An analysis of the mechanisms governing species replacements in crayfish.

We investigated mechanisms governing replacement of the native crayfish Orconectes sanborni by an invading cryafish, Orconectes rusticus. The two species had similar life histories, habitat preferences, and feeding patterns in allopatric and sympatric stream areas. Orconectes rusticus young-of-year (YOY) grew faster than O. sanborni YOY in the field. Adult O. rusticus were larger and, hence, dominant over adult O. sanborni; YOY were non-aggressive. In laboratory experiments, adult crayfish (about 28 mm carapace length or larger) were not susceptable to predation by largemouth bass (Micropterus salmoides, 30 cm total length) and did not alter shelter use when fish were present. Orconectes rusticus YOY were less susceptible to predation than O. sanborni YOY. Orconectes rusticus YOY reduced their vulnerability to largemouth bass by occupying shelters more often than YOY O. sanborni. In mixed-species mateselection experiments, male O. rusticus and male O. sanborni preferentially mated with O. rusticus females. Inappropriate mate selection in sympatry may have caused the 90% reduction in recruitment for both species in 1982. Orconectes rusticus probably maintains greater population growth than O. sanborni, because (1) more gravid O. rusticus females occurred in sympatry, (2) O. rusticus produced more young than O. sanborni, and (3) O. rusticus young grew faster. Reproductive interference, acting synergistically with differences in aggressive dominance and young-of-year susceptibility to predation, appears to serve as the major mechanisms regulating replacement of O. sanborni by O. rusticus in Ohio streams.