Ecology and evolution of life history variation in the marine amphipod Jassa marmorata Holmes

Organisms respond to environmental variation by altering their demographic rates. The life history or life cycle of an organism is the result of interactions among genetics, development, ecology and evolution. This study examined life history variation of a tube-dwelling marine amphipod, Jassa marmorata. Three separate approaches were used to quantify causes and consequences of life history variation in two populations of Jassa marmorata from Narragansett Bay, Rhode Island. Populations were sampled at weekly intervals during 1993 and 1994 as numbers of individuals per size class in a fixed area of substrate. I modified an inverse matrix technique to estimate vital demographic rates. Modifications included incorporation of a number of reproductive size classes, an empirical estimate of egg production and survival, and development of a bootstrap technique which corrects for bias and provides confidence intervals for the estimates of size-specific growth and survival probability. In this way, I assessed temporal (within and between years at one site) and spatial (between two sites) variation in vital rates. Field growth experiments provided independent estimates of growth for one size class. Allozyme surveys within and beyond Narragansett Bay were conducted using starch gel electrophoresis. F-statistics were used to evaluate the extent of genetic divergence among populations. Results include a time series of size-specific growth, survival and fertility at two sites, Fort Wetherill (1993 and 1994) and Point Judith (1994). There were no consistent seasonal or interannual differences in growth and survival probabilities. Vital rates varied between sites and resulted in different, asynchronous dynamics. Egg production was higher at Point Judith but size-specific recruitment rates of adult stages (measured as fertility) did not vary. Reciprocal transplant studies provided evidence for phenotypic plasticity in growth but the range of response varied between source populations. Lastly, allozyme data provided evidence for genetic differentiation among populations at two loci. Taken together, this research demonstrates local population differentiation for important life history traits in Jassa marmorata. The observed life history variation has both genetic and environmental sources