Sea turtles (Chelonioidea) are marine reptiles that inhabit all of the world's oceans except the Arctic. Their relatives on land are tortoises, of which there are 37 living species, although none live in Australia. There are 181 species of freshwater turtles of which 20 are found in Australia. Fresh water turtles are smaller than marine turtles (15-45cm), and have webbed feet allowing them to be mobile in water and on land.
Marine turtles migrate long distances between their feeding grounds and nesting sites. They have a large shell (carapace), four strong, paddle-like flippers and, like all reptiles, lungs for breathing air. Hard scales (scutes) cover all but the leatherback. The characteristic beak-like mouth is used to shear or crush food.
They hear best at low frequencies, and their sense of smell is excellent. Their vision underwater is good, but they are nearsighted out of water. Their streamlined bodies and large flippers are adaptations to life at sea.
Although all sea turtles breathe air, under extreme circumstances they may divert to anaerobic respiration for long periods of time. When surfacing to breathe, a sea turtle can quickly refill its lungs with a single explosive exhalation and rapid inhalation. Their large lungs have adapted to permit rapid exchange of oxygen and to avoid trapping gases during deep dives.
Sea turtles possess a gland to remove excess salt from their bodies as their diets are high in salt. The gland is found at the corner of the eye, in the nostrils, or in the tongue depending on the species.
Sea turtles are generally solitary creatures that remain submerged for much of the time they are at sea, they feed and rest off and on during a typical day. They can sleep at the surface while in deep water or on the bottom wedged under rocks in nearshore waters.