The leatherback sea turtle (Dermochelys coriacea) is the largest living and most widespread species of turtle. Adults can grow to just over 6 feet (2 meters) in total body length and weigh as much as 2,016 pounds (913 kilograms)! The only living reptile that can surpass the size of these marine behemoths is the saltwater crocodile, which can grow to 23 feet (7 meters) in length and weigh more than 3,000 pounds (1,400 kilograms).
This unique species is the most specialized of all the living turtles in the world. In fact, it was once classified in a group of reptiles separate from all other turtles. The most distinguishable characteristics of D. coriacea are a carapace that lacks a hardened layer of horny scutes and a completely scale-free body. The bony elements of a leatherback’s shell have been reduced to little more than a mosaic of small, irregularly shaped bones called ossicles. The resulting feel of their shell is that of thick, rubbery leather. The coloration of the carapace may be slate gray, black, bluish black, or brown, with scattered white to yellowish specks. The head may be slate gray to black with speckling present. The chin and ventral portion of the neck are noticeably lighter in color and have a higher concentration of speckling. The limbs are grayish black with the heaviest amount of spotting on the anterior portions of the flippers.
The carapace is elongated and lyre-shaped; it tapers to a point just above the tail. The highest and widest point of the carapace is just above the shoulders. Five longitudinally oriented ridges formed of tightly fitted osteoderms run the length of the carapace, while a prominent medial keel extends the entire length of the shell. Shallow troughs are present between the dorsal ridge. Leatherback hatchlings bear scales on the limbs and head. These scales are lost after the young turtles shed their skin at twenty-six and forty-six days of age. The limbs are devoid of any claws such as those found on the flippers of other sea turtles. The ventral surface of the leatherback sea turtle is also relatively soft. In Suriname, South America, this characteristic has played a role in the occasional death of females. While venturing out of the Atlantic Ocean and onto beaches at night, they have accidentally impaled themselves on driftwood. The head of the leatherback is large, and the neck is short and wide. This sturdy design affords very little flexibility but enhances the body’s aerodynamics in the water.
The leatherback’s digestive system is specialized to feed on the vast number of jellyfish that drift afloat in the oceans. The turtle’s beak is tricuspid, and the inside of the mouth is lined with spiny, horny projections directed toward the rear of the mouth. These projections continue from the inside of the mouth into the looped esophagus, which can measure over 7 feet (2 meters) in length and has a descending and ascending portion. The internal organs, such as the esophagus and stomach, appear to be quite flexible considering that leatherbacks ingest vast amounts of seawater while consuming jellyfish, and then eject the former. The projections lining the esophagus and mouth help prevent the loss of the turtle’s slippery feast when the water is expelled. Given the meager constitution of most jellyfish, it is mind-boggling to imagine the sheer volume of the invertebrates these enormous turtles must consume to satisfy their nutritional requirements.
Beneath the leathery flesh and ossicles of the carapace is a large amount of cartilage that is rich in blood vessels. Unlike other turtle species, the ribs of Dermochelys are not fused to the inside of the carapace. Instead, they are buried within a layer of cartilage, fat, and ossicles. The family Dermochelyidae is known from fossil deposits from the Eocene Epoch. Recently, scientists have begun to understand more regarding the past diversity of leatherback turtles; however, one major obstacle challenges these paleotological pursuits. The habitat and body of a prehistoric creature greatly affects how and where its remains settled. Dermochelys has always dwelled in the open oceans, and its carapace has always been comprised of a mosaic of bony plates. While the remains of some ancient leatherbacks fossilized, it is likely that many more simply decomposed amid the elements and the scavengers of the ocean.
What is known about these ancient turtles is that there were at least four genera and nine species belonging to the family Dermochelyidae. Just as more leatherback fossils are likely to be discovered eventually, a better insight into the past of this now monotypic family will be gained.
While Dermochelys is a type of sea turtle, it can be more accurately described as a marine reptile. Unlike the other sea turtles, the leatherback does not inhabit the continental shelf or reefs. Instead, it is truly a restless wanderer of the open seas.
The leatherback sea turtle has a worldwide distribution and occurs in all temperate and tropical oceans. These turtles routinely travel great distances and can move at a rate of 112 miles (70 kilometers) per day. One specimen that was tagged during a study of nesting females in Suriname was found four months later and 2,700 miles (4,345 kilometers) away in the cold waters off the coast of Newfoundland, Canada!
