THE ROLE OF HYBRIDIZATION IN A SPECIES INVASION AND EXTIRPATION OF RESIDENT FAUNA: HYBRID VIGOR AND BREAKDOWN IN THE RUSTY CRAYFISH, ORCONECTES RUSTICUS

The rusty crayfish, Orconectes rusticus (Girard, 1852), is native to the Ohio River basin, but was introduced into northern Midwestern lakes through the bait trade about 50 years ago. Since then, the rusty crayfish has spread rapidly through lakes in northern Wisconsin, displacing two resident species of crayfish, O. propinquus (Girard, 1852), the northern clearwater crayfish, and O. virilis (Hagen, 1870), the virile crayfish. Previous studies have shown that as O. rusticus invades, it hybridizes extensively with O. propinquus, but not with O. virilis. Here, we examine why stable hybrid zones do not form between O. rusticus and O. propinquus, but instead hybridization accelerates the decline of O. propinquus. We report that although putative F1 hybrids tend to grow faster and survive better than other crayfish, backcross hybrid crayfish tend to have fewer young (at a given age) that grow more slowly and experience lower survivorship than F1 hybrids and both parental species. Parental O. rusticus, in comparison, have offspring numbers and growth rates more similar to those of F1 hybrids, while survivorship is intermediate between F1 hybrids and backcrosses. Consequently, hybrid breakdown appears to occur in latter generation crayfish of mixed ancestry, resulting in unstable hybrid zones in lakes that speed the extirpation of O. propinquus. Our findings underscore the negative effects that hybridization can have on conserving native gene pools, a danger that may be particularly detrimental for the great diversity of North American crayfishes.