Aggressive interactions between two invasive species: the round goby (Neogobius melanostomus) and the spinycheek crayfish (Orconectes limosus)

The invasion success of introduced species may be limited by competitive interactions with phylogenetically unrelated invaders. The round goby (Neogobius melanostomus) and spinycheek crayfish (Orconectes limosus) are invasive benthic predators that occupy and defend similar cryptic microhabitats, and thus may compete for shelter. The round goby expanded throughout the North American Great Lakes within 8 years of introduction, whereas another 6 years passed before it had spread through the upper St. Lawrence River. Here, we explore the premise that dense established populations of the invasive spinycheek crayfish slowed round goby colonization of the St. Lawrence River. We performed a series of videotaped laboratory experiments to determine if round gobies suffer from aggressive attacks or alter their behaviour (e.g. use of shelter and movement) in the presence of spinycheek crayfish. We also assessed the prolonged effects of food and shelter competition by comparing changes in the submerged mass of juvenile round gobies and spinycheek crayfish in conspecific and heterospecific pairs. Contrary to our predictions, round gobies more frequently initiated aggressive encounters with spinycheek crayfish, whereas the crayfish were more likely to flee or be evicted from their shelters. Furthermore, round gobies gained more body mass than spinycheek crayfish, regardless of conspecific or heterospecific pairing. Rather than impeding round goby colonization, spinycheek crayfish appear more likely to suffer energetic costs and an increased exposure to predation in the presence of round gobies.