Effects of temperature and salinity on body fluid dynamics and metabolism in the estuarine Diamond-backed Terrapin (Malaclemys terrapin)

The diamondback terrapin is the only temperate turtle species that exclusively inhabits estuarine habitats. Morphological, behavioral and physiological features contribute to the terrapin's ability to regulate body fluid osmotic pressure in a euryhaline environment. Low integument permeability combined with aquatic-terrestrial shuttling behavior limits passive exchange of water and salts with the environment, and terrapins regulate active uptake of salts via alterations in drinking and feeding behavior. The lachrymal salt gland facilitates excretion of excess sodium (Na+) and chloride (Cl−) ions through active transport mechanisms. We investigated body fluid dynamics, oxygen consumption (V̇O2), and osmotic status of terrapins exposed to an acute increase in salinity (12 to 35 psu) at 10°C and 25°C to gain insight into the relative importance of behavioral vs. physiological osmoregulatory adjustments over a range of seasonally-relevant temperatures. Linear mixed models were used to evaluate the effects of experimental temperature, salinity, and mass. Overall, temperature effects were stronger than salinity effects. Terrapins acclimated to 25°C had significantly lower blood osmolality and Na+, and higher water turnover rates, daily water flux (DWF), and V̇O2 compared with terrapins acclimated to 10°C. Salinity effects were restricted to DWF, which significantly decreased in response to acute exposure to 35 psu. Our results support the notion that behavioral adjustments predominate in the osmoregulatory strategy of terrapins.