Green Turtle

Green turtle close up Maldives ©Jasmine Pape
Green turtle close up, Maldives ©Jasmine Pape

Green turtles (Chelonia mydas) are the largest of all the hard-shelled sea turtles. They can grow to about 134 cm in length and weigh between 135 and 250 kg as fully grown adults. The green turtle has a smooth, sub-circular to heart-shaped carapace with colour varying from greenish-yellow to greyish-brown. The plastron is yellowish-white in colour. In the Maldives, the colouration of adults is darker than that of green turtles in the western Indian Ocean.

Green Turtle Biology and Behaviour

Profile of Pia green sea turtle Maldives Indian Oean
Pia, an identified green turtle on a reef in Maldives ©Lisa Bauer

Green turtles reach adulthood between 15 and 50 years of age. Turtles that eat a diet rich in nutrients grow and mature faster than those with a poor diet.

Mature females return to their nesting beaches once every 2-4 years to nest. Females migrate huge distances between feeding grounds and nesting areas, but tend to follow coastlines rather than cross open waters. A female green turtle leaves a symmetrical track with a tail drag down the middle. Tracks are 85-90 cm across. The females lay between 1-6 clutches of between 70 and 125 eggs. Larger females tend to lay larger clutches of eggs. The incubation period lasts between 50 to 70 days.

Green sea turtle hatchlings Maldives Indian Ocean turtle life cycle
Green turtle hatchlings, Maldives ©Chiara Fumagalli

Green turtle hatchlings measure around 50 mm long at birth. Their carapaces is black and they have a light coloured plastron.

Green Turtle Diet

Green sea turtles eat sea grass as adults. image.
Adult green sea turtle eating sea grass in the Maldives.

Adult green turtles are vegetarians, feeding primarily on seagrasses and algae. The greenish coloured fat deposits under their carapaces is probably due to this diet, and the source of their name. Juvenile green turtles, however, feed on small crustaceans and jellyfish. As they mature, their diet shift from omnivorous to herbivorous.

Turtles actively select fresh green growth near the base of the seagrass and there is some evidence that turtles actively farm certain areas of seagrass to ensure a steady supply of younger, protein-rich growth. When seagrass pastures are absent, as they are in much of the Maldives, they eat various species of red and green algae.

 Green Turtle Habitat and Distribution

Adopt a turtle in Kenya.
G028, on Kisima Mungu reef, Diani, Kenya

The green turtle can be found worldwide in tropical and sub-tropical waters down to 20°C. They have been recorded as far north as the English Channel and as far south as Polla Island, Chile. Green turtles are highly migratory and as a result undertake complex migrations through drastically different habitats. Some times they migrate thousands of kilometres to lay their eggs.

Both males and females migrate from benthic foraging areas to mainland or island nesting beaches. The best feeding grounds rarely coincide with the best nesting beaches. Green turtles can travel 20-40 km per day when migrating.

Green turtles are not picky about their nesting beaches; sand size, composition, and the presence of beach vegetation do not seem to be factors in choosing a nesting site. Mating generally occurs around one kilometre offshore of nesting beaches.

Although nesting occurs in more than 80 countries worldwide, only 10-15 populations are of significant size (greater than 2,000 females yearly). The Indian Ocean is home to some of the largest nesting populations of green turtles in the world, particularly Oman, Pakistan, and Reunion Island. The nesting population of green turtles in the Maldives is currently unknown, but populations in India and Sri Lanka are severely depleted. Nesting appears to happen all year-round.

Green Turtles in the Maldives

Green sea turtle swimming in the sea in Maldives Indian Ocean
Green sea turtle, Maldives ©Lauren Arthur

In the Maldives, the green turtle is known as “velaa”. Both juveniles and adults forage on seagrass beds throughout the archipelago. Furthermore, they nest in several atolls, including Baa, North Malé, Laamu, Noonu, Addu, and Lhaviyani. Previous reports have noted more nesting on the northern and eastern sides of the archipelago and a peak in nesting between June and December. Marine biologists and citizen scientists have photographed more than 830 individuals throughout the archipelago until June 2020. Green turtles seem to stay on their chosen “home reefs”, being seen year after year in the same spots. Most observed turtles at coastal foraging grounds are juveniles.

Green Turtle Conservation Status

IUCN Red List Status: Endangered (2004)

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Sea Turtles FAQ – The Answers to All Your Sea Turtle Questions

Male hawksbill popping head above surface for air, Maldives. Image.
Marvin, male hawksbill, coming up for a breathe, Maldives.

Sea turtles can hold their breath for several hours, depending on their level of activity.

If they are sleeping, they can remain underwater for several hours. In cold water during winter, when they are effectively hibernating, they can hold their breath for up to 7 hours. This involves very little movement.

