Occurrence and Distribution of the Freshwater AmphipodsGammarus pseudolimnaeusandGammarus fasciatusin Southeastern Virginia

The freshwater amphipod, Gammarus fasciatus, and a population that keys to Gammarus pseudolimnaeus are broadly sympatric in southeastern Virginia. By documenting the successful formation of pre-copulatory mate-guarding pairs between individuals collected from Virginia and New York, we confirmed the occurrence of a G. pseudolimnaeus population strongly disjunct from the previously described range in the Mississippi and St. Lawrence River drainage basins. Gammarus pseudolimnaeus appears restricted to high-quality, spring-fed streams that occur at low density across an increasingly fragmented natural landscape in Virginia. Gammarus fasciatus, however, occurs in lakes and streams of developed landscapes that typically are more degraded, and this species does not co-occur with G. pseudolimnaeus in high-quality habitat. Gammarus pseudolimnaeus reproduces year-round, with adults and juveniles of all size classes continuously present. Gammarus fasciatus reproduces primarily from February through June, at...