Cutaneous Surface Area and Bimodal Respiration in Soft-Shelled (Trionyx spiniferus), Stinkpot (Sternotherus odoratus), and Mud Turtles (Kinosternon subrubrum)

Abstract

Total mass-specific cutaneous surface area was determined for three species of freshwater turtles: Trionyx spiniferus, Sternotherus odoratus, and Kinosternon subrubrum. Similarly sized individuals of the three species exhibited high, intermediate, and low values of cutaneous surface area, respectively. Respiratory gas exchange in both air and water was measured in unrestrained individuals of each species. Because the T. spiniferus used in these experiments were on the average over an order of magnitude heavier than the S. odoratus, these two species had similar mass-specific cutaneous surface areas, whereas both had considerably higher values than K. subrubrum. All species were bimodal breathers, exchanging gases with both air and water. The percent aquatic V̇o2 and V̇co2 were highest in soft-shells (38% and 85%, respectively), intermediate in stinkpots (26% and 56%), and lowest in mud turtles (14% and 46%). The inverse pattern was observed for aerial gas exchange. Regression analysis (cutaneous surface area vs. percent aquatic V̇o2) with ANOVA on the residuals indicated that T. spiniferus was more reliant on aquatic gas exchange than could be explained by cutaneous surface area alone. A similar analysis using V̇o2 as the dependent variable indicated that cutaneous surface area explained most of the differences in V̇o2. The interpretation of these results was not substantially altered when differences in proportional skeletal mass between the species were taken into account. The results show that aquatic gas exchange is important even in species with few adaptations that facilitate such exchange and that the degree of aquatic gas exchange is directly related to the mass-specific cutaneous surface area across which gases can diffuse.