Green sea turtle

• 1. Introduction

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  The green sea turtle, Chelonia mydas, is named for a color that nobody sees from the outside. The green belongs to a layer of fat tucked between the inner organs and the shell. A diet of seagrass tints that fat. The animal's own carapace runs from olive to black.

  In Hawaii the turtle is called honu, and it is known there as a symbol of good luck and longevity. Across tropical and subtropical seas, females drag themselves onto beaches at night to bury their eggs. Those that survive to maturity may live to 90 years in the wild.

  This documentary follows three questions. How does an animal that lives almost entirely underwater find one particular beach? Why can the temperature of sand decide whether a hatchling is male or female? And how have people lived alongside this turtle, and how are they trying to protect it?

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• 2. Built For The Sea

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  A mature green turtle can grow to 1.5 m long, and most adults weigh between 68 and 190 kg. The average carapace measures 78-112 cm. Only the leatherback is larger among sea turtles in the United States. Exceptional animals pass 315 kg. The largest one ever recorded weighed 395 kg and had a carapace 153 cm long.

  The body is flat from back to belly, topped by a teardrop-shaped shell and driven by two paddle-like front limbs. Unlike its relative the hawksbill, the green turtle has a very short snout and an unhooked beak. Its neck cannot be pulled into the shell. Each front flipper carries a single claw, where the hawksbill has two. Young animals sometimes show a second claw. The shell itself is built from five central plates flanked by four pairs of side plates.

  Color changes with age. Hatchlings are mostly black on top with pale undersides. Juveniles turn dark brown to olive. Adults may be plain brown, spotted, or marbled with streaks, and the underside is yellow. In the eastern Pacific, parts of the shell can look almost black. Adult males are easy to pick out, because their tails extend visibly past the shell and their front claws are longer.

  Breathing is a matter of speed. In routine activity, green and loggerhead turtles dive for about four to five minutes. They then spend only one to three seconds at the surface. One explosive exhale and a rapid inhale replace the air in the lungs. Stress and exertion shorten the time a turtle can hold its breath, which is why turtles caught in shrimp trawlers and other gear drown so quickly.

  Resting turtles can stay under for hours. At night, adults wedge themselves beneath rocks and under ledges in reefs and coastal rock. Many have been seen returning to the same sleeping spot night after night.

  The senses are tuned to water. Green turtles see best between violet and yellow, in wavelengths of 400-600 nanometers, and they barely register orange and red. On land they become nearsighted, because their spherical lenses are adjusted for underwater light. They have no external ear, only a single ear bone called the columella. It lets them hear low sounds between 200 and 700 Hz. They also smell by pumping water in and out of the nose, using a structure known as Jacobson's organ.

  Warmth is managed too. Sea turtles can shunt blood away from tissues that tolerate low oxygen and toward the heart, brain, and central nervous system. They also bask on warm beaches. A basking turtle can look as if it is crying. The tears come from a gland behind the eye that expels excess sea salt. In winter, turtles at higher latitudes can hibernate briefly in mud.

  A body this capable still needs fuel, and the fuel is unusual for a sea turtle.

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• 3. Seagrass Gardeners

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  Green turtles were among the first sea turtles ever studied, so much of what science knows about sea turtle ecology comes from them. Their diet shifts dramatically with age. Newly emerged hatchlings are carnivorous animals that drift in the open ocean. For about their first five years, the young stay in convergence zones in bare open water, swimming through deep, pelagic seas. They are rarely seen.

  Juveniles eat fish and their eggs, sea hare eggs, jellyfish, molluscs, sponges, worms, insects, crustaceans, and plant matter. As they mature they become omnivores, and most adults end up strictly herbivorous. No other aquatic turtle is a plant eater once fully grown. Plants offer little nutrition, so green turtles grow slowly.

  The shift reshapes the skull. A serrated jaw helps adults chew algae such as sea lettuce, Ulva lactuca, along with seagrasses. They also eat large quantities of salt marsh plants, including Avicennia schaueriana and Sporobolus alterniflorus.

  The way they graze benefits the meadow. In seagrass beds, the turtles clip only the tops and leave the roots in place. That habit improves the health and growth of the beds. Healthy beds in turn shelter and feed many fish and crustaceans.

