Morphology Mirrors Trophic Niche in a Freshwater Amphipod Community

Amphipod crustaceans are ubiquitous in temperate freshwaters, where they radiated multiple times in a broad range of habitats. This has led to significant morphological and ecological disparity. However, a limited knowledge of functional morphology and its relation to ecology has often led to the view that freshwater amphipods are omnivorous shredders. Herein, we hypothesise that trophic differentiation is related to functional morphology in a sympatric assemblage of three species belonging to three widespread families: Crangonyctidae, Gammaridae, and Niphargidae. We quantified seasonal diet composition by examining gut contents, and measured 42 functional morphological traits. Our results reveal a clear trophic and morphological differentiation among these sympatric species. The carnivorous Niphargus valachicus exhibited predatory traits exemplified by a smaller molar surface and stomach length, and larger/longer grasping, locomotory, and sensory appendages in comparison to the herbivorous Gammarus dacicus. The third species, Synurella ambulans, was omnivorous given its intermediate diet and morphology. We did not find consistent evidence for seasonal diet variation, except for the summer and winter diet of G. dacicus. The observed niches stretched along a broad trophic spectrum, indicating that even small riverine amphipod communities can exhibit significant functional disparity. The apparent connection between morphology and diet is similar to that observed in other crustaceans, strengthening our mechanistic understanding of the niche-morphology link. However, the generality of these conclusions needs to be tested on larger species datasets.