Offspring Size Variation in Eastern Mosquitofish (Gambusia holbrooki: Poeciliidae) from Contrasting Thermal Environments

Seasonal patterns of offspring size variation in eastern mosquitofish (Gambusia holbrooki, Poeciliidae) are described from a thermally ambient environment and an unpredictable, thermally elevated system in South Carolina. Newborn mosquitofish (n = 1371) were individually lengthed and weighed shortly after birth; mean offspring size (both length and dry weight) varies among females, seasons, and thermal environments. Size variance was high in all cases but less than half was explainable by environment, season, maternal size, or clutch size; environment and season explain the most variation. Although offspring size is certainly subject to selection (i.e., optimality arguments) and even size variation may be selected for (i.e., plasticity arguments) it should not be overlooked that some portion of variation may be due to uncontrollable noise (i.e., the female cannot precisely control offspring size). All three aspects of size variation in mosquitofish remain to be tested. ONE focus of life history studies is the evolution of offspring size. Possible fitness differences among different sized offspring, and relationships between offspring size, clutch size and other life history parameters have resulted in divergent views of the action of selection on