  On coral reefs, red, brown, and green algae feed the turtles, and the reef shelters them from predators and storms. There the turtle has a cleaning partner in the yellow tang. This fish swims alongside and eats algae, barnacles, and parasites from the shell and flippers. The tang gets a meal. The turtle gets a smoother shell, which reduces drag and helps its health.

  Adults face few enemies. Only people and the larger sharks prey on them. In Hawaiian waters, tiger sharks hunt adult turtles of every size, often near nesting beaches, where the turtles are penned in and vulnerable. Hatchlings and juveniles face many more predators, including crabs, small marine mammals, and shorebirds. Eggs are taken by scavengers such as red foxes and golden jackals.

  Parasites add to the toll. Barnacles cling to the carapace, and leeches feed on flippers and skin, damaging tissue and causing blood loss. Protozoans, tapeworms, and roundworms infect the liver and intestines and kill many turtles.

  The worst disease is fibropapilloma, which grows lethal tumors on the scales, lungs, stomach, and kidneys. A herpesvirus causes it. The leech Ozobranchus branchiatus carries the virus, and that leech feeds almost entirely on green sea turtles.

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• 4. Finding The Way Home

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  Some green turtles swim more than 2600 km between their feeding grounds and the beaches where they breed. Mature animals often return to the exact beach where they hatched. Scientists call this natal homing, and salmon do something similar. The behavior probably pays off. A birth beach has proved it has sand, easy access to the water, suitable incubation temperatures, and few egg predators. Males return to their birthplaces as well. They can expect to find mates there, because females born on the same beach come back too.

  Navigation in open water draws on several cues. Turtles read wave directions, sunlight, and temperature. They also carry what amounts to an internal magnetic compass. Magnetic crystals in the brain let them sense the intensity of Earth's magnetic field. That sense helps them reach nesting or feeding grounds.

  How they recognize the home beach is still a puzzle. Many researchers think imprinting is involved, a special kind of learning that happens when a turtle first hatches. One idea is chemical imprinting, in which a turtle remembers the smell of its beach, as salmon are thought to do. The problem is distance. Some turtles travel thousands of miles, and scent from the beach is unlikely to carry that far. The second idea is geomagnetic imprinting. A hatchling would record the magnetic signature of its birth beach, which fits well with the way turtles navigate.

  The map of these journeys splits the species in two. The Atlantic and eastern Pacific subpopulations are genetically distinct, each with its own nesting and feeding grounds. Turtles from Atlantic and Mediterranean rookeries share one type of mitochondrial DNA. Those from the Pacific and Indian Oceans share another. The likely reason is that the southern tips of South America and Africa offer no warm water for turtles to cross.

  The native range sits between 30 degrees north and 30 degrees south. Ocean temperatures below 7 to 10 degrees Celsius limit it. Green turtles live along the coasts of more than 140 countries and nest in over 80. Strays have turned up as far north as Canada, southern Alaska, and the Cimbrian peninsula. A newer discovery came from Rangaunu Harbour in New Zealand.

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• 5. Nests And Hatchlings

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  Ascension Island in the South Atlantic sees 6,000 to 25,000 nests dug each year by green turtle females. Many of these females are among the largest in the world. They often exceed 1 m in length and weigh up to 300 kg.

  Males visit the breeding areas every year. Females mate only every two to four years. Mating seasons vary by place. In the Caribbean it runs mostly from June to September. In French Guiana, nesting runs from March to June. In Pakistan turtles nest year-round but prefer July to December.

  The female controls the process. After mating in the water, she crawls above the high tide line. She digs a hole 11-22 inches deep with her hind flippers and lays between 85 and 200 eggs. Clutch size depends on her age. The eggs are round, white, and about 45 mm across. She then covers the nest and returns to the sea. The work takes an hour to an hour and a half, and she repeats it three to five times in a season.

  The sand decides the sex. Sex is fixed around days 20-40 of incubation, and nests above 30 degrees Celsius produce females. Cooler nests produce males. Eggs in the center of the nest are warmest, so they tend to become females. Warmer nests also hatch faster.

