sea turtle, name for several species of large marine turtles found in tropical and subtropical oceans. These turtles are modified for life in the ocean by having flipperlike forelimbs without toes and lightweight shells. Their heads are too large to be withdrawn into the shell. They spend most of their lives in the water, but come ashore to lay their eggs. Nearly all species have a circumglobal distribution although there are differences between the Atlantic and the Pacific populations. Most sea turtles are found in North American waters as far N as the U.S. S Atlantic coast and Baja California.

All sea turtle species are declining in numbers, owing in large part to the destruction of the eggs, which are widely used as food in tropical regions. Sea turtle meat is also eaten, and there is a market for turtle oil, hide, and shell. In a few places, such as Sarawak, harvesting of eggs is regulated by law to insure propagation of the species. The size of sea turtles has also decreased, owing to the hunting of large specimens.

The green turtle, Chelonia mydas, with greenish to brownish skin and shell, formerly reached weights of 1,000 lb (450 kg); the largest now found are about 4 ft (120 cm) long and weigh about 500 lb (225 kg). The green turtle feeds chiefly on marine vegetation and is most abundant in shallow water. The loggerhead, Caretta caretta, is a large-headed brown to reddish turtle. Chiefly carnivorous, it ranges from open oceans to coastal salt marshes and stream mouths. Like the green turtle it sometimes comes ashore in uninhabited places to bask.

The hawksbill, or tortoiseshell turtle, Eretmochelys imbricata, may reach 30 in. (75 cm) in length and weigh 100 lbs (45 kg). The horny plates of its shell are translucent and have a variegated color pattern, chiefly brown and yellow. These plates have long been valued for the making of ornamental objects. Although tortoiseshell has to a large extent been replaced in many applications by plastic, a renewed demand for the genuine material poses a serious threat to the hawksbill.

The ridleys, the smallest sea turtles, inhabit shallow offshore waters. The 2-ft (60-cm) long, gray Atlantic ridley, Lepidochelys kempii, breeds only in the Gulf of Mexico, although the young are often carried by the Gulf Stream to the Carribean Sea and Europe. The slightly larger, greenish Pacific ridley, or oliveback (L. olivacea), is found in the Indian and Pacific oceans. The leatherback is the largest of all turtles; it belongs to a separate family from the other sea turtles.

All sea turtles other than the leatherback are classified in the phylum Chordata, subphylum Vertebrata, class Reptilia, order Chelonia, family Chelonidae; the leatherback belongs to the family Dermochelidae.

See J. R. Spotila, Saving Sea Turtles (2011).