A new invader in freshwater ecosystems in France: the rusty crayfish Faxonius rusticus

Crayfish are of major ecological importance within freshwater ecosystems, due to their central position in the trophic niche, acting as regulators of food webs. However, the introduction of exotic crayfish species threatens native species. Indeed, these introduced crayfish often outcompete local species and can even disseminate pathogens (i.e. exotic American species with the crayfish plague, Aphanomyces astaci Schikora, 1906), leading to rapid population losses. We report here the first record of rusty crayfish Faxonius rusticus in the wild in France and in Europe so far. The situation is alarming because F. rusticus is native to North America, potentially carrier of A. astaci, and classified among the most invasive species of crayfish in the world, due to its impressive dispersal capacities. This study reports high densities of individuals from a pond along the Inières Brook, where all size classes are represented, both adults and juveniles. A total of thirty-seven specimens were also observed over 2.5 km of the Inières brook as well as the Briane (main stream) in 2019 where this species co-occurs with another invasive crayfish species, the signal crayfish Pacifastacus leniusculus. In these sites, all specimens of both species were positive for crayfish plague presence with some individuals showing high level of infection (A5) (on an A0 to A7 scale), rarely observed in specimen of other invasive crayfish species. Since these first observations, the situation seems to have worsened, with 847 individuals captured, in the pond along the Inières brook, during survey sessions in 2021.