Testing whether adrenal steroids mediate phenotypic and physiologic effects of elevated salinity on larval tiger salamanders

Abstract

Salinity (sodium chloride, NaCl) from anthropogenic sources is a persistent contaminant that negatively affects freshwater taxa. Amphibians can be susceptible to salinity, but some species are innately or adaptively tolerant. Physiological mechanisms mediating tolerance to salinity are still unclear, but changes in osmoregulatory hormones such as corticosterone (CORT) and aldosterone (ALDO) are prime candidates. We exposed larval barred tiger salamanders (Ambystoma mavortium) to environmentally relevant NaCl treatments (<32-4000 mg·L −1 ) for 24 days to test effects on growth, survival, and waterborne CORT responses. Of those sampled, we also quantified waterborne ALDO from a subset. Using a glucocorticoid antagonist (RU486), we also experimentally suppressed CORT signaling of some larvae to determine if CORT mediates effects of salinity. There were no strong differences in survival among salinity treatments, but salinity reduced dry mass, snout-vent length, and body condition while increasing water content of larvae. High survival and sublethal effects demonstrated that salamanders were physiologically challenged but could tolerate the experimental concentrations. CORT signaling did not attenuate sublethal effects of salinity. Baseline and stress-induced (after an acute stressor, shaking) CORT were not influenced by salinity. ALDO was correlated with baseline CORT, suggesting it could be difficult to decouple the roles of CORT and ALDO. Future studies comparing ALDO and CORT responses of adaptively tolerant and previously unexposed populations could be beneficial to understand the roles of these hormones in tolerance to salinity. Nevertheless, our study enhances our understanding of the roles of corticosteroid hormones in mediating effects of a prominent anthropogenic stressor.

Effects of Salinity on the Egg Fertilization, Hatching, and Tadpole Growth and Survival Rates of the Thailand Frog (Rana tigerina Dubois, 1981)

“…Amphibians are sensitive to habitat variations due to their skin permeability and their poor osmoregulatory capacity during the aquatic larval stage when they are highly dependent on the resources available in the aquatic environment [46,58,59]. Te osmoregulatory process in amphibians is mediated by endocrine hormones from their adrenal glands, and corticosterone (CORT) and aldosterone (ALDO) are prime candidates [60,61]. CORT in aquatic vertebrates increases with salinity exposure [62][63][64][65][66] and is linked to ionization and osmotic regulation [67][68][69].…”

“…ALDO in amphibians often decreases when exposed to saline conditions [60,73], presumably to reduce sodium uptake [30]. In addition, it is recorded that the ALDO of Northern leopard (Rana pipiens) frog larvae marginally decreased with increasing salinity and was positively correlated with stress-induced waterborne CORT, and CORT may be a substrate for ALDO synthesis [61]; so it also seemed difcult to decouple the roles of CORT and ALDO in the osmoregulatory process [61]. Te Cuban treefrogs (Osteopilus septentrionalis) in the Gosner stages could respond to high salt levels by decreasing aldosterone secretion when they were exposed to salinities of 1.0, 2.5, and 3.5%, and the regulation process of osmotic functions in high salt concentrations must involve tradeofs in their growth and development [60,74].…”