Marine turtles are often called the ancient mariners of the sea. They have been swimming in the sea for more than 150 million years, first appearing during the age of the dinosaurs.
Turtles have changed little over the millennia, now only coming ashore to lay eggs, producing another generation to swim the seas.
Much of the information known about marine turtles in the Great Barrier Reef World Heritage Area comes from research conducted over the past 30 years by the Queensland Turtle Conservation project of the Department of Environment and Heritage Protection and from the local knowledge of Indigenous people and fishers.
Marine turtle descriptions
Six of the world’s seven species of marine turtle live in the waters around Australia, and all occur within the Great Barrier Reef World Heritage Area. Some species such as the loggerhead and green turtle are seen frequently, while others such as the olive ridley and leatherback are known to occur in the Great Barrier Reef but are seldom seen. You can find out how to distinguish each species with the marine turtle identification sheet.
The conservation status of marine turtle species found in the Great Barrier Reef World Heritage Area has been assessed by IUCN (the World Conservation Union), and by the Australian and Queensland governments.
|Scientific name||IUCN (World Conservation Union)1|| Australian |
Conservation Act 19992
| Queensland |
|Loggerhead|| Caretta |
| Critically |
|Olive ridley|| Lepidochelys|
1IUCN Red List categories: Extinct, Extinct in the Wild, Critically Endangered, Endangered, Vulnerable, Lower risk, Data deficient (Source: 2008 IUCN Red List of Threatened Animals).
2Australian Environment Protection and Biodiversity Conservation Act 1999 categories: Extinct, Extinct in the eild, Critically endangered, Endangered, Vulnerable, Lower risk, Data deficient.
3Queensland Nature Conservation (Wildlife) Regulation 1994 schedules: Presumed extinct, Endangered, Vulnerable, Rare, Common.
Marine turtle life cycle
All species of marine turtles have the same general life cycle. They grow slowly and take decades to reach sexual maturity.
As immature turtles, they may drift on ocean currents for many years or live for years in the one place before maturing and making a long breeding migration of up to 3000km from their feeding ground to a nesting beach.
At an unknown age (believed to be between 20 and 50 years) male and female turtles migrate to a nesting area located in the region of their birth. Both male and female turtles mate with a number of partners. The females store sperm in their bodies to fertilise the three to seven clutches of eggs laid during the season.
Mating generally takes place offshore a month or two prior to the turtle's first nesting attempt for the season, usually in summer.
Male turtles generally return to their foraging areas once the females commence their fortnightly trips to the beach to lay eggs. When ready, a female turtle crawls out of the sea and uses her front flippers to drag herself up the beach to a nest site.
The female digs out a body pit with her front flippers and then excavates a vertical egg chamber (between 30 and 60cm deep) with her hind flippers. If the sand is too dry and unsuitable for nesting, the turtle moves on to another site.
For most turtles, digging the nest takes about 45 minutes. Another ten to 20 minutes are then spent laying the clutch of leathery-shelled eggs. Each clutch contains about 120 eggs, ranging in size from the golf ball-sized egg of the hawksbill to the billiard ball-sized egg of the flatback.
After laying, the turtle fills the egg chamber with sand using her hind flippers, and then fills the body pit using all four flippers. The turtle finally crawls back to the sea about one to two hours after emerging, entering the surf exhausted.
In this offshore area she begins to make the next clutch of eggs, fertilising them from her sperm store. After the nesting season, females return to their distant foraging areas and may not nest again for two to eight years.
The temperature of the nest during incubation determines the sex of hatchlings. Warm, dark sand produces mostly females. Eggs laid in cool, white sand result mostly in males and generally take longer to hatch.
After about seven to 12 weeks the eggs hatch. The hatchlings take two or more days to reach the surface where they emerge as a group, usually at night.
To find the sea, hatchlings orient towards the brightest direction and use the topography of the surrounding horizon line. Once in the sea, hatchlings use a combination of cues (wave direction, current, and magnetic fields) to orient themselves to deeper offshore areas. Crossing the beach and swimming away is believed to imprint the hatchlings with the cues necessary to find their way back when they are ready to breed.
Once in the ocean, hatchlings are believed to enter regions where ocean currents meet. There they associate with floating seaweed mats and other flotsam caught up in ocean currents. Here they feed on tiny sea animals.
The hatchlings are rarely seen again until their shell length is 20-40cm, which may be five or 10 years after hatching. At this time, the young, free-swimming turtles migrate back to inshore foraging areas. They remain in these areas until they are ready to breed and the cycle begins again.
Adapted from Lanyon, J. M., Limpus, C. J., and Marsh, H. (1989). Dugongs and turtles - grazers in the seagrass system. pp.610-634. In. Biology of seagrasses. A. W. D. Larkum, A. J. McComb and S .A. Shepherd. Elsevier, New York.