VARIATIONS IN THE LIFE HISTORY STRATEGY OF DAPHNIA PULEX ALONG A LATITUDINAL GRADIENT

Observations of life history strategies of populations of a species along a latitudinal gradient allows comparisons of organisms with similar morphological, physiological, and genetic constraints but adapted to different environments. Four predictive models for life history characteristics of the cladoceran Daphnia pulex along a latitudinal gradient were tested. These models were based on temperature physiology, the size-selectivity of predation, and deterministic and stochastic life history theory. Two clones of cladocerans were isolated from animals collected at Frobisher Bay (63(DEGREES)48\u27N), Churchill (58(DEGREES)40\u27N), and Windsor (42(DEGREES)19\u27N), Canada, and at Lincoln, Nebraska (40(DEGREES)49\u27N). After acclimation newborn individuals were isolated and reared in environmental chambers which simulated arctic (24:0 LD, 14(DEGREES)C) or temperate (14:10 LD, 18(DEGREES)C) conditions of photoperiod and temperature. Each pair of clones was found to have a unique suite of life history characteristics. The arctic clones took longer to mature and had few, small broods of large offspring; the temperate clones took less time to mature and had many broods of small offspring. Arctic clones appeared to be sensitive to the difference in photoperiod in the two simulations, producing only resting eggs in the temperate simulation. The particular suite of traits remained the same in both environments for all sets of clones which suggests that there is a genetic component to the life history strategy of Daphnia. The stochastic model was least accurate in predicting the life history characteristics observed. The other models predict similar traits so that the observed suite of traits could be the result of interactions between many factors including temperature physiology, the effects of size-selective predation, and density dependent/independent mortality factors (the deterministic model)