Turtles are reptiles and obligate air-breathers, using lungs for primary gas exchange. They must surface to inhale oxygen from the atmosphere. However, many aquatic species have evolved physiological and anatomical adaptations that allow them to remain submerged for extended periods, leading to the misconception that they can breathe water.
Primary Respiration: Lung Structure and Function
The rigid structure of a turtle’s shell (carapace and plastron) presents a unique mechanical challenge to breathing. Unlike other reptiles and mammals that use their ribs to expand the chest cavity, a turtle cannot move its ribs because they are fused to the shell. This bony enclosure necessitates a different mechanism for pulmonary ventilation, the process of moving air in and out of the lungs.
Turtles use specialized muscle sheets deep within their body cavity to change internal pressure. The transverse abdominis muscle pulls the viscera upward, compressing the lungs and forcing air out during exhalation. Conversely, the pectoralis and other shoulder girdle muscles pull the viscera downward, expanding the lung volume to draw air in during inhalation.
These movements create a muscular aspiration pump that substitutes for a mobile ribcage. The lungs are complex, multi-chambered organs known as faveoli, which provide a large surface area for gas exchange, similar to the alveoli in mammals. This reliance on muscle-driven pressure changes makes air-breathing an active, energy-consuming process.
Surviving Underwater: Metabolic Adaptations
When aquatic turtles dive for prolonged durations, they employ internal mechanisms to conserve their limited oxygen supply. This is achieved by drastically reducing overall energy consumption, a state known as hypometabolism. The decrease in body temperature for ectothermic reptiles further aids this process by slowing cellular activity.
A coordinated physiological response manages the distribution of oxygenated blood away from less-demanding tissues. This involves peripheral vasoconstriction, where blood vessels constrict to restrict flow to the extremities and non-vital organs. Simultaneously, the heart rate slows significantly (bradycardia), conserving the oxygen contained within the circulating blood.
Once stored oxygen is depleted, the turtle switches to anaerobic respiration, a process that does not require oxygen but is far less efficient and produces lactic acid. Although this metabolic byproduct is highly acidic, turtles can tolerate massive accumulation. Their shell and skeleton, rich in calcium carbonate, act as a buffer to neutralize the lactic acid, sequestering it until the turtle can surface and recover.
Supplementary Aquatic Oxygen Uptake
Certain freshwater turtle species, particularly those inactive or hibernating in cold, oxygen-poor water, can extract dissolved oxygen directly from the water. This process, known as aquatic respiration, is supplementary to lung-based breathing and is only effective when the metabolic rate is low. It cannot sustain a turtle during high-energy activity.
One method is pharyngeal breathing, which involves pumping water over highly vascularized papillae and membranes in the mouth and throat. The thin skin covering these tissues allows for oxygen diffusion into the bloodstream. This gas exchange is most common in softshell turtles, which have a reduced shell and a higher surface area of permeable skin.
A more unusual form of aquatic respiration is through the cloacal bursae, colloquially called “butt breathing.” These are paired, thin-walled sacs extending from the cloaca and richly supplied with capillaries. The turtle rhythmically pumps water into and out of these bursae, extracting oxygen through the highly permeable cloacal lining.
The Australian white-throated snapping turtle, which uses this adaptation extensively, can obtain up to 70% of its total oxygen requirements through aquatic respiration when resting. For most other species, like the common snapping turtle, cloacal and pharyngeal respiration provides a smaller, yet significant, portion (5 to 31% of total oxygen consumption). These mechanisms maximize submerged time, especially in cold water where oxygen demand is naturally reduced.