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Patterns of variability in growth, development and mortality in early life histories were examined at two levels--general and specific. At the general level, I examined the functional significance of larvae in metazoan life cycles. This was achieved by surveying the metazoa at the class level and categorizing mode of development by habitat. Three long-standing hypotheses for the paucity of larval development in fresh water were rejected. I argue that viewing metazoan larvae as a means for feeding and growth provides a unifying framework for evaluating the features of habitats which correlate with the range of variation expressed in development modes. I offer a modelling framework to investigate variation in developmental modes within and between habitats. The suggested model requires input on larval growth rates, larval period duration (development time), and size-specific mortality rates. These variables, and the interactions between them, are also central to the specific analysis which addresses the ecological processes and mechanisms which affect survival and hence recruitment during the early life histories periods of marine fishes. Winter flounder (Pleuronectes americanus) was used as a model species to investigate phenotypic variability in patterns of growth and development and their survival consequences. Using laboratory-reared fish, I demonstrate that size-at-age does not diverge continuously during the larval and juvenile periods. The results show that fish which grow slowly as larvae, compensate for their slow growth by growing rapidly as juveniles. This compensation in growth rate causes juvenile size-at-age to converge, or at least, prevents divergence in juvenile size-at-age. In addition, I provide some of the first estimates of individual variability in larval growth trajectories for a marine fish. I conducted the first experiment which separates the effects of both size and age on the vulnerability to predation in recently metamorphosed fishes. Th