Evolution of Growth Rate: Tradeoffs Between Costs of Growth and Benefits of Increased Size

Suny At Stony Brook, Stony Brook NY

Investigators

Abstract

Genetic variation and local adaptation may be responsible for many of the differences in somatic growth rate that are common among populations of fishes and other organisms. The existence of genetic variation in juvenile growth is an enigma, challenging the belief that organisms should maximize energy intake and utilization in the early life stages. If higher rates of juvenile growth and larger size translate into greater fitness, sub?maximal rates can only be explained by the existence of tradeoffs associated with rapid growth. Recent evidence suggests that a major tradeoff exists between the amount of energy allocated to growth as opposed to locomotory performance and vulnerability to predators. Using a marine fish, the Atlantic silverside (Menidia menidia) as a model system, this project will explore the mechanisms responsible for this tradeoff and its life history implications. In silversides, the genetic capacity for growth varies inversely with length of the growing season across latitudes (termed "countergradient variation"). Northern genotypes grow at up to twice the rate of southern forms by consuming more food and converting it more efficiently into somatic tissue. But northern fish are poor swimmers and are more vulnerable to predators compared with southern fish. And within populations, juveniles that consume large meals and/or grow more rapidly are also more vulnerable to predators. This project will combine modeling, experimental, and field studies to explore the trade?off between the cost of growth and the benefits of increased size in different environments. There are five major objectives. First, 1) a life history model will be constructed to evaluate the optimal growth strategy when both grow rate?dependent and size-dependent sources of mortality and reproductive success affect fitness. The model will be used to predict how the optimal growth strategy changes across a latitudinal gradient. 2) To parameterize the model, experiments will be conducted to determine the functional response of swimming performance to feeding level, and 3) long?term mesocosm experiments will be used to test the model's predictions. 4) To validate the importance of these trade?offs under natural conditions, tests for an association between growth rate of silversides in the field (as estimated from otoliths) and those found within the stomachs of their major predator, young bluefish (Pomatomus saltatrix), will be conducted. 5) Finally, in order to generalize results to other species, experiments will be conducted to determine the physiological basis of the trade?off between grow rate and swimming performance. The results of this project will greatly increase understanding of the strategies and tactics that larval and juvenile fishes employ during recruitment to the adult stage and will further illustrate the capacity of marine organisms to adapt to abiotic and biotic change.