Long-term salinity exposure reveals site-specific physiological and behavioural responses in coastal and inland toads

Salinization is a global phenomenon affecting many coastal and inland water bodies at unprecedented rates, and having multiple consequences for wildlife, mostly due to the high metabolic cost of osmoregulation. Most research on salinity effects has focused on juvenile life stages and short-term exposures. In this study, we evaluated the impact of chronic exposure (7 months) to environmental salinity (0, 2 or 4 g l-1) in adult individuals of a widespread amphibian species, the spined toad (Bufo spinosus), originating either from coastal (salt-exposed) or inland (naive to salinity) environments. Coastal and inland individuals, irrespective of their experimental exposure to salinity, differed post-experimental exposure in telomere length, cutaneous permeability, defensiveness and foraging behaviours, but unexpectedly not in triiodothyronine and testosterone levels, and mass-specific metabolic rate. As pre-exposure data were unavailable for some of these traits (metabolic rate, cutaneous permeability and behaviour), these differences may reflect either intrinsic population-level variation or population-specific responses to treatment. Coastal individuals exhibited compensatory growth in freshwater, suggesting differential energy allocation. Chronic exposure to moderate salinity increased osmolality and triiodothyronine levels while reducing growth and preferred temperature. Interestingly, defensiveness and thermal preference were more affected by exposure to salinity in coastal individuals. Despite their saline environment, these toads also showed higher skin permeability to water flux, potentially facilitating water uptake to counter salinity stress. Our study demonstrates that salinization can have negative effects on critical traits of adult anurans, and emphasizes the importance of increasing research effort on the impact of salinity on wildlife including both experimental and long-term field studies.