  Between 50 and 70 days after laying, the eggs hatch at night. The hatchlings stay buried for days until all of them are ready, then emerge together. They head straight for the water on instinct. This is the most dangerous stretch of a turtle's life. Gulls and crabs feed on them as they scramble across the sand, and a significant percentage never reach the sea.

  The survivors spend three to five years in open ocean before settling in shallow water. It is speculated that they need twenty to fifty years to reach sexual maturity. Only about 1% of hatchlings are estimated to get there.

  Temperature-dependent sex has a modern consequence. Scientists see signs that rising heat in the northern Great Barrier Reef region is limiting how many males are produced in Australia.

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• 6. A Name In Dispute

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  In 1758, Carl Linnaeus described the species in the tenth edition of his Systema Naturae. He called it Testudo mydas. A century later, in 1868, Marie Firmin Bocourt described a turtle he named Chelonia agassizii, honoring the Swiss-American zoologist Louis Agassiz. People called it the black sea turtle.

  Later research found that Bocourt's turtle was not genetically distinct from C. mydas. The two names were merged, and the populations were first given subspecies status. That split was eventually ruled invalid. C. agassizi survives only as an invalid junior synonym.

  Genus-level order was settled in 1993. A study placed the carnivorous hawksbill, loggerhead, and ridley turtles in the tribe Carettini. The plant-eating Chelonia earned its own genus. The flatback, Natator, turned out to be more distant from the others than anyone had thought.

  The dispute lingers over one Pacific population, the Galápagos green turtle. William Dampier recorded these animals in the 17th century. For a long time they drew little attention, because Galápagos giant tortoises overshadowed them. Extensive behavioral studies are only about 30 years old.

  The Galápagos turtles do look different. Adults are considerably smaller and much darker on the shell, belly, and limbs. The shell is more dome-shaped and curves above each hind flipper. Adult males have very long tails. Three explanations were offered. The turtle might be a separate species, a subspecies, or just a color mutation.

  DNA settled much of it. An analysis of mitochondrial and nuclear DNA from 15 nesting beaches found no significant distinction. Recognizing the Galápagos turtle as a separate species would make the group paraphyletic. At a meeting of sea turtle scientists in 2000, a majority backed treating it as a population or subspecies. The source suggests this may be a case of political taxonomy. Three major checklists, the Reptile Database, the 2007 checklist of Fritz and Havas, and the 2017 IUCN checklist, all treat the name as a junior synonym.

  The animal's genome was sequenced in 2013, to study how the turtle body plan developed and evolved.

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• 7. Turtles And People

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  When Christopher Columbus found the Cayman Islands in 1503, he named them Las Tortugas because of the sea turtles around them. Many early visitors came to catch turtles for fresh meat on long voyages. Today the green turtle appears on the coat of arms and flag of the Cayman Islands. A turtle is the watermark on the banknotes. A stylised turtle nicknamed Sir Turtle serves as the mascot of Cayman Airways and decorates its aircraft.

  The Cayman Turtle Farm began in 1968 as Mariculture Ltd. It sold turtle meat, oil rendered from the fat, shell, and leather. Much of its first stock came from doomed eggs taken from nests threatened by erosion, flooding, or chemically hostile soil. At its peak it held as many as 100,000 turtles at once. Regulations then closed the international markets, even for farm-bred products. The surviving farm became mainly a tourist attraction with 11,000 turtles. In 2010 it took the name Cayman Turtle Farm: Island Wildlife Encounter.

  Appetite for the turtle ran across continents. Ancient Chinese regarded the flesh of sea turtles as a delicacy. In the 19th century, English and American diners prized turtle soup. Cooks made it from the fat, cartilage, and flesh known as calipee. In Hawaii, skin was tanned to make handbags.

  Indonesia shows how religion shaped the trade. In Java, turtle eggs were popular, though Islamic law treats the flesh as unclean. In Bali, turtle meat featured at ceremonial and religious feasts, and the island has imported turtles since the 1950s as its own supply ran short. The mostly Hindu Balinese do not eat the eggs. They sell them to local Muslims. On Java's south coast, green turtles are the most commonly traded species. They are sold whole, stuffed, or as oil known as minyak bulus.