Although turtles can hold their breath for 45 minutes to one hour during routine activity, they normally dive for 4-5 minutes and surfaces to breathe for a few seconds in between dives.

However, a stressed turtle, entangled in a ghost net for instance, quickly uses up oxygen stored within its body and may drown within minutes if it cannot reach the surface.

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References:

  • Hays GC, Akesson S, Broderick AC, Glen F, Godley BJ, Luschi P, Martin C, Metcalfe JD & Papi F 2001. The diving behaviour of green turtles undertaking oceanic migration to and from Ascension Island: dive durations, dive profiles and depth distribution. Journal of Experimental Biology 204: 4093-4098.
  • Hays GC, Hochscheid S, Broderick AC, Godley BJ & Metcalfe JD 2000. Diving behaviour of green turtles: dive depth, dive duration and activity levels. Marine Ecology Progress Series 208: 297-298.
  • Hochscheid S, Bentivegna F & Hays GC 2005. First records of dive durations for a hibernating sea turtle. Biology Letters 1: 82-86.
  • Lutz PL and Musik JA (eds.) 1996. The Biology of Sea Turtles Volume I. CRC Press.

The actual documentation of a sea turtle’s age in the wild is difficult or nearly impossible. Individual turtles can be tracked for a shorter time of six month to three years with the help of satellite transmitters. Longterm studies rely on capture-recapture principle, just like our turtle photo id project. Each photo of a turtle represents a recapture event documenting that the individual is still alive.

A study of nesting green turtles in Hawaii observed female turtles returning to nest for up to 38 years after they were first identified. Assuming the average age at first nesting activity of 24 years, this would show that green turtles can live to up to at least 62 years.

Similar estimates have been made for loggerhead turtles.

An adult green turtle resting on a reef in Maldives, image
An adult green turtle
Hawksbill turtle resting on the reef, Maldives
An adult hawksbill turtle

References:

  • Dodd C 1988. Synopsis of the biological data on the loggerhead sea turtle. Ecology 88.
  • Humburg IH and Balazs GH 2014. Forty Years of Research: Recovery Records of Green Turtles Observed or Originally Tagged at French Frigate Shoals in Northwestern Hawaiian Islands, 1973-2013. NOAA Technical Memorandum NMFS-PIFSC-40.

When sea turtles are juveniles, it is very difficult to tell their sex by eye as they do not differ externally. However, after reaching sexual maturity male sea turtles develop a long tail, which houses the reproductive organ. The tail may extend past the hind flippers.

Female turtles have a short tail, which generally doesn’t extend more than 10 cm (4 inches) past the edge of the carapace. Male sea turtles (except leatherbacks) have elongated, curved claws on their front flippers to help them grasp the female when mating.

The sex of a sea turtle embryo is determined by the temperature of the sand: warm temperatures result in more females while cooler temperatures result in more males.

Male green turtle tail
Male green turtle tail
Female green turtle tail
Female green turtle tail

The olive and kemp’s ridley sea turtles are the smallest species, growing only to about 70 cm (just over 2 feet) in shell length and weighing up to 45 kg (100 lbs). Leatherbacks are the largest sea turtles. On average leatherbacks measure 1.5 – 2m (4-6 ft) long and weigh 300 – 500 kg (660 to 1,100 lbs). The largest leatherback ever recorded was 2,56 m (8.4 ft) long and weighed 916 kg (2,019 lbs) !

Kemp’s Ridley

Nesting female kemp's ridley turtle, nicknamed
Nesting female kemp’s ridley turtle, nicknamed “Mij”, laying 116 eggs on Galveston’s East Beach. ©Ron Wooten, Wildscreen Exchange.

55.6-66.0 cm carapace length, weight range of 25-54 kg for nesting females.

References:

  • Marquez-M R 1994. Synopsis of Biological Data on the Kemp’s Ridley Turtle, Lepidochelys kempi (Garman, 1880). NOAA Technical Memorandum NMFS-SEFSC-343.

Olive Ridley

Female Oliver ridley turtle nesting on the beach during arribada
Nesting female olive ridley turtle ©Susie Gibson.

Curved carapace length 52.5-80.0 cm, weight less than 50 kg (average 35.7 kg) for nesting females.

References:

  • Qureshi M 2006. Sea turtles in Pakistan. In: Shanker K and Choudhury BC (Eds.). Marine Turtles of the Indian Sub- continent. Heydarabad: India Universities Press, pp. 217–224.Reichart HA 1993.
  • Reichart HA 1993. Synopsis of biological data on the olive ridley sea turtle Lepidochelys olivacea (Eschscholtz 1829) in the western Atlantic. NOAA Technical Memorandum NMFS-SEFSC-336.

Hawksbills

Adult male hawksbill turtle swimming in the blue, Maldives. Image
Adult male hawksbill, Maldives.

