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Understanding the forces that govern allocation to growth, asexual, and sexual reproduction is a major goal for understanding life history diversity in the sea. Environmental conditions influence the costs and benefits of various life history strategies, so different strategies may be favored across geographic gradients. We describe latitudinal patterns in population density, body size, and gamete production in the clonal sea anemone Diadumene lineata. Fission rate in this species is highly correlated with temperature, so the rate of asexual reproduction declines with latitude. We measured the relationships among body size, gamete production, and environmental variables using individuals from 20 sites in intertidal estuaries and harbors from Florida to Maine across the US Atlantic and Gulf coasts. Average body size increased with latitude and decreased with temperature. Body size strongly predicts both the probability of having gametes and the number of gametes produced, such that there is a latitudinal transition from low per capita gamete production in the south to higher production in central and northern populations, with a peak in the Mid-Atlantic region. This gradient in per capita gamete production in this species runs counter to a known gradient in fission rate along the US Atlantic coast, supporting a latitudinal dine in the reproductive strategy in this species.