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Turtle distribution

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Picture of Green Sea Turtle

Size and body mass

Diagnostic features

Geographical distribution

Habitat and Biology

Typical habitats

Migration

Nesting areas

Nesting periods

Nesting behaviour

Clutch size,egg size and weight

Incubation period

Size and weight of hatchlings

Maturity

Courtship and mating

Sex determination

Hatching

Hatchling mortality and predation

Commensalism and disease

Feeding

Threats to survival

Chelonia mydas

En - Green turtle; Fr - Tortue verte; Sp - Tortuga blanca

Size (cm) and body mass (kg):

N.B. All figures are FAO (Food and Agriculture Organization) unless otherwise stated.
Figures for females were assumed to be obtained during nesting.
SCL = Straight Carapace Length
CCL = Curved Carapace Length

Min. and Max. recorded sizes for adults:

Min: (at maturity) 78 cm (SCL?) Solomons )

Max: 139.5 cm (CCL) Ascension Island

n.b. CCL gives a figure which is approximately 3-4 cms longer than SCL.

World variation: (mean)figures, unless stated, are for mature females.

      Surinam          112cm     Pritchard (1979)
      Ascension Island 108cm     Carr and Hirth (1962)
      Guyana           107cm     Pritchard (1979)
      Aldabra          103cm     Frazier (1971)
      Florida          101cm     Ehrhart (1979)
      Costa Rica       100cm     Carr and Hirth (1962)
      Yemmen           96cm      Hirth (1971)
      Sarawak          95cm      Hendrickson (1958)
      Turkey(Cukurova) 90cm      Geldiay & Koray (1982)
      Turke (Mersin)   95cm      Baran,Durmas & Atatur (1988)
      N.Cyprus         92cm      Broderick & Godley (1992-95)

      Yemen       71-104 cm      (FAO) for males,
                                 which are generally smaller

      Most figures taken from a paper by LM Ehrhart (1984?).
     

n.b. CCL gives a figure which is approximately 3-4 cms longer than SCL.

      Australia        186 kg    FAO
      Solomons         89.9 kg   FAO
      Surinam          235 kg    FAO

      West Indies      395 kg    FAO Claim from the 1960's ! 
                                 (CCL - 150 cms) 
        West Indies figures are unconfirmed and unsexed
                             

Diagnostic features:

• Carapace oval shaped in dorsal view, its width approximately 90% of its length.
• Head relatively short and blunt - approximately 20% of Carapace length
• Tomium of lower jaw has sharply serrated rim corresponding to strong ridges on the inner surface of the upper tomium.
• The carapacial scutes are thin, smooth and flexible when removed.
• Five central scutes, low keeled in juveniles, but lacking a median keel in adults and sub adults.
• Each flipper has one visible claw.
• Colour:
  

  On the upper side, the colour can vary from pale to dark and from plain colour to vivid combinations of yellow, brown and green. These may form radiated stripes or a blotch effect.

  The Pacific populations (namely Chelonia mydas japonica) are more melanistic than the Atlantic ones (Chelonia mydas mydas) and easily confused with the Black turtle (Chelonia agassizii). Recent genetic studies by Bowen and Karl in Lutz & Musick suggest that C.agassizii is not a separate species to C.Mydas.

  The underside in Atlantic forms is plain white through to a yellowish white. Pacific populations have a dark grey, blue or blue/green underside.

  Juveniles have a yellow border to scales on the head and upper side of flippers.

  Hatchlings are dark brown/black. The rear edges of the flippers and the rim of the carapace is white. Underneath they are white.

The colours used are to aid identification, not actual colours

Geographical Distribution:

• Widely distributed in tropical and subtropical waters, near continental coasts and around islands.
• Rare in temperate waters.
• The green turtle, together with the hawksbill, is the most tropical of all the marine turtles.
• Normal latitudinal range remains within the Northern and Southern limits of the 20°C isotherm.
• Range in Summer: Western Oceans: 40N 35S; Eastern Oceans: 30N 25S
• Range in Winter: Western Oceans: 30N 25S, Eastern Oceans: 20N 15S.
• Individuals do stray out of these limits but tend to be in a non reproductive phase.
• In the Mediterranean Sea, green Turtles are thought to spend most of their time in the Eastern end. They may have feeding areas on the coast of Tunisia.

Click here to go to World Distribution Maps

Habitat and Biology:

Typical habitats:

Found in warm sea water on both continental and island coasts. Typically a solitary animal, it does occasionally form feeding aggregations in shallow water areas where sea grass and algae are plentiful. Some feeding areas support an entire population, others are seen only to contain turtles of a certain age. Nesting sites are always found in places with a sea water temperature of over 25°c.

Migrations:

The Green Turtle migrates from rookeries to feeding grounds, which can be several thousand kilometres away. These migrations are usually carried out along the coast, but in the case of some populations, e.g. that of Ascension island, the turtles undertake a transoceanic migration of over 2200 Kilometres from their nesting beaches on the island to their feeding grounds in Brazil (the means of navigation remains a mystery). The exact nature of migrations of the hatchlings and juveniles are unknown. It is known that they converge in the open ocean after hatching and leave the pelagic habitat for the bethnic feeding grounds only when the carapace is 20-25cm long. What happens in the interlude is unknown. The population of green Turtles in the Mediterranean is thought to be isolated from other populations. There is evidence to show that turtles enter and exit the Mediterranean, but it is thought that this is due to navigational error.

