Reptiles, a diverse group including snakes, lizards, turtles, and crocodiles, often spend significant time in aquatic environments. Unlike fish, reptiles do not possess gills; they rely on lungs for respiration. This means they cannot breathe underwater in the conventional sense. Despite this, many aquatic reptiles have evolved remarkable adaptations that allow them to remain submerged for extended periods.
The Core Truth: Air-Breathers with Lungs
Reptiles are air-breathing animals, equipped with lungs to extract oxygen from the atmosphere. Their lungs, featuring small sacs called alveoli, allow for efficient gas exchange, similar to mammals. This necessitates periodic surfacing to replenish their oxygen supply.
Even highly aquatic reptiles must eventually return to the surface. Their respiratory systems are adapted for terrestrial life, meaning they cannot sustain themselves indefinitely underwater. For instance, crocodiles must regularly surface to breathe.
Mastering the Dive: How Reptiles Stay Submerged
While reptiles cannot breathe underwater, many have developed sophisticated physiological adaptations enabling them to endure prolonged periods beneath the surface. One primary mechanism is efficient breath-holding, allowing aquatic reptiles to remain submerged for considerable durations. For example, sea turtles can hold their breath for 4-7 hours while resting or sleeping, and some can stay submerged for up to 45 minutes to an hour during activity.
Their ability to significantly reduce their metabolic rate plays an important role in conserving oxygen during dives. As ectotherms, their body temperature and, consequently, their metabolic rate, can decrease in colder water, further slowing oxygen consumption. This metabolic slowdown is often accompanied by bradycardia, a significant reduction in heart rate. For instance, a diving leatherback sea turtle’s heart rate can drop to as low as 1.05 beats per minute, compared to 27 beats per minute at the surface.
Reptiles also possess enhanced oxygen storage capabilities. They have higher concentrations of hemoglobin in their blood and myoglobin in their muscles, allowing them to store more oxygen than many terrestrial animals. Crocodilians, for example, have a high blood volume and a greater oxygen-carrying capacity.
Furthermore, many aquatic reptiles exhibit circulatory shunts, where blood flow is redistributed to prioritize vital organs like the brain and heart, while reducing supply to less critical tissues, thereby conserving oxygen. Marine iguanas, found in the Galapagos, demonstrate these adaptations, with their heart rates dropping from 40 to 10 beats per minute during dives, enabling them to forage underwater for up to 30 minutes, and sometimes an hour. Crocodiles can hold their breath for 1 to 2 hours under normal conditions, and even longer, potentially exceeding 6-8 hours, when inactive or in cold water.
Beyond Holding Breath: Specialized Aquatic Respiration
Beyond general physiological adaptations for breath-holding, some reptiles utilize unique, supplementary methods to absorb oxygen from water. One such method is cloacal respiration, primarily observed in certain freshwater turtles. These turtles can absorb dissolved oxygen from water through specialized, highly vascularized sacs within their cloaca. This process involves pumping water over these vascularized surfaces, allowing gas exchange to occur. While not true breathing in the pulmonary sense, cloacal respiration can be important during periods of inactivity, such as underwater hibernation in cold environments, where oxygen demands are lower.
Another less common, but notable, supplementary method is cutaneous respiration, or gas exchange through the skin. Although most reptiles have thick, scaly skin that limits gas exchange, some species, like sea snakes, have adapted to absorb a significant amount of oxygen directly through their skin. This can account for up to 33% of their total oxygen needs, especially during prolonged dives. Sea snakes possess a dense network of blood vessels close to the skin’s surface, facilitating the diffusion of oxygen from the water into their bloodstream. While these specialized methods offer additional oxygen uptake, they do not eliminate the need for reptiles to surface and breathe air with their lungs.