Ecology and life history
As one of the oldest sea turtle species studied, much of what is known of sea turtle ecology was gleaned from studies of green turtles. The ecology of Chelonia mydas changes drastically with each succeeding stage of its life history. For instance, newly-emerged hatchlings are carnivorous, pelagic organisms part of the open ocean mini-nekton. In contrast, immature juveniles and adult turtles are commonly found in seagrass meadows closer inshore as herbivorous grazers
Habitat
Green turtles alternate between three habitat type depending on their current life history stage. Nesting beaches are where the turtles return to lay eggs. Mature turtles spend most of their time in coastal, shallow waters with lush seagrass beds. Seagrass meadows within inshore bays, lagoons and shoals are common locations where adult Chelonia mydas can often be found. This particular species is known to be very selective about their feeding and mating sites and entire generations will often alternately migrate between the same feeding and nesting areas.
After hatching, turtles in their first five years are known to spend a majority of their early life stages stage in convergence zones within the open ocean. These young turtles are rarely seen as they swim in frequent deep, pelagic waters where they spend the first few years of their lives
Trophic ecology
As large and well-protected animals, adult green turtles have few enemies and even fewer predators. Only human beings and the larger sharks are known to feed on C. mydas adults. Specifically, tiger sharks (Galeocerdo cuvier) are known predators of adult green turtles in Hawaiian waters. Juvenile turtles and recently-emerged hatchlings have significantly more predators, including crabs, small mammals and shorebirds.
Adult Chelonia mydas are obligately herbivorous. They almost-exclusively feed on various species of seagrasses and seaweed. They have been observed grazing on various species of macroalgae, specifically Caulerpa, Turbinaria, Spyridia, Codium, and Ulva. While mature green turtles are entirely herbivorous, juveniles are known to subsist on a plethora of marine invertebrates. Select preferred prey items include smaller cnidarians and crustaceans. Their digestive intake of plant matter grows larger as they age, until as mature adults they become obligate herbivores. While it has been previously stated that green turtles do not feed while at their respective nesting areas, it has been shown that gravid turtles do in fact feed while in the waters surrounding their nesting grounds
Life history
Unlike most sea turtles, which spend most of their adult lives in the ocean, Pacific green turtles are known to willingly crawl onto secluded beaches during the day to bask in the sun.
Green turtles migrate long distances between their chosen feeding sites and the beaches from where they hatched. Some C. mydas are known to swim distances of greater than 2,600 kilometers to reach their spawning grounds. Mature turtles will often return to the same exact beach from which they hatched from. Individual female green turtles usually mate every two to four years. Males on the other hand, are known to make the trip to their breeding areas every year. As with many species that are found across a wide range of latitudes, mating seasons vary between populations. For most Chelonia mydas in the Caribbean, mating season is from June to September. The French Guiana nesting subpopulation nests from March to June. In the tropics, green turtles are known to nest throughout the year, with some subpopulations preferring particular times of the year. In Pakistan, Indian Ocean C. mydas nest all year-round but prefer to nest during the months of July and December.
Green turtles reproduce in the typical way that marine turtles do so. Female turtles control mating; males cannot force females to mate. While it does not seem to offer increased survival among the hatchlings, a few green turtle populations are known to undergo polyandry when mating. After mating in the water, the females haul themselves onto the beach above the high tide line. Upon reaching a suitable nesting site, the gravid female then digs a hole with her hind flippers and deposits a number of eggs in the nest. The number of eggs laid per litter depends on the age of the female and differs from species to species, but C. mydas clutches range between 100 to 200 eggs. After laying eggs, the female then covers the nest with sand and returns to the sea.
After around 45 to 75 days, the eggs hatch. As with other marine turtles, C. mydas eggs hatch during the night and the newly-emerged turtles instinctively head directly towards the water's edge. This undoubtedly is the most dangerous time in a turtle's life, as the hatchlings make their way to the water, various predators such as gulls and crabs pick off many turtles. A significant percentage of turtle hatchlings never make it to the ocean. Just like other sea turtles, little is known of the early life history of newly-hatched green turtles. After this trek to the ocean juvenile green turtles spend from three to five years in the opean ocean as carnivores before they settle as immature juveniles into a more herbivorous, shallow-water lifestyle.It is speculated that they take twenty to fifty years to reach mature size. Individuals of the species are known to live up to eighty years in the wild.
One of the most significant mass-nesting sites for this species is located on Ascension Island in the South Atlantic. Each year on the island, thousands of C. mydas create between 6,000 and 15,000 nests. These particular turtles are among the largest green turtles in the world, many more than a meter in length and weighing up to 300 kilograms.
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