Activity not submergence explains diving heart rates of captive loggerhead turtles

Abstract

Marine turtles spend their life at sea and can rest on the seafloor for hours. As air-breathers, marine turtles' breath-hold capacity 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 examines cardiovascular responses in loggerhead turtles, which have even fewer adaptations to diving than other marine turtles, but can dive for extended durations. Heart rates (fH) 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 fH while resting on the bottom of the tank, diving, or resting in shallow water with their head out of water. Heart rate 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 fH is not reduced by submergence under these conditions.