The blood biochemistry of overwintering diamondback terrapins (Malaclemys terrapin)

Estuarine ectothermic vertebrates, such as diamondback terrapins (Malaclemys terrapin, Schwartz 1955), inhabit a dynamic environment, and many aspects of their biology reflect their ability to withstand and respond to these environmental challenges. The physiological adjustments necessary to maintain water and salt balance and the metabolic adjustments that accompany seasonal changes in activity and behavior have not been well-characterized for overwintering terrapins under field conditions. To investigate terrapin osmotic and metabolic physiology during winter when activity levels are depressed, we obtained repeat blood samples from 10 radio-tagged female terrapins maintained in a semi-natural, open-air salt marsh enclosure in southeastern North Carolina, USA. From November 2011 to April 2012, we measured monthly plasma osmolality, plasma concentrations of inorganic osmolytes (Na+, K+, Cl−), and protein catabolic indices (urea and uric acid), as well as monthly plasma concentrations of total Ca2 +, lactate, and glucose as metabolic indices. We used linear mixed models to determine the best predictors of blood chemistry, where time (i.e., day) and environmental variables were fixed factors and individual terrapins were random factors. Day was a poor predictor of blood chemistry concentrations, indicating that the progression of winter did not elicit corresponding changes in biochemical indices as documented in other semi-aquatic turtles exposed to more severe winter and/or laboratory conditions. Carapace temperature was the most common predictor of blood chemistry concentrations in all models, underscoring its relative influence on physiology. In contrast to previous laboratory-based studies on the overwinter physiology of terrapins, our study demonstrates that terrapins in their natural environment are able to maintain biochemical homeostasis throughout winter. The use of evasive behavioral strategies may be an important factor for terrapins to reduce the passive exchange of water and salts with the estuarine environment.