Sea turtles, or marine turtles, are shelled reptiles with streamlined bodies found in all oceans except in polar regions. They belong to the order Testudines and superfamily Chelonioidea (under the suborder Cryptodira). The seven species of sea turtles are classified under two families: Dermochelyidae (leatherback sea turtles) and Cheloniidae (green turtles, flatback sea turtles, loggerhead sea turtles, hawksbills, Kemp’s and Olive ridley sea turtles). While members of the first family have leathery shells, those of the latter are hard-shelled.
These turtles are long-lived and are extremely susceptible to both anthropological and environmental factors, like poaching for skin, bycatch, and climate change, among others. Currently, six species are listed as endangered, critically endangered, or vulnerable on the IUCN Red List.
The leatherback is the largest sea turtle, which measures about 4.6 and 5.9 ft (1.4 to 1.8 m) in length and weighs approximately 661 to 1,411 lb (300 to 640 kg). The green sea turtle, the second largest species, reaches about 3.9 ft (1.2 m) in length. All other species are smaller, with the Kemp’s ridley being the smallest, measuring just 2 ft (0.6 m) in length. For all species, the females and males are of the same size.
As aquatic animals, these turtles have fusiform or spindle-shaped bodies, streamlined shells, forelimbs modified into flippers, and webbed hindlimbs that function as rudders. Unlike their freshwater cousins, sea turtles cannot retract their heads and limbs into their shells.
An adult male sea turtle is distinguished from an adult female by its long tail, with the cloacal opening located near the tip. In contrast, the female has a shorter tail, with the cloacal opening positioned closer to the base.
The most notable feature of a sea turtle’s body is its shell. Its top, the carapace, and its bottom, the plastron, each consists of two separate layers. The first layer comprises flattened bone plates, while the second layer contains firm yet flexible keratin scales called scutes that prevent water loss from their bodies. Besides the shape of the shell and scales, the different species of sea turtles are also distinguished by the number and pattern of scutes.
The leatherback has a distinct rubbery shell with a waxy, leathery, and semi-flexible carapace. They lack the second layer of scutes and instead have a blubbery fat layer in which thousands of small bone plates are embedded.
Their eyes feature flat corneas and nearly spherical lenses containing two types of cells: rods and cones. Rods are responsible for detecting movement in dim light, while cones enable the detection of colored light. To explore the darker depths of the ocean, these turtles have large pupils, which help them locate bioluminescent prey.
Though not visible, sea turtles possess ears covered by a protective layer called the tympanum. Additionally, a layer of fat unique to sea turtles lies beneath the tympanum that helps transmit sound vibrations to the inner ear.
The name of the superfamily Chelonioidea is derived from the Ancient Greek word khelōnē, which means tortoise. The seven species of sea turtles are grouped under two families, Dermochelyidae and Cheloniidae. All six species except the Leatherback sea turtle are part of the family Cheloniidae.
A list of all extinct genera of sea turtles is given below:
The earliest sea turtle fossils, such as those of the genus Plesiochelys, date back to the Late Jurassic Period, around 150 million years ago. In Africa, the first marine turtle discovered was a member of the genus Angolachelys, unearthed in Angola. It is believed that modern sea turtles evolved from a single radiation event originating from one group of ancestral marine turtles. The oldest known species from which modern sea turtles are thought to have evolved is Desmatochelys padillai, from the Early Cretaceous Period. This species belonged to the extinct family Protostegidae, which disappeared by the end of the Cretaceous. The earliest true sea turtles are attributed to the genus Nichollsemys, also from the Early Cretaceous. Recently, other significant specimens have been found, including Leviathanochelys (an extinct genus) in Spain.
They are found in all oceans (except in the polar regions) but prefer inhabiting the waters that lie above continental shelves. However, some species show regional localization. For example, the flatback sea turtle is exclusively found on the northern coast of Australia, Papua New Guinea, and Indonesia, whereas the Kemp’s ridley is restricted to the Gulf of Mexico and along the East Coast of the United States.
In their early life (first 3 to 5 years), sea turtles inhabit the pelagic zone, floating in seaweed mats. For example, green sea turtles are particularly found in Sargassum mats.
However, as they become adults, they move toward the shore and reside on coastal beaches for egg-laying. The green sea turtle is also found basking on these beaches and other terrestrial habitats.
Sea turtles use their large, powerful front flippers like paddles to propel themselves through the water, while their hindlimbs function as rudders to help with steering. Although they typically swim at speeds of 0.9 to 5.8 mph, they can quickly accelerate to 22 mph when startled or escaping a predator.
These turtles spend most of their time underwater and are thus experts at holding their breaths for long periods. For instance, a foraging sea turtle can hold its breath for up to 40 minutes underwater, whereas a sleeping sea turtle can remain underwater for up to seven hours.
However, since these turtles possess lungs, they need to swim to the surface for respiration. When a turtle reaches the surface, it rapidly fills its lungs through quick inhalation. The large size of their lungs helps in the rapid exchange of oxygen and prevents the trapping of gases at greater depths.
Sea turtles attain sexual maturity around 10 to 20 years, and both males and females migrate miles to the breeding site at the sea. After mating, thousands of females (most notably the ridley turtles) travel to the shore (arribada), usually at night, to lay their eggs on the beach. They mostly exhibit philopatry and return to the same nesting place at the same time each year.
After arriving at the beach, the female finds a suitable nesting site on the sand and digs a deep, circular hole (16 to 20 in) in the ground with her hind flippers. She then lays about 50 to 350 eggs (depending on species) per clutch in the hole and re-fills it with sand, smoothing its surface and dumping vegetation to camouflage the nest and keep it safe from predators. This entire process takes about 30 to 60 minutes, after which the female leaves for the sea.
The eggs incubate in the heat of the sand for about 50 to 60 days. The sand temperature determines the hatchling’s sex (temperature-dependent sex determination). All eggs in a single nest hatch within a short period. The juvenile turtles of most species hatch at night, except the Kemp’s ridley sea turtle, which usually hatches during the day. These hatchlings dig upward through the sand and venture into the sea.
The hatchlings immediately drink a lot of seawater to replenish water lost during hatching. They remain pelagic for about 5 to 10 years and then return to warmer shoreline waters to continue their growth and attain maturity.
Like all turtles, sea turtles have a long lifespan, averaging 50 to 100 years. The oldest recorded sea turtle was around 400 years old and was kept in captivity at an aquarium in Guangzhou, China.
Although adult sea turtles have few predators, their eggs and hatchlings are extremely susceptible to predation. Raccoons, foxes, and sea birds often raid their nests and kill their hatchlings, whereas large fish and other sea turtles eat those that venture into the water. As per estimates, only one out of 1,000 hatchlings survives and grows into an adult.
Sharks (especially tiger sharks) and crocodiles have been found feeding on adult sea turtles, while jaguars also smash open their shells and scoop out their flesh.
According to the IUCN Red List, the flatback sea turtle is data deficient (DD), whereas the hawksbill and Kemp’s ridley turtles are critically endangered (CR). Moreover, the green turtle is considered endangered (EN), and the remaining three species (Olive ridley, loggerhead, and leatherback turtles) are vulnerable (VU). Similarly, all seven species have been listed in CITES Appendix I, and all populations inhabiting the waters of the United States are considered threatened or endangered by the US Endangered Species Act (ESA).
As the above data hints, there are multiple factors adversely affecting sea turtle populations worldwide.
To prevent further decline in the sea turtle population, conservation and management measures are being undertaken globally.
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