Activity, not submergence, explains diving heart rates of captive loggerhead sea turtles

Marine turtles spend their life at sea and can rest on the seafloor for hours. As air-breathers, the breath-hold capacity  of marine turtles is a function of oxygen (O₂) stores, O₂ consumption during dives and hypoxia tolerance. However, some physiological adaptations to diving observed in mammals are absent in marine turtles. This study examined cardiovascular responses in loggerhead sea turtles, which have even fewer adaptations to diving than other marine turtles, but can dive for extended durations. Heart rates (f_H) of eight undisturbed loggerhead turtles in shallow tanks were measured using self-contained ECG data loggers under five conditions: spontaneous dives, resting motionless on the tank bottom, resting in shallow water with their head out of water, feeding on squid, and swimming at the surface between dives. There was no significant difference between resting f_H while resting on the bottom of the tank, diving or resting in shallow water with their head out of water. f_H rose as soon as turtles began to move and was highest between dives when turtles were swimming at the surface. These results suggest cardiovascular responses in captive loggerhead turtles are driven by activity and apneic f_H is not reduced by submergence under these conditions.