Underwater and undetected: Aquatic respiratory capacity of a secretive aquatic turtle
Aquatic turtles vary widely in their capacity to remain submerged in water for extended periods. Ecologically, remaining below the water’s surface can have great benefits to individuals by prolonging foraging bouts and reducing the likelihood of detection by predators. However, prolonged submersion presents obvious physiological challenges for a group that classically relies on aerial respiration. Two options for metabolically supporting long diving bouts include, 1) relying on anaerobic metabolic pathways to supplement or supplant aerobic metabolism, and 2) engaging in aquatic gas exchange across non-traditional respiratory surfaces, including skin, oral and buccal surfaces, and cloacal bursae. In general, ecology tends to trump phylogeny in predicting species’ capacity for aquatic respiration; therefore, highly aquatic species that are seemingly ill-suited for aquatic respiration warrant extra scrutiny. One such species is the Alligator Snapping Turtle (Macrochelys temminckii), a secretive denizen of rivers, oxbows and swamps in the southeastern United States. Therefore, we measured oxygen consumption in air and water as a means of estimating whole-animal metabolic rate and non-pulmonary metabolism, respectively. These trials were conducted across a range of size classes (all juveniles) and temperatures to determine whether there was a threshold below which metabolic demands could be wholly supported via aquatic respiration.
Predictably, both pulmonary and cutaneous respiration pathways demonstrated increased rates at higher temperatures. However, oxygen consumption via cutaneous respiration pathways had a smaller response to temperature than oxygen consumption via pulmonary means. Furthermore, oxygen consumption via pulmonary respiration pathways was significantly greater than oxygen consumption via cutaneous respiration (P < 0.0001). Contrary to what we predicted based on the species’ ecology, we found that even juvenile Alligator Snapping Turtles—individuals whose small size results a relatively high surface area-to-volume ratio—had very low capacity for aquatic respiration. Therefore, extended submersion bouts by this species are likely supported anaerobically. Future investigations will examine the length of submersion bouts juvenile alligator snapping turtles are capable of completing before they need to surface. These studies will further contribute to the knowledge of juvenile turtle behavior, which is difficult to study in the wild.