Implications of Early Life Histories on Competition Between Virile and Rusty Crayfish in the Monocacy River, Maryland

Rusty crayfish (Orconectes rusticus) are currently displacing the virile crayfish (Orconectes virilis) in the Monocacy River. Our research examined young of the year life history traits of both crayfish species to identify potential mechanisms of competitive dominance and eventual displacement of O. virilis. The distribution of species along Monocacy River provided a unique location to investigate young of the year (YOY) life histories in the presence and absence of inter-specific competition. We sampled at sites dominated by O. rusticus and O. virilis respectively, and at a site where both were present and in direct competition. In competition with O. rusticus, O.virilis experienced higher mortality, reduced growth rate, and smaller year-end size than compared to the virile dominant site. The O. virilis population also had a large YOY collapse of its juvenile population by early summer in competition, but this was not observed at the O. virilis dominant site.  This suggests that displacement of O. virilis by O. rusticus is occurring in the juvenile age class. Orconectes rusticus juveniles showed increased growth rate and larger year-end size at competition sites than at O. rusticus dominant sites suggesting the invasion front may provide juvenile O. rusticus new resource availability. Orconectes rusticus juvenile mortality was unchanged by the presence of the virile crayfish. Adult female O. rusticus had a lower individual fecundity than O.virilis but was investing larger amounts of energy per egg as seen by increased egg volume. The larger energetic investments per individual and earlier egg extrusion may be additional mechanisms explaining O. rusticus' successful invasion of O. virilis.