Sea turtle shells (carapace) are strong, light and streamlined and provide protection, buoyancy and the ability to slip through the water with minimum effort.
The back or top shell is called the carapace and the bottom is called the plastron.
The shell is made of flattened bones covered in horny plates called scutes, with the exception of leatherback turtles that have a shell made from many small bones embedded in a leathery skin.
Unlike most land turtles, sea turtles cannot retract their heads into their shell for safety. They have exchanged this feature for powerful fore (front) flipper muscles that propel them at great speed through the water. Their muscles take up so much room inside their shell that there is no room for their heads.
Their strong, paddle shaped fore flippers power them through the water whilst the smaller rear flippers are used for steering and digging nest chambers.
The ‘tears’ often seen around a sea turtle’s eyes when it is on land come from special glands located near the tear ducts that excrete excess salt ingested while feeding underwater.
Sea turtles have no teeth. Their mouths have evolved to either shear food such as sponges and sea grass in the case of hawksbill and green turtles, or to crush food such as crustaceans and gastropods in the case of loggerheads and olive ridley turtles.
Although much is known of their activities when they come ashore to nest, little is known of their ocean activities.
The young of most sea turtle species drift and feed in the open ocean until they reach about 5-10 years old and about 30 centimetres in length when they settle at inshore feeding grounds.
Sea Turtles grow slowly and – depending on species – take between 11–40 years to reach sexual maturity.
They can live for years in the one place, often returning to the same location to sleep.
They make long migrations, which can be hundreds and even thousands of kilometres, from feeding grounds to nesting beaches. Males mate with females along the route and close to nesting beaches.
Females return to nest on the same beach, or beaches nearby, on which they hatched. How they achieve this remarkable feat of navigation is poorly understood, however it is thought they use a combination of clues such as ‘reading’ the earth’s magnetic field and ocean currents, and even judging the angle of the sun.
Most female sea turtles breed only every 2-3 years, as an annual breeding pattern would be too exhausting due to long migrations and the energy required to produce and lay many hundreds of eggs.
Hatchling gender is dependent on sand temperature. Cooler sands produce more males, warmer sands produce more females.
Recommended further reading: Sea Turtles: A complete guide to their biology, behaviour and conservation (James R. Spotila; Johns Hopkins University Press).