Native crab and crayfish co-occurrence: First evidence in Europe

Cases of co-occurrence among freshwater decapods are few. In their European range, the crab Potamon fluviatile and the white-clawed crayfish Austropotamobius pallipes complex do not usually share the same stream section or pond even if they live in sympatry, suggesting a strong segregation of the two species. The coexistence was hypothesized during the Pleistocene era, but without evidence of recent contact between them. The co-occurrence of these two species (i.e., occupying a common area) is reported in a stream of the Central Italy, being the first case in Italy and in Europe. To assess this co-occurrence, in 2015 we investigated three transects (only crabs, only crayfish and both species), collecting biotic and abiotic data. The three transects were similar for canopy cover, substrate composition and density of potential shelters (but not for the abiotic parameters), and characterized by a good water quality and abundance of preys, being suitable for the two species. No agonistic behavioural patterns have been observed, leading to hypothesize that co-occurrence may be favoured by the slightly smaller size of crayfish and crabs in the shared area than the size of animals in the other transects or by the different microhabitats chosen by crayfish and crabs when sharing the same stream section. Other possible explanations are discussed here, but further studies are necessary to better understand the co-occurrence of these two native crustaceans. Moreover, urgent measures are demanded to guarantee the survival of these protected species hampered by several threats.