Green Sea Turtle, Chelonia mydas

Green Sea Turtle, Chelonia mydas. Underwater photographs taken in Zihuatanejo Bay, Guerrero, March, November and December 2018. Photographs courtesy of Ron Woheau, Zihuatanejo.

Phylogeny: The Green Sea Turtle, Chelonia mydas (Linnaeus, 1758) is a member of the Chelonidae Family of Sea Turtles. The genus Chelonia is one of seven genera in this family. This is the only species in this genus. Green Sea Turtles are known by various local names including: Pacific Green Sea Turtle, Atlantic Green Sea Turtle, Japanese Sea Turtle, and Black Sea Turtle. In Mexico they are known as tortuga marina verde or tortuga marina negra. Several important sources state that there are no valid subspecies of Chelonia mydas. Many of the researchers and residents of Western Mexico disagree and consider the Pacific Black Sea Turtle, Chelonia mydas agassizii sufficiently different to constitute a valid subspecies. The Pacific Black Sea Turtle differs from the Green Sea Turtle in that the Green has a  flatter carapace, a lighter color, and a larger head.

Morphology:  The Green Sea Turtle has a smooth, oval, carapace that consists of 5 medial scutes, 4 pairs of lateral or costal scutes, and 11 pairs of marginal scutes. As they age, their carapaces become more elongated. The carapace is mottled shades of brown and olive color. The plastron is whitish to light yellow. Green Sea Turtles get their name from the greenish fat layer beneath their carapace, not their shell color. The green fat is cause by their diet of green sea grasses and algae. Hatchlings have a black carapace, a white plastron and a white margin. Their head is small and blunt with a short snout, strong beaks and serrated jaws. There is one claw visible per flipper. Unlike other Sea Turtles, Green Sea Turtles have only one pair of large scales (prefrontals) between their eyes. Green Sea Turtles can reach a maximum carapace length of 1.3 m (4 feet ) and 200 kg (440 pounds) in weight.

Habitat and Distribution:  The Green Sea Turtle spends the majority of its time in coastal waters including bays and estuaries in tropical and sub-tropical waters. Most of their time is spent in shallow water where algae growth is greatest, but they can dive to depths in excess of 50 m (165 feet).  They are found in the Atlantic, Indian, and Pacific Oceans. In Mexico the Green Sea Turtle is found in the Caribbean, Gulf of Mexico, Gulf of California, and the entire Pacific coast. They are known to nest on the beaches of the State of Michoacán, and along the X’Cacel and Sian Ka’an coasts of the Yucatán Peninsula.

Diet:  Adult Green Sea Turtles are herbivores that primarily consume sea grasses and  algae. Juveniles have an omnivorous diet, eating crustaceans, worms, jellies, and mollusks, in addition to vegetation.

Predators:  Green Sea Turtles are eaten by sharks. Juveniles and hatchlings are also preyed upon by octopuses, crabs, sea birds, and fishes .

Reproduction: Green Sea Turtles are gonochoric (male or female for life). Reproduction is sexual with fertilization occurring internally. After mating at sea, the female visits a wide beach, at night. She clears an area of debris and digs multiple nesting holes, using her rear flippers. She selects one hole, then lays clutches of 70 to 100 eggs and covers them with sand. After the hours-long process, the female returns to the sea leaving asymmetrical tracks that are easy to identify. She will return multiple times with nesting intervals of twelve to fourteen days. Babies hatch at night within fifty days to fifty-five days. The babies instinctively crawl into the sea, under the cover of darkness; baby turtles that do not reach the water by daybreak are preyed upon by shore birds, shore crabs, and other predators. Studies conducted in Michoacán on the nests of Green Sea Turtles reveal that nest temperatures below 27.1°C (81°F) do not produced females while temperatures of 31°C (88°F) or higher produced only females.

Ecosystem Interactions:  Green Sea Turtles can host several types of internal and external parasites including: Sporochid flukes, protozoa, helminths, trematode eggs, leeches, and even ticks. At times they  have  commensal relationships with barnacles, crabs, shrimp, and algae.

Human Interactions:  From a conservation perspective the Green Sea Turtle is currently considered to be ENDANGERED primarily due to exploitation by humans. Present population estimates state that there are about 85, 000 to 90,000 nesting females worldwide. They have slow growth rates, are slow to reach maturity, and have slow reproductive rates. Major factors that affect their longevity include artificial lighting that affects their nesting locations, poaching of turtles and eggs, destruction of seagrass beds, entrapment in shrimp trawls and other bottom trawling devices, boat strikes, and plastic pollution. In addition, for human consumption turtle eggs and turtle meat have been exploited in many parts of the world.  At present, major efforts are on-going to provide protection of sea turtles including the ban of their trade and sale, agreements to help conserve habitats, enhanced public awareness, and enhanced scientific focus.

Synonyms:  Chelonia agassizii, Chelonis japonica, Chelonia mydas agassizii, Cheloni mydas mydas, Chelonia viridis, and Testudo mydas.