Invasion is accompanied by dietary contraction in Ponto-Caspian amphipods

A species expansion beyond the native range is often assumed to be associated with an increased dietary niche breadth. However, empirical evidence remains limited due to a scarcity of studies comparing both the parental and invaded ranges. Here, we test the trophic niche expansion hypothesis by examining stable isotopes and functional morphology across native (NW Black Sea) and invaded (SE Baltic Sea) ranges of two amphipods, Dikerogammarus villosus and Pontogammarus robustoides, originating from the Ponto-Caspian region - a major source of species invading Holarctic inland waters. Stable isotopes revealed that both species underwent a twofold contraction of the dietary niche with a shift towards decreased carnivory in the invaded range. This dietary shift was morphologically mirrored by an overall reduction of prey grasping appendages, antennae, and mouthpart palps. The magnitude of dietary and morphological change was greater in D. villosus. Our findings indicate that previous experimental reports of aggressive predation in D. villosus reflect opportunistic foraging and align with local stable isotope studies which generally indicate a low trophic position. We conclude that Ponto-Caspian species can undergo rapid, if non-intuitive, changes in both diet and functional morphology outside the native range, likely contributing to their invasive potential.