This large, thoroughly aquatic turtle rarely comes to land except to bask and sleep and to lay eggs. Males have slightly longer, narrower carapaces than females and enlarged curved claws on the front flippers for gripping the female when mating. Green turtles are primarily herbivorous animals and have serrated jaw surfaces, well suited to feeding on sea grasses and seaweed; some crustaceans and jellyfish may also be eaten.
The best feeding grounds, where there are vast underwater pastures of plants, are often far away from the best nesting beaches, and green turtles have evolved astounding migratory habits. At nesting time they travel hundreds of miles to the beach of their birth to lay eggs, and as a result, there tend to be a limited number of important nesting sites, to which hundreds of turtles go. One such site is Ascension Island in the mid-Atlantic. Every second or third year, green turtles travel to their nesting site and mate. The female heaves herself up the beach well away from the tidal area. With her foreflippers she sweeps away sand to create a hollow to lie in, her shell flush with the beach. She then uses her hind flippers to dig a hole about 40 cm (16 in) deep, immediately beneath her tail. She deposits her eggs into the hole, covers the area with sand and returns to the sea. The average clutch contains about 106 eggs. Sometimes a female lays several clutches in a season at 2-week intervals. After a 2- to 3-month incubation period, the young turtles hatch and dig their way through the sand to the surface. Having oriented themselves, they rush for the sea, past a horde of eager predators. Mortality is high, and those which do reach the sea will have to face yet more predators.
The green turtle is now an endangered species, and the population has been eliminated in some areas, although it is still reasonable in others. The turtles have been overexploited for their meat, hides and eggs, and the predictability of their nesting habits has made them easy victims. Exploitation is now strictly controlled, and imports are banned in many countries. The closely related flatback turtle, C. depressa, is a little smaller than the green turtle and lives off the coast of Northern Australia.