Everything about the physical composition of the leatherback is designed to enhance its ability to live in a marine environment and make adjustments for dramatic changes in pressure and temperature.
These turtles are powerful swimmers and possess a unique physiology that allows their muscles to work at an almost constant rate. Many reptile species completely tire out after even a relatively short burst of physical activity. This reaction is due to the depletion of oxygen levels in the muscles and the increase of lactic acid levels in the blood. Unlike other sea turtles, leatherbacks have a physiology that allows their muscle tissues to remain oxygenated and capable of working for long periods of time. The turtle’s large lung capacity is the main reason that they can survive in this manner. In fact, leatherback sea turtles have the capacity for twice the air volume as do green sea turtles.
When female leatherbacks move on land, they do so more efficiently than other sea turtles. Once again, their special physiology proves helpful. Even during the process of laying eggs, the leatherbacks utilize their versatile physiology and breathe with Lamaze-like patterns.
Aside from their special adaptations for enhancing respiration and supplying muscles with oxygen, these turtles also possess an enhanced circulatory system. Their blood has high levels of hemoglobin. Hemoglobin is a protein transported by the red blood cells to the lungs. The hemoglobin then carries the oxygen to the cells in need.
Unlike other turtle species, leatherbacks do not need to bask at the surface between dives. Instead, they maintain a stable body temperature through their low metabolic rate, large body size, and ability to alter their blood circulation. Changing the circulation of the blood is necessary in freezing waters and during dives in freezing temperatures. Blood flow can be restricted from certain parts of the body to maintain a core temperature. Adaptations such as these allow the leatherback to descend to more than 3,000 feet (1,000 meters) up to fifty times a day!
Female leatherbacks have a tail that is smaller than the hind feet. They also feature a considerable suffusion of pink coloration on the head. The pink is due to an increase in blood flow. Males have a concave plastron and a tail that is longer than their hind feet. Males also have a narrower carapace and lack any pink pigmentation on the head.
The reproductive cycle of D. coriacea is a well-studied and predictable event. When the time to lay eggs arrives, the female makes her way to the beach. After selecting a desirable location for her nest, she begins the excavation process using her hind flippers. Upon completion of digging the nest, she deposits her eggs. Nesting females emit a range of sounds. Hisses, sighs, and even sounds similar to human belches have been recorded. The female produces 100 to 120 spherical eggs per clutch, six to seven times annually. The incubation period requires fifty to seventy-five days. Upon hatching and emerging from the nest, the young turtles instinctively head toward the water. This event attracts a large number of opportunistic predators, such as birds, mammals, lizards, fish, and crabs. The gauntlet of predators eliminates many hatchlings before and after they enter the water. The high mortality rate faced by hatchlings is counteracted by the large number of eggs that this species produces.
Due to their large proportions, leatherbacks have few natural enemies except for humans. However, at least one specimen fell victim to a natural predator. The turtle was a nesting female in India’s Andaman Islands. A herpetologist witnessed the turtle on the beach and saw that it did not move throughout the night. Closer inspection revealed that the turtle was dead. Its hind flipper had been bitten off and the turtle had bled to death. The footprints of a large saltwater crocodile led away from the scene. Over the course of several more days, the crocodile returned to feed upon its feast.
D. coriacea is currently listed as one of the most endangered species by the Convention of International Trade of Endangered Species Act. It is also listed as endangered by the International Union for the Conservation of Nature and Natural Resources (IUCN). The grave reality concerning this species is that it seems to be on an irreversible path toward extinction. Surveys conducted in 1995 revealed between 26,000 and 42,000 nesting females worldwide. These numbers were considerably fewer than the 1980 estimate of 115,000 nesting leatherbacks.
Maintaining this species in captivity long-term is currently beyond the scope of modern zoological institutions. One of the many factors challenging their suitability in captivity is that leatherbacks remain in almost constant motion. Hatchlings continuously swim against the sides of their enclosure and pause only when offered food.
This species is extremely sensitive to human activities and people are completely responsible for its endangered status in the wild. Current leatherback populations cannot tolerate any further pressures without going extinct. We should all endeavor to witness the ancient cycle of sea turtles returning to land in order to nest while we still can and, if possible, help preserve the existence of this fascinating, ancient species.