Sea turtles, like all reptiles, must breathe air to survive. They possess lungs for respiration and cannot extract oxygen from water as fish do with gills. This need for atmospheric oxygen governs their life in marine environments, despite spending significant portions of their lives underwater.
The Fundamental Need for Oxygen
Sea turtles require oxygen for cellular respiration, a process that generates adenosine triphosphate (ATP), the primary energy for all bodily functions. This energy fuels swimming, foraging, digestion, and body temperature maintenance. Their lungs are the primary organs for gas exchange, taking in oxygen and releasing carbon dioxide.
While primarily reliant on aerobic respiration, sea turtles can temporarily engage in anaerobic metabolism. This allows them to produce some energy without oxygen, useful during extended dives. However, this is a less efficient process and cannot sustain them indefinitely.
Respiratory Adaptations for Diving
Sea turtles possess remarkable physiological adaptations for long durations submerged. Their lungs are highly efficient, designed for effective gas exchange to maximize oxygen uptake during brief surface intervals. These large, powerful lungs store a considerable volume of air for underwater excursions.
A key adaptation is bradycardia, the ability to significantly slow their heart rate during dives. For instance, a loggerhead sea turtle’s heart rate can drop from 20.9 beats per minute at the surface to 12.8 beats per minute underwater. During prolonged dives, their heart rate can decrease further, sometimes to 1-4 beats per minute, or even one beat every nine minutes in green sea turtles.
This heart rate reduction is coupled with a lowered metabolic rate, which conserves oxygen and energy. As cold-blooded animals, sea turtles naturally have slower metabolisms than mammals, and this rate can decrease further when resting or in colder waters. This allows them to extend their time underwater significantly.
During dives, sea turtles exhibit selective peripheral vasoconstriction, shunting blood away from less oxygen-demanding organs to vital areas such as the brain, heart, and central nervous system. They also have higher concentrations of hemoglobin in their blood and myoglobin in their muscles, which enhance oxygen storage for prolonged submersion. These combined mechanisms allow them to hold their breath for varying periods.
While foraging, sea turtles typically dive for 4-5 minutes, surfacing for only a few seconds. During routine activity, they can hold their breath for 45 minutes to an hour. When resting or sleeping, their oxygen consumption drastically decreases, allowing them to remain submerged for several hours, with some species staying underwater for up to 7 hours, especially in colder water where metabolism is further reduced.
Risks of Oxygen Deprivation
Despite their remarkable adaptations, sea turtles are vulnerable to oxygen deprivation, which can lead to severe consequences. A primary threat is entanglement in fishing gear, such as nets, or marine debris like plastic. When entangled, a sea turtle may be unable to reach the surface to breathe, leading to rapid oxygen depletion and drowning within minutes.
Another danger is cold stunning, a hypothermic reaction occurring when sea turtles are exposed to prolonged cold water temperatures, typically below 50°F (10°C). As ectothermic animals, their body temperature mirrors their surroundings, and a significant drop slows their heart rate and circulation, reducing oxygen delivery. This condition causes lethargy, making them unable to swim, feed, or surface effectively.
If left untreated, cold stunning can progress to shock, pneumonia, brain damage, and ultimately death. Human activities, such as leaving discarded fishing gear or altering coastal habitats, can inadvertently increase the risk of oxygen deprivation for these marine reptiles.