Role of Thyroid Hormones in Adaptation to Volcanic Contamination of Freshwater Habitats in Charr of the Genus Salvelinus (Salmonidae)

The balance between phenotypic plasticity and adaptive specialization in response to environmental pressures remains a hot topic in evolutionary biology. In fish, one of the strongest impact factors is the chemical pollution of habitats. In an attempt to assess the consequences of heavy pollution of fresh waters for resident fishes, we studied Kamchatkan charr, which undergo paedomorphosis in the case of isolation in streams of volcanic areas contaminated with heavy metals. Experiments were carried out on the resistance of charr to metal mixtures during normal development and in six experimental groups with therapeutically altered intensity of metabolism and the secretory activity of thyroid gland. Water from volcanically contaminated streams was found to be lethally toxic for embryos and early juveniles of unadapted charr. The success of acclimation to toxic exposure was correlated with an increase in thyroid status. In experiments, the group with significantly elevated thyroid status showed a significant decrease in mortality and attenuation of oxidative stress in solutions of heavy metals. Under natural conditions, hyperthyroidism provokes a redistribution of the charr’s organism resources from somatic growth and morphological differentiation to stress counteracting and accelerated maturation, which is necessary for the long-term survival of the population under conditions of increased risk of individual mortality. Our experiments highlight the role of thyroid hormones in the rapid response to habitat pollution and the subsequent adaptation of fish populations to chronic deterioration.