Differences in energy storage in sympatric salmonid morphs with contrasting lifestyles

Although physiology ensures homeostasis and fitness in a particular environment, and ecological shifts cannot be realized without physiological changes, metabolic transformations during animal adaptive radiations still remain unexplored. We present a study of energy reserve storage in the salmonid assemblage inhabiting a cold-water Lake Kronotskoe. This assemblage diversified from Salvelinus malma and includes eight distinct ecomorphs with contrasting lifestyles and trophic specializations. We hypothesized that ecomorphs differ in energy storage and expenditure, and that their metabolic phenotypes should be among the primary targets of natural selection. To test this hypothesis, we compared the stored amount and ratio of carbohydrates, lipids, circulating peptides that supply the citric acid cycle, as well as proxy indicators of metabolic rate, the blood levels of plasma proteins (including albumin) and hemoglobin. Among ecomorphs, numerous significant differences in physiological parameters were found, closely related to the composition of food, the depth of habitat, and determined by internal factors, probably genetics. Each ecomorph has a specific metabolic phenotype corresponding to its tropho-ecological specialization and lifestyle. Metabolically advanced predators accumulate lipids; littoral insectivorous morphs grow slower and accumulate glycogen; amphipod feeders do not accumulate spare substances; the deepwater consumer of silt benthos differs in the most divergent physiological characteristics. We assume a specific selection on endocrine regulators of energy metabolism during the adaptive radiation of the assemblage, among which the most plausible candidates are thyroid hormones and leptin.