarum (collected from another waterbody) had an intermediate trophic position. 4. Stomach content analysis of P.robustoides, D.haemobaphes and G.fossarum collected in the field, as well as laboratory feeding experiments, was used to compare diet and feeding preferences among the two invasive and one native species. All three species were omnivorous and predacious. However, the two invasive species (P.robustoides and D.haemobaphes) were more effective predators than G.fossarum and showed a clear preference for animal prey and tissue. 5. Pontogammarus robustoides and D.haemobaphes may, like D.villosus, also be called killer shrimps' and could have a similar impact as invaders of European freshwater and brackish waterbodies.