Plasticity of Life-History Characters in Eastern Mosquitofish (Gambusia holbrooki: Poeciliidae) in Response to Thermal Stress

Eastern mosquitofish (Gambusia holbrooki) from a population exposed to abnormally high temperatures for 28 years were raised from birth in environmental chambers at 25 C (a normal temperature) or 32 C (a stressful temperature). Fish grew faster and larger at 25 C and were in better somatic condition than were fish grown at 32 C. There were no differences in ovum production at the two temperatures. At the higher temperature, fish matured at a younger age and a smaller size than did those at the normal temperature. This particular age-size at maturity relationship is not included as one of the predicted norms of reaction of a model by Stearns and Koella (1986) that deals with age and size at maturity under various growth rates. Gambusia holbrooki is a highly plastic species, which makes it adaptable to a variety of conditions, but also complicates interpretations of its interpopulation life-history differences; much of its life-history variation may be a consequence of different developmental environments rather than adaptive responses to demographic conditions.