Marine turtles are among the longest and deepest diving of the air-breathing vertebrates. In fact, these animals spend less than 10% of their time at the sea surface. The leatherback sea turtle (Dermochelys coriacea), the best adapted to diving of sea turtles, reaches depths of over 1,000m, and voluntary dives of between 2 and 5 hours have been recorded. How do they do it? This is not fully understood today – it is very complicated to investigate these animals’ physiology in the wild without capturing them, which would in turn completely modify the animal’s behavior due to stress.

In this post, we try to summarize the most important adaptations of sea turtles to deep diving.

Sea turtles are anatomically adapted to life at sea: completely hydrodynamic, with a shorter neck and a smaller carapace mass than its land and sweet water relatives, and non-retractile head and limbs. Their front flippers are flattened and are used for propulsion, while the back flippers, short and flat, work as a rudder. The nares are fully closed under the water to avoid water entering the mouth and respiratory system.

But it is their unique physiological adaptations that allow a very efficient and fast gaseous exchange when the animal breaths at the surface, and maximum storage of oxygen in the body while diving.

Great capacity to store oxygen

When a sea turtle carries out shallow dives most of the oxygen is stored in the lungs, from where it goes to the bloodstream and tissues. However, for the deep dives, sea turtles depend on the oxygen stored in the blood and muscles. Sea turtles (especially the leatherback) show a high concentration of hemoglobin and red blood cells in the blood, and of myoglobin in muscles, which allows them to store a high amount of oxygen in the body, easily available for the tissues during the dive. The lungs also show a large surface for gas exchange, facilitating the entrance of oxygen into the blood with each breath. All this makes a sea turtle breath much more efficient than ours.

Good management of oxygen

Sea turtles, as cold-blooded animals (they cannot thermoregulate and depend on the external temperature – with different aspects that we’ll explain in another post), show a slower metabolism than mammals. This allows them to resist longer with the same amount of oxygen than, for example, a seal. In addition, when preparing for deep dives, sea turtles activate a “diving reflex”, which allows them to reduce the heart rate and limit the blood supply to those organs and tissues that are essential for diving, thus further reducing oxygen consumption. Lastly, they are extremely tolerant to hypoxia. Especially the brain, which at least in the loggerhead sea turtle, and contrary to that of marine mammals, has been proved to survive up to several hours without oxygen.

And how can they stand the pressure at depth, which affects us so much?

Contrary to what it seems, the carapace of sea turtles is not completely fused with the plastron (the ventral part); there is a small space with connective tissue which allows for some degree of movement and compression. This comes to maximum expression in the leatherback turtle, which does not have an osseous carapace. Furthermore, the respiratory system (lungs and airways) of sea turtles collapses at depths of around 80-160m, reducing the problem of narcosis due to dissolved nitrogen, or decompression syndrome during ascend after long, deep dives. Some air remains in them, but in areas where there’s no gas exchange. Blood circulation into the lungs is also interrupted during diving. However, leatherback turtles have been seen to modify the time spent at different depths and to ascend slowly from deeper dives, avoiding thus the formation of nitrogen bubbles on the ascent.