Sea turtles belong to the Class Reptilia and Order Testudines. The seven species of living sea turtles are a monophyletic group (derived from a common ancestor that has not given rise to other living turtles) of the suborder Cryptodira. Fossil records indicate that turtles first appeared on land about 200 million years ago. Sea turtles are believed to have originated in the lower Mesozoic era and evolved from land-based turtles. During the late Cretaceous period (about 65 million years ago), four distinct families of sea turtles are believed to have existed, of which two families survived into the current era: Dermochelyidae and Cheloniidae.
Sea turtles are distributed mostly in tropical and subtropical waters of the world’s oceans and they depend on land only during the reproduction stage. They are air breathing vertebrates like birds and mammals but have returned to a near complete life in the water. They are considered highly derived morphologically and have many adaptations for life in the sea.
Swimming: All species share features such as paddle-shaped limbs and a streamlined shape with an enlarged shoulder girdle and well-developed pectoral muscles which aid in swimming. Sea turtles are also excellent divers. Leatherbacks routinely dive to depths exceeding 1000 feet in search of jellyfish. During long dives, blood is shunted away from tissues tolerant of low oxygen levels toward the heart, brain, and central nervous system. Unlike other cryptodires, sea turtles have a reduced ability to retract their heads; the shell adaptations necessary for retractile limbs would impede rapid swimming.
Respiration and metabolism: Sea turtles are known to have a reduced metabolic rate which allows them to stay underwater for long periods of time before needing to resurface to breathe. They are also equipped with enlarged lacrimal or tear glands modified to remove excess salts from body fluids. They are able to live in seawater without the need for a freshwater source as they can obtain sufficient water from their diet and from metabolising seawater.
Thermoregulation: A sea turtle’s large size leads to a low surface area to volume ratio, so the heat exchange rate is low compared to total size. The thermoregulatory adaptations of leatherback turtles in particular, such as a counter-current heat exchange system, high oil content, and large body size, allow them to maintain a core body temperature higher than that of the surrounding water (up to 180C above their surroundings), thereby allowing them to tolerate colder water temperatures.
Navigation: A wide range of theories have been suggested to account for the ability of some sea turtles to migrate in the open ocean between feeding and nesting grounds sometimes separated by thousands of kilometers. Commonly held among the suggested theories is that when hatchlings emerge, they are “imprinted” on the earth’s geomagnetic field. Using their ability to differentiate magnetic field intensities and inclination angles, they are able to migrate back to their natal beaches as adults.