Co-occurrence between signal crayfish and italian native crayfish: how long yet?.

In the first Italian stream invaded by signal crayfish (Pacifastacus leniusculus) since 2009, we found a relict population of native white-clawed crayfish (Austropotamobius pallipes) in the headwater of the same stream. The co-occurrence of these two Astacidae, sharing the same pool, is here reported for the first time in Italy. We examined sex ratio and body condition in adult crayfish during the reproductive season in order to assess animal health and fitness and compare them between these two species. Signal crayfish females were significantly in a better body condition compared to the native ones, whereas signal crayfish and whiteclawed males were not different. Moreover, white-clawed crayfish females showed a worse body condition in the mixed sector compared to the resident ones. Our results suggest that this signal crayfish population benefited from better condition of females during its range expansion and that the native crayfish population need the utmost conservation actions.