Leatherback Turtle

The leatherback sea turtle (Dermochelys coriacea), also known as the lute turtle, leathery turtle, or luth, holds the title for being the largest living turtle and the heaviest non-crocodilian reptile. It can grow up to 1.8 meters (5 ft 11 in) in length and weigh up to 500 kilograms (1,100 lb). It is the sole species in the genus Dermochelys and the family Dermochelyidae. Its unique feature is the absence of a bony shell, replaced by flexible, leather-like skin, which gives it its name.

Leatherback turtles have maintained a similar appearance since the first true sea turtles evolved over 110 million years ago in the Cretaceous period. They are closely related to the family Cheloniidae, which includes the other six extant sea turtle species. However, their closest extinct relatives come from the Protostegidae family, which lacked a hard carapace.

This species possesses a highly streamlined body, with a large, teardrop-shaped physique. Its front flippers, the largest compared to its body among all sea turtles, provide the power for efficient swimming. Similar to other sea turtles, the leatherback has flattened forelimbs adapted for open ocean swimming and lacks claws on its flippers. Its carapace is covered by a layer of oily flesh and flexible, leathery skin, distinguishing it from other sea turtle species. The turtles' dorsal surface is dark grey to black, with scattered white blotches and spots, while the underside is lightly colored, demonstrating countershading. Instead of teeth, the leatherback has spines in its throat (esophagus) to facilitate swallowing and prevent prey from escaping.

The carapace of the leatherback sea turtle has a distinct design that enables it to endure high hydrostatic pressures when diving to depths of 1200 meters. Instead of a hard, keratinous shell, it has a soft, leathery skin covering its osteoderms. The jagged-edged osteoderms, composed of bone-like hydroxyapatite/collagen tissue, are linked by interpenetrating extremities called sutures, providing flexibility and allowing movement between osteoderms. This flexibility is crucial as the lungs and carapace expand during breathing and contract during deep dives. The triangular tooth geometry evenly distributes load and absorbs energy, resulting in high tensile strength. Cracks in the carapace are impeded by the sutures, which resist crack propagation through crack bridging.

As a cosmopolitan species, the leatherback sea turtle has a global range. It can be found as far north as Alaska and Norway and as far south as Cape Agulhas in Africa and the southernmost tip of 🇳🇿 New Zealand. It occupies tropical and subtropical oceans, extending its range into the Arctic Circle. Distinct populations exist in the Atlantic, eastern Pacific, and western 🌊 Pacific Oceans. However, leatherback populations in the 🌊 Indian Ocean remain largely unassessed. Global nesting population estimates have shown a dramatic decline from 115,000 in 1980 to 26,000 to 43,000 females nesting annually.

Leatherback turtles primarily inhabit open ocean environments. They track their jellyfish prey, which influences their depth preferences. To avoid predation and heat stress, they tend to occupy deeper waters during the day and shallower waters at night, when jellyfish rise to the water surface. This hunting behavior exposes them to frigid temperatures, with turtles actively foraging in waters as cold as 0.4 °C (32.7 °F). They return to warmer surface waters to regain body heat between foraging dives. Leatherbacks dive to depths exceeding 1000 meters, surpassing the physiological limits of most other diving tetrapods, aside from beaked whales and sperm whales. They exhibit a preference for mainland nesting beaches facing deep water, avoiding reef-protected sites.

Leatherback turtles feed primarily on jellyfish and play an essential role in controlling jellyfish populations. They also consume other soft-bodied organisms, including other cnidarians, tunicates, cephalopods, small crustaceans, fish, sea urchins, and snails. The species migrates approximately 6,000 miles across the Pacific from 🇮🇩 Indonesian nesting sites to California, where it feeds on jellyfish. A significant threat to the leatherback population arises from the ingestion of plastic bags, which are mistaken for jellyfish. Up to one-third of adults have ingested plastic, contributing to their endangered status. Plastic marine debris obstructs sea turtles' digestive tracts, and even small amounts can be fatal. Ingestion impacts nutrient gain and growth, potentially delaying sexual maturation and affecting future reproductive behaviors. The highest risk of encountering plastic bags occurs offshore of San Francisco Bay, the Columbia River mouth, and Puget Sound.

The lifespan of leatherback turtles remains largely unknown, with estimates varying from "30 years or more" to "50 years or more." Some suggest that they may live upwards of 100 years.

When dead leatherbacks wash ashore, their decomposing bodies become microecosystems. Beetles and flies, including carrion-eating species and those feeding on decaying matter, infest the carcasses. Organisms from over a dozen families participate in consuming the remains.

Mating in leatherback turtles occurs at sea. While males remain in the water, females come ashore to nest. The male uses various behaviors, such as head movements, nuzzling, biting, or flipper movements, to determine the female's receptiveness after encountering her, possibly facilitated by a reproductive status pheromone. Males can mate annually, while females mate every two to three years. Fertilization takes place internally, with multiple males often mating with a single female. Polyandry in mating does not confer any special advantages to the offspring. Female leatherbacks have the shortest internesting interval among sea turtles, nesting up to 10 times in a single breeding season.

Although most sea turtle species return to their hatching beach, leatherbacks may choose a different beach within the region. They prefer beaches with soft sand to protect their softer shell and plastron from damage caused by rocks. Nesting beaches should also have a gentle slope from the ocean. However, these beaches are susceptible to erosion. Leatherback nesting occurs at night when the risk of predation and heat stress is lowest. Their eyes, adapted to ocean conditions, do not perform well in low-light terrestrial environments. Ideal nesting locations comprise dark, forested areas bordering the beach, providing contrast between the dark land and the brighter, moonlit ocean. The females navigate towards the dark forest when nesting and head back to the ocean and its light.

Using their flippers, females excavate nests above the high-tide line. A female may lay up to nine clutches in a breeding season, with approximately nine days between nesting events. The average clutch size is around 110 eggs, of which 85% are viable. Once laid, the female carefully covers the nest, disguising it from predators with scattered sand. However, around 50 percent of the eggs do not develop into hatchlings, posing challenges for management and conservation efforts.

Embryonic development in leatherback sea turtles begins shortly after fertilization, with cleavage of the cells. Gastrulation, characterized by movements and infoldings of embryonic cells, occurs while the eggs are being laid, causing development to pause. Resuming development, the embryos remain highly susceptible to movement-induced mortality until the membranes fully develop after approximately 20 to 25 days of incubation. Organogenesis, the structural differentiation of body and organs, follows. The eggs hatch after around 60 to 70 days. Similar to other reptiles, the temperature of the nest determines the sex of the hatchlings