Microhabitat selection by the invasive amphipod Echinogammarus ischnus and native Gammarus fasciatus in laboratory experiments and in Lake Erie

1. The amphipod Echinogammarus ischnus was first reported in the Laurentian Great Lakes during 1995. However, analysis of archived samples revealed the presence of the species from western Lake Erie in 1994 and possibly as early as 1993. 2. Surveys conducted in Lake Erie between 1995 and 1998 revealed that Echinogammarus was the dominant amphipod on rocks covered by Dreissena molluscs compared with those fouled by the filamentous alga Cladophora , while Gammarus fasciatus used both Dreissena and Cladophora substrata extensively. 3. In laboratory habitat selection studies, Echinogammarus chose Dreissena - over Cladophora -encrusted rocks and bare rocks, while Gammarus occupied the more complex substrata equally. 4. Field colonisation experiments demonstrated that the densities of Echinogammarus and Gammarus were positively correlated when the total density of the species was low, in contrast to the large-scale natural distribution of the species that revealed a strong inverse relationship. 5. The on-going replacement of Gammarus by Echinogammarus in the Laurentian Great Lakes may be related to the stronger affinity of the latter for substrata fouled by Dreissena , a genus with which it co-evolved.