Osmoregulatory Adaptations of the Spiny Softshell Turtle, Trionyx Spiniferus, from Brackish and Fresh Waters

Two geographically separated populations of the Texas spiny softshell turtle, Trionyx spiniferus emoryi, were collected in New Mexico and their osmoregulation and body fluid compositions were studied. One population inhabits fresh water (0.09% saline) and the other inhabits brackish water (0.79% saline). Turtles were maintained in laboratory water analogous to their field environments and then acclimated to tap water or 0.7% saline. Weight changes and urine flow were measured. Blood and urine samples were obtained from turtles in each aqueous environment and analyzed for sodium, potassium, calcium, magnesium, urea, and protein. Osmotic pressure of these samples was also measured. The two populations of softshell turtles do not differ in their weight or urine volume responses to being maintained in tap water or 0.7% saline. However, populational differences were observed for urea, potassium; and osmotic pressure of plasma, and also for potassium, calcium, magnesium, and osmotic pressure of urine. Relative proportions of different plasma solutes change when total concentrations change in response to different environmental salinities. Blood from both populations of T. spiniferus had seasonal protein fluctuations. Turtles kept at constant laboratory temperature and fed throughout the year had higher plasma protein concentrations in fall than in winter