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The Life Cycle of Green Sea Turtles in the Great Barrier Reef

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Green sea turtles are olive-green with brown, reddish-brown or black markings above; they're a whitish color below. The shield-shaped carapaces or shells of mature green turtles can grow to 3 feet long; the turtles weigh about 330 pounds. Green sea turtles are found worldwide in tropical and temperate waters, including the Great Barrier Reef off the coast of Queensland in north-eastern Australia. There and worldwide, green turtle life cycle center around long migrations from their feeding sites to their nesting grounds.


The Great Barrier Reef is an important foraging area for the green turtles nesting in the area and for turtles who feed at the reef and migrate to different regions and countries to nest. They travel long distances -- average migrations are about 250 miles but some exceed 1,615 miles -- between their feeding grounds and the beaches where they nest. The adults feed mainly on seagrass, along with algae, mangrove fruit and occasionally jellyfish.


Green turtles from feeding grounds around Indonesia, Papua New Guinea, the Solomon Islands, Fiji, western and northern Australia as well as Queensland will breed at the Great Barrier Reef. They have 13 nesting sites, called rookeries, concentrated in the Capricorn Bunker group of islands in the southern Great Barrier Reef, with an annual nesting population of about 8,000 females. Five other rookeries, with approximately 30,000 nesting females, are in the northern Great Barrier Reef, mostly around Raine Island and Moulter Cay.


The green turtles who nest in the Great Barrier Reef area breed in the summer from late October to February. The females are about 45 years old when they first mate; they breed every two to eight years. Males and females mate with several partners. The females store the sperm in their bodies to fertilize eggs when needed, and they lay clutches of about 115 ping-pong-ball-size eggs at fortnightly intervals. They will make around five trips to the beach to nest. When ready, the females crawl up the beach where they were born and use their flippers to excavate vertical chambers to lay their eggs in, refilling the chambers with sand to bury the eggs. The whole process takes one to two hours.


After 7 to 12 weeks, depending on the temperature of the sand, the eggs hatch. Sand temperature influences the sex of hatchlings, with mainly females developing in warm dark sand and males in cooler white sand. It takes a couple of days for the hatchlings to reach the surface, usually emerging at night as a group. The hatchlings, which are black above and white below, and 2 inches long, feed on tiny marine invertebrates. They find the sea by crawling toward the brightest direction; in the water they use wave direction, currents and magnetic fields to negotiate deeper areas offshore. It's believed these cues will be used to find the beach when the fully grown turtles are ready to breed.

Immature Turtles

Not much is known about green turtles' early years. They probably drift on ocean currents feeding. When they are about 6 years old, the immature turtles return to coastal waters, where they feed and grow until they reach sexual maturity. With their low-protein diets, this can take up to 40 years. Once the green sea turtles mature, the cycle of long-distance migrations between foraging grounds and their breeding sites begins. Green sea turtles can live more than 80 years in the wild.