Ampipods are important littoral benthic invertebrates in the Great Lakes, being used as food by almost every fish species at some point in their life cycle. Recently, an amphipod invader, Echinogammarus ischnus, has become numerically dominant in several locations throughout the lakes. The native amphipod has life history characteristics that should enable it to outnumber the invasive species; it reproduces earlier, has larger broods, and grows larger. Yet, in some habitats and locations, the invasive species far outnumbers the native. It is possible that the invasive species simply does better in certain habitat conditions, like mussels beds, than the native. Other researchers have shown that both species prefer mussel bed habitats over other habitat types, and these habitats likely provide excellent refuge from fish predators.
Researchers at the GLC have been investigating amphipod behavioral responses to fish and crayfish predators to determine if differences in predator-avoidance behavior might explain why the invasive species is out competing the native species in mussel beds. Both species seem able to identify fish predators, avoiding fish that prey on invertebrates, but not avoiding fish that do not prey on invertebrates. Both species respond more when more fish are present, but the invasive species responds more to round gobies that the native species. The invasive amphipod is from the Ponto-Caspian region, just like the round goby is. Thus, these two species share an evolutionary history that is not shared by the native amphipod. Possibly, the high density of round gobies in the Lakes, coupled with the prevalence of mussel beds, is enhancing the success of invasive amphipods. Counteracting this finding is the response of these amphipod species to crayfish predators.
Crayfish are omnivorous, benthic organisms that seem to shift their diet preferences with age; eating more animal matter as juveniles and more plant matter as adults (though there is certainly some controversy on this). Different sized crayfish (and presumable different ages) differ in their ability to capture and consume amphipods and the amphipod species appear to have differences in their ability to avoid crayfish predators. In studies performed at the GL Field Station, the invasive amphipod was able to avoid large crayfish predators better than the native amphipod, but there was no difference in their ability to avoid small crayfish. Thus, changes in the size distribution, or population age structure of crayfish in the lakes may have implications for amphipod invasion success.
