Functional equivalence across deep evolutionary divergence in a cryptic amphipod species complex

Abstract Cryptic species are genetically distinct yet morphologically similar, raising the critical question of whether they perform equivalent ecological roles. We tested this using the freshwater amphipod Gammarus roeselii , a species complex with a diversification hotspot in the Balkan Peninsula and a widespread lineage in Central Europe. We followed a three-stage experimental approach, by analysing gut contents from 35 populations (five cryptic lineages), quantifying leaf-litter decomposition in laboratory trials, and validating shredder activity in situ . Our results reveal that while minor dietary variations exist—driven primarily by local land use—detritus remains the dominant food source across all lineages. These findings demonstrate that identical morphological traits suggest functional equivalence among lineages in allopatry, despite deep genetic divergence. However, because our study focussed on sites with single highly divergent genetic lineages, the observed niche overlap implies that secondary contact would likely trigger intense competition. While evidence from other taxa suggests that syntopy can drive character displacement, it remains to be seen if G. roeselii follows this pattern. Investigating rare contact zones will be essential to determine whether these lineages maintain functional redundancy or undergo niche differentiation upon coexistence, providing a critical next step in understanding the evolutionary ecology of cryptic complexes.