Nesting females reported between 53.3 and 95.5 cm carapace length, with weight between 27.2 and 86.2 kg.

References:

  • Witzell WN 1983. Synopsis of biological data on the hawksbill turtle, Eretmochelys imbricata (Linnaeus, 1766). No. 137. Food & Agriculture Org.

Green turtles

Adult green turtles mating, Malsdives. Image.
Adult green turtles mating, Maldives.

Nesting green females reported curved carapace length 75-134 cm, weight (after egg deposition) 45-250 kg (!).

References:

  • Hirth HF 1997. Synopsis of the Biological Data on the Green Turtle Chelonia mydas (Linnaeus, 1758). Vol. 2. Fish and Wildlife Service, US Department of the Interior.

Flatbacks

Nesting flatback sea turtle ©Lyndie Malan / CC BY-SA.

Ones study (Ref. 1) found nesting females have a mean curved carapace length 86.3 cm, and mean weight of 67.4 kg. Another study (Ref. 2) found flatbacks to be between 87.5-96.5 cm.

References:

  • Schäuble C, Kennett R and Winderlich S 2006. Flatback Turtle (Natator depressus) nesting at Field Island, Kakadu National Park, Northern Territory, Australia, 1990-2001. Chelonian Conservation and Biology 5: 188-194.
  • Limpus CJ 1971. The Flatback Turtle, Chelonia depressa Garman in Southeast Queensland, Australia. Herpetologica 27: 431-446.

Loggerheads

Adult female loggerhead, Oman.

The largest loggerhead was reported stranded in 1938 on the welsh coast (Tenby, Pembrokshire) with a carapace length of 146.7 cm. The turtle was highly emaciated and missing a front flipper. It was reported to weigh only 27.8 kg, which is severely underweight for a turtle of that size. On average nesting and therefore adult female loggerheads have a curved carapace length of 65.1-114.9 cm and weigh between 40.0 and 180.7 kg. Males fall into the same size range (79.0-104.0 cm curved carapace length).

References:

  • Brongersma LD 1972. European Atlantic turtles. Zoologische Verhandlingen 121, Leiden.
  • Dodd C 1988. Synopsis of the biological data on the loggerhead sea turtle. Ecology 88: 1-119.

Leatherbacks

Leatherback Turtle, Claudia Lombard, USFWS
Adult leatherback turtle ©Claudia Lombard, USFWS.

143.8-169.5 cm curved carapace length, weight 259-506 kg recorded for nesting females all around the world. Largest ever recorded specimen was found dead on a beach on the coast of Wales. The adult male turtle weighed 916 kg and its shell was 256.5 cm long. An autopsy revealed that it had drowned.

References:

  • Eckert KL and Luginbuhl C 1988. Death of a Giant. Marine Turtle Newsletter 43: 2-3.
  • Eckert KL, Wallace BP, Frazier JG, Eckert SA and Pritchard PCH 2012. Synopsis of the Biological Data on the Leatherback Sea Turtle (Dermochelys coriacea). Biological Technical Publication BTP-R4015-2012, US Fish & Wildlife Service.

Each sea turtle species feeds on a specific diet and all lack teeth:

  • Loggerheads feed mainly on hard-shelled organisms such as lobsters, crustaceans, and fish.
  • Green turtles are vegetarian and prefer sea grasses, sea weeds and algae as adults, however, green turtle hatchlings are omnivorous, eating jellyfish, snails, crabs, and shrimp.
  • Leatherbacks feed mostly on jellyfish.
  • Hawksbills have a bird-like beak that is used to cut through soft coral, anemones and sea sponges.
  • Olive ridleys are omnivorous, mostly eating jellyfish, snails, crabs, and shrimp but they will occasionally eat algae and seaweed as well.

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References

  • Bjorndal KA, Wetherall JA, Bolten AB and Mortimer JA 1999. Twenty-Six Years of Green Turtle Nesting at Tortuguero, Costa Rica: An Encouraging Trend. Conservation Biology 13: 126-134.
  • Hirth HF 1997. Synopsis of the Biological Data on the Green Turtle Chelonia mydas (Linnaeus, 1758). Vol 2. Fish and Wildlife Service, US Department of the Interior.
  • Hudgins J, Mancini A and Ali K 2017. Marine turtles of the Maldives – A Field Identification Guide. Gland, Switzerland: IUCN and Government of Maldives. 90 pp.
  • Florida Fish & Wildlife Conservation Commission. Guidelines for Marine Turtle Permit Holders: Nesting Beach Surveys: Crawl Identification Guide. 2014.
  • Moran KL and Bjorndal KA 2007. Simulated green turtle grazing affects nutrient composition of the seagrass Thalassia testudinum. Marine Biology 150: 1083-1092.