Genotype-by-environment interaction for salinity tolerance in the freshwater-invading copepod Eurytemora affinis

This study examined the extent of phenotypic plasticity for salinity tolerance and genetic variation in plasticity in the invasive copepod Eurytemora affinis. Euryemora affinis is a species complex inhabiting brackish to hypersaline environments but has invaded freshwater lakes and reservoirs within the past century. Reaction norm experiments were performed on a relatively euryhaline population collected from a brackish lake with fluctuating salinity. Life history traits ( hatching rate, survival, and development time) were measured for 20 full-sib clutches that were split and reared at four salinities ( fresh, 5, 10, and 27 practical salinity units [PSU]). On average, higher salinities (10 and 27 PSU) were more favorable for larval growth, yielding greater survival and faster development rate. Clutches differed significantly in their response to salinity, with a significant genotype-by-environment interaction for development time. In addition, genetic ( clutch) effects were evident in response to low salinity, given that survival in fresh (lake) water was significantly positively correlated with survival at 5 PSU for individual clutches. Clutches raised in fresh water could not survive beyond metamorphosis, suggesting that acclimation to fresh water could not occur in a single generation. Results suggest the importance of natural selection during freshwater invasion events, given the inability of plasticity to generate a freshwater phenotype, and the presence of genetic variation for plasticity upon which natural selection could act.