Environmental conditions can modulate the phenotype of an organism through evolutionary and plastic developmental mechanisms, with potential energetic consequences due to growth and maintenance costs. In this thesis, I evaluate habitat differences in a suite of morphological and physiological traits using ecologically diversified pumpkinseed sunfish ecotypes and investigated when habitat differences arise in development. Sunfish occupying the in-shore littoral habitat exhibit greater investment in relative brain, spleen, liver, gastrointestinal and gonad size compared to sunfish occupying the open-water pelagic habitat. Habitat differences in gastrointestinal and gonad size arose around age three in correspondence with changing ecology over ontogeny and sexual maturation. Furthermore, positive correlations among visceral organs suggest an organ investment syndrome. Littoral sunfish presumably face greater maintenance costs than their pelagic counterparts, particularly regarding the metabolically expensive brain. I evaluate the mechanisms used by sunfish with a large relative brain size to compensate for the high costs of neural tissue through reductions in energy allocation to other traits and/or an increase in resting metabolic rate (RMR). After accounting for the organ investment syndrome, brain size was negatively associated with immune function. Sex-specific tradeoffs were also observed between brain size and other organ size traits. The relationship between brain size and RMR depended on habitat and investment in visceral tissues. Furthermore, brain size was positively related to maximum metabolic rate and aerobic scope. My findings suggest that several mechanisms may be simultaneously operating to compensate for the increased energetic costs of neural tissue in large-brained individuals.