  On the Big Island of Hawaii, in the Pu'u Loa lava fields, a ki'i pōhaku, or petroglyph, shows a green turtle. It may date to when the islands were first settled. The turtle stands for a navigator who finds the way home again and again, just as the real animal swims hundreds of miles to lay eggs where it was born. Some legends say these turtles guided the first Polynesians to the islands. Another tells of Kailua, a turtle who could take the form of a girl and watched over children at Punaluʻu Beach.

  The geographer James J. Parsons wrote a book called The Green Turtle and Man. It played a special role in the movement to save the species.

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• 8. Threats And Rescue

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  In 1982 the IUCN officially listed the green sea turtle as endangered. The 1986, 1988, 1990, and 1994 editions of the Red List kept that listing, as did the landmark 1996 edition. In 2001, Nicholas Mrosovsky petitioned to delist the species, arguing that some populations were large, stable, or even growing. The IUCN's standards subcommittee ruled that visual counts of nesting females were not direct observation. The listing stayed. In 2004 the IUCN reclassified the turtle as endangered under criteria that cite a probable population drop of more than 50% over the past decade.

  Trade controls moved in parallel. The species joined CITES Appendix II in 1975. In 1977 the whole sea turtle family moved to Appendix I, except Australia's population, which followed in 1981. On the 3rd of May 2007 the species was listed on Appendix I, which bars commercial international trade. In October 2025 the IUCN changed the global status to least concern. Some groups remain at risk. The Mediterranean population is critically endangered, and the Florida population is endangered. The eastern Pacific, Hawaiian, and Southern California groups are threatened. The World Wide Fund for Nature calls Pakistan's turtles rare and declining.

  The dangers are mostly human. Hunting, poaching, and egg harvesting continue. More damaging are boat strikes, nets without turtle excluder devices, pollution, and habitat loss. Light pollution disorients hatchlings. Tar balls can be mistaken for food and swallowed. They release toxins and can block the gut. Beach-front construction, land reclamation, and tourism strip nesting beaches. In the Río de la Plata in Uruguay, invasive rapa whelks have fouled young turtles, adding drag and injuring shells.

  In the eastern Pacific, shrimp trawlers in Mexico are the main cause of death, and many deaths go undocumented. The only foraging area identified in the region is San Diego Bay, which is heavily polluted with metals and PCBs. There, one adult was found dead with monofilament netting packed tightly in its esophagus.

  On Raine Island in Australia, up to 100,000 nesting females have been seen in a single season. The island produces 90% of the region's green turtles. Hatching rates fell in the 1990s, and thousands of females died struggling to climb small sandy cliffs. Between 2011 and 2020, the Queensland Government, BHP, the Great Barrier Reef Marine Park Authority, the Great Barrier Reef Foundation, and the Wuthathi and Meriam traditional owners reshaped the island with heavy machinery. Females gained a smoother path, and nests faced less risk of flooding. Monitoring with 3D modelling, satellites, and drones continues. A separate effort, the Turtle Cooling Project, studies how global warming affects northern green turtle breeding. It works around Raine Island, Heron Island, and Moulter Cay.

  Other places have their own programs. Costa Rica has limited egg collection at Tortuguero since the 1950s, and Tortuguero National Park was established in 1976. Ascension Island runs an active conservation program, and the group Karumbé has monitored juveniles in Uruguay since 1999. Indonesia restricted turtle trade and consumption in 1999. In November 2009 it rejected a request from Bali's governor for a quota of 1,000 turtles for Hindu ceremonies. In the Philippines, the Turtle Islands Wildlife Sanctuary covers an entire municipality. In Sabah, Malaysia, tourists help release hatchlings at Pulau Selingan. Hawaii's population has made a remarkable comeback, and students at Hawaii Preparatory Academy have tagged thousands of turtles since the early 1990s.

  Since 1999, the Florida Aquarium has led sea turtle rehabilitation. Over 20 years it received 200 turtles and released 180. The Cayman Turtle Farm has released over 31,000 turtles since 1968. On the 19th of February 2012 it released the first second-generation captive-bred turtle fitted with a satellite tag. Captive-bred turtles marked with living tags have started returning to nest on Grand Cayman. In Tuvalu, green turtles nest on islets in the 33 square kilometer Funafuti Conservation Area. Whether the 2025 reclassification holds will depend on beaches like those.

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