Mouthparts of the ponto-caspian invader Dikerogammarus villosus (Amphipoda: Pontogammaridae)

The pontogammarid amphipod Dikerogammarus villosus, originally a Ponto-Caspian faunal element, has, in the recent 15-20 years, successfully invaded various aquatic systems in Europe including Lake Constance. In these rivers and lakes it had and still has severe ecological impact on native macro-invertebrates, often eliminating the native and earlier established gammaridean species. In order to test the hypothesis that the mode of food acquisition of D. villosus is of significance for this phenomenon, we focused on the mouthparts of D. villosus, i.e., mandibles, the two pairs of maxillae and the maxillipeds using SEM. Contrary to expectations, provoked by field and laboratory observations, the results of this study show that the mouthparts of D. villosus are not highly specialized just for carnivory and predation. Indeed, the stout mandibles, with their well-developed incisors enable to kill even prey with robust integument, but other modes of feeding are possible. On the maxillulae, maxillae, and maxillipeds we found setae that can be used, together with the gnathopods and the antennae, for filtering suspended algae and other small particles from the respiration current. The same structures are involved in collecting detritus. In contrast, D. villosus does not possess any specific tools for scraping periphyton from the substrate. Feeding on macrophytes may be possibly but not very effective because the surfaces of the molars are not well suited for grinding such plant material. It is shown that D. villosus is neither I shredder, as traditionally predicated for most gammarideans, nor is it a specialized carnivore, as predation experiments proposed, but rather unspecialized. Its ability to be carnivorous and to use a wide spectrum of other food may be an important reason for the success of this invader, being an advantage compared to mainly herbivorous gammarideans, which have been eliminated in many places by D. villosus.