Turtles are reptiles, meaning they are fundamentally air-breathers equipped with lungs, just like humans and other terrestrial vertebrates. This seems to contradict their aquatic lifestyle, especially for species that spend months or even years submerged in water. The central question then becomes how these shelled creatures manage to survive extended periods underwater without having to surface constantly for air. The answer lies in a combination of efficient breath-holding techniques, specialized biological structures that can absorb oxygen from water, and the ability to control their internal energy consumption.
Pulmonary Respiration and Breath-Holding
When a turtle is at the water’s surface, it engages in pulmonary respiration, using its lungs to take in a large volume of air. Sea turtles, for example, can perform a rapid exhalation followed by a quick inhalation to maximize air exchange before diving again. The duration a turtle can remain submerged depends heavily on the amount of oxygen stored in its lungs and bloodstream.
Turtles have developed physiological adaptations to make this stored oxygen last during a dive. A common strategy is bradycardia, where the turtle significantly slows its heart rate to conserve energy and oxygen. This allows them to ration the oxygen supply, directing blood flow primarily to the brain and other oxygen-sensitive organs. When resting, a sea turtle can stay underwater for several hours, far exceeding the 5 to 40 minutes they typically spend foraging actively.
Specialized Non-Pulmonary Respiration
While breath-holding is the primary tactic, certain freshwater turtles possess unique biological structures that allow for a form of aquatic respiration. This process supplements lung breathing by absorbing dissolved oxygen directly from the water through thin, highly vascularized tissues. This adaptation is common in species inhabiting cold water or fast-flowing rivers where surfacing is difficult.
Cloacal Respiration
Cloacal respiration, utilized by turtles like the Fitzroy River turtle, involves specialized sac-like extensions called bursae, connected to the cloaca. These bursae function like auxiliary aquatic lungs and are lined with papillae, which are projections rich in tiny blood vessels. By rhythmically pumping water into and out of the cloacal opening, the turtle facilitates the diffusion of oxygen across these vascularized membranes into its bloodstream.
Pharyngeal Respiration
Another method is pharyngeal respiration, used by some softshell turtles. These species have permeable, vascularized tissues lining the mouth and pharynx (throat area). They pump water over these surfaces, allowing gas exchange to occur similarly to how fish use gills. Both cloacal and pharyngeal respiration are forms of cutaneous, or skin-based, gas exchange that use specialized internal surfaces to extract minimal oxygen required for survival underwater.
Metabolic Factors Governing Submerged Time
Submerged time is heavily influenced by the turtle’s metabolic rate. Turtles are ectotherms; their body temperature and rate of energy use are governed by the ambient environment. This physiological characteristic plays a significant role in diving endurance.
In cold water, the turtle’s metabolism slows dramatically, drastically reducing the demand for oxygen. This is evident during brumation, a state of winter dormancy when ponds freeze over. During brumation, oxygen consumption can drop so low that the minimal amount absorbed through aquatic respiration is sufficient to sustain life for months.
Activity level also directly impacts submerged time. A turtle actively swimming or foraging consumes oxygen at a rate up to 3.2 times higher than a turtle at rest. This increased metabolic demand shortens the dive duration, forcing the turtle to surface more frequently. The overall time a turtle spends underwater balances available oxygen, water temperature, and physical exertion.