Nesting areas:

The Mediterranean population of Green Turtles is very small compared to those world-wide. Within the Mediterranean, the main nesting sites are in:

• South Eastern Turkey
• Cyprus
• Syria? (WCMC list)
• Egypt? (WCMC list)
• Israel? (WCMC list)
• Albania; last sighting before 1900

The estimated total population of green Turtles in the Mediterranean is a maximum of 400 females (Groombridge 1990) nesting annually.

Generally, the green turtle nests within the tropical latitudes, on gently shelving sandy beaches protected from the sea.

Nesting periods:

This usually occurs in Spring or Summer. 
Times vary slightly depending on exact location of nesting beach.

     Caribbean              April to October
     Gulf of Mexico         May to September
     NW Atlantic Ocean      May to October
     SW Atlantic Ocean      Throughout the year
     SE Atlantic Ocean      November to February
     Western Indian Ocean   Throughout the year
     NW Indian Ocean        May to October
     Central Indian Ocean   July to March
     Eastern Indian Ocean   May to November
     Western Pacific Ocean  Throughout the year
     Central Pacific ocean  Throughout the year
     Mediterranean Sea      May to September

Nesting behaviour:

Females usually show nesting site fixity, often returning near to the spot that the last clutch was laid, perhaps even where they emerged as hatchlings. The female comes ashore at night on a gently sloping sandy beach. The site of the nest is always well above the high tide mark. A flask like pit is dug and the clutch of eggs deposited in it. The nest is then camouflaged with a covering of sand. The turtle then returns to the sea. A picture of the tracks left by a Green Turtle is shown below.

Clutch size,egg size and weight:

Most females will lay between two and five clutches each breeding year, with the mean being slightly over 2.5 times. On average the time between each clutch being laid is two weeks. This varies with the population. Females usually have an interval of two years between successive breeding years. However, this varies, and they can lay every year or every four years. This is thought to vary depending upon diet and nutrition.

The number of eggs per clutch varies greatly from population to population. Clutch size depends on weight, age and size of the turtle, distance of migration, and time of season.

                               egg number/clutch
                     Minimum clutch        Maximum clutch
South Yemen                38                    -
Ascension Island            -                   195
Mediterranean              33                   190

Cyprus           mean clutch - 115.5 (n=347) Broderick & Godley (1996) 

The weight and size of the eggs varies from population to population.

                     Minimum size(mm)      Maximum size(mm)
South Yemen                 42.3                   -
Ascension Island             -                    54.6

                     Minimum weight(g)      Maximum weight(g)
SE Africa                   47.7                   -
Comoro Islands               -                    52.9

Incubation period:

Incubation period varies from 48 to 70 days in length. This is dependant on humidity and temperature change during the season. The cooler the weather, the longer the incubation time.

In Cyprus the mean incubation period (1995) was 51.1 days (N = 121)

Broderick & Godley (1996)

Size and wieght of Hatchlings:

The resultant size and weight of the hatchlings depends on the size of the egg.

                               Hatchlings
                  min. weight(g)          max weight(g)
SE Africa              18.4                    -
Hawaii                   -                    35
Comoro Islands         21.6                    -

                 min size(mm)             maz size (mm)
                    (SCL)                    (SCL)
Yemen                 44                       -
NE Australia           -                       54

Maturity:

Recent estimates point to the age of sexual maturity to be between 25 to 30 years or more. These figures take into account average populations of nesting turtles, not just maxima and minima. In captivity on a high protein diet, sexual maturity can be reached in as little as 10 years.

Courtship and Mating:

This occurs at sea near the nesting beaches. A single female can be courted by several males. Copulation starts early in the mating season and usually stops when nesting begins. A female will not usually mate once she has laid her first clutch. It has been hypothesised that the fertilisation of the eggs laid in any one season may have taken place several years before, and that sperm may also be stored and used for clutches in the next season.

Sex determination:

The sex of the turtle is temperature dependant. The pivotal temperature of any location is specific to that population. In the Mediterranean, the pivotal temperature is 28°C (Mrosovsky et al. 1984). If the average temperature is greater than this then the more hatchlings will be female than male; if the average temperature is lower than this, then more of the hatchlings will be male than female.

Male..........................................Female

Hatching:

The hatchlings from one nest emerge together at night and race toward the lightest part of the horizon: the sea. The hatchlings move quickly down the beach and swim to open sea. It is believed that it is during the "race" to the sea that the hatchlings become imprinted and then return to that same (natal) beach in future years.

In Cyprus the mean hatching success rate (1995) was 84.2% (N = 341)

Broderick & Godley (1996)

Hatchling mortality and predation:

There is high level of predation throughout the life cycle of the green turtle. Only when it reaches a size that is large enough to avoid being swallowed does it cease being preyed upon by all but sharks. Before the hatchlings emerge, they are often eaten by small mammals, such as racoons, mongooses, dogs or jaguars, or by other animals, such as monitor lizards, ghost crabs, ants, or