Environmental and population-specific contributions to growth rate variation in the marine amphipod Jassa marmorata Holmes

Abstract

Local selective regimes and limited dispersal may contribute to genetic differentiation of life history traits among local populations of the marine amphipod Jassa marmorata Holmes. Reciprocal transplant experiments were used to explore growth rate variation and to determine the effects of environment versus source population (genotype) on the observed variation. Gravid females were collected at two sites in Narragansett Bay, Rhode Island. Hatchlings were measured and reared in field enclosures following a split brood design. Measurements of total body size (length in mm) were made initially and at two additional time intervals. Experiments were conducted in spring and summer of 1994 and again in the spring of 1995. Growth curve analysis was used to test the effects of two independent variables, source population and environment, on growth rate. Results of the spring 1994 experiment revealed both source population (genetic) and environmental effects on growth rate. Individuals collected from and grown at Point Judith attained the highest body size. In the summer of 1994, there were no significant effects of environment or source population on growth rates. In the spring of 1995 only environmental effects were detected; animals grew faster at Point Judith. These results suggest that divergence between populations may occur with respect to growth rate but this divergence is not consistent interannually. An alternative mechanism, phenotypic plasticity, may also be employed by these amphipods to cope with environmental heterogeneity.