Turtles are found in temperate and tropical waters around the world. Worldwide there are eight species of marine turtle, three of these, the Loggerhead, the Hawksbill and the Green Turtle are often seen at Julian Rocks and in the Bay

Turtles grow very slowly and take a long time to mature. All three species are endangered. Their life patterns are broadly similar. Occasionally turtles nest in this area but the main rookeries are further north. Turtle eggs are about the size of a golf ball, and white with soft shells. They are a traditional food in many societies. From the time that they are laid the eggs take 8-10 weeks to hatch. The exact time depends on the species and on the temperature of the nest. When the temperature is higher the incubation period is shorter. The gender of the hatchlings is also temperature dependent. More males result from lower temperatures and more females from higher ones. By the time the 4-5cm hatchlings emerge, their parents have left and so, without instruction, they scurry down the beach to the ocean and disappear. They are rarely seen again until they are much bigger and return to inshore waters as juveniles. Little is known about where they go or what they do during the Lost Years, but it is suggested that they live on the surface near sargassum mats.

Turtles have two shells, the one underneath is called the plastron and the one on top (on the back) is called the carapace. The size of a turtle is usually described by its curved carapace length (CCL), that is, the length of the shell measured along the curved surface, down the centre line. Green and Hawksbill turtles return to inshore waters as juveniles at about 35cms CCL which is perhaps 5 years old, Loggerheads reappear at about 80 cms CCL, which may be 20 years old.

The turtle population at Julian Rocks is largely juvenile, although there are some adults, especially among the male Loggerheads. Turtle growth rates depend on food supply and on other environmental conditions. Green turtles have a growth spurt at about 60cms CCL, when they may grow by as much as 3cm/yr. From there on growth slows and after maturity growth rates are negligible.

The external appearance of male and female juveniles is very similar. Adult male turtles can be identified by their large tails and long claws. Adult turtles return to the place of their birth to breed. Whilst showing a great fidelity for place, they are quite promiscuous. The males will mate with several females and vice-versa. A male will mount a female in the water and the female will store the sperm in the oviducts. After fertilisation the female will remain in the water for about two weeks as the eggs are prepared for laying. The female will then go ashore and dig a nest in the sand in which to lay the eggs. A clutch will consist of 100-150 eggs depending on species. The eggs in the nest belong to one mother but possibly several fathers. After laying, the female returns to the water to produce another clutch of eggs. A female may produce between three and five clutches per season. Turtle sex can be a bit rough. The females can incur bite marks on their necks. The males damage their flippers and can incur tail bites from competing suitors. At the end of the breeding season the turtles return to their own preferred feeding grounds which may be a considerable distance away. Males breed every year or two and females at intervals of two to eight years. There is no parental care of the young.