Biologists have feared that besides outcompeting local crayfish, the rusty crayfish also hybridizes with them. However, so far this assessment has been based on morphological studies, which can be ambiguous.
To determine definitively whether introduced rusty crayfish have been hybridizing with local species, Perry and his colleagues compared the DNA of crayfish from 30 sites in the midwestern U.S. The researchers found that hybridization is common. One of the worst cases is Birch Lake, Wisconsin, where only a fourth of the crayfish still have northern clearwater crayfish genes and the entire population already looks exactly like rusty crayfish.
"Wherever the two taxa meet, [the rusty crayfish] ecologically displaces and, through hybridization, genetically assimilates and morphologically extirpates [northern clearwater crayfish] populations," say Perry and his colleagues.
Several states have restricted using the rusty crayfish for fish bait and there are calls for banning the use of live crayfish as bait. This can't come too soon. "We are rarely if ever able to get rid of an exotic species and thus introduced species like the rusty crayfish represent a permanent alteration to the system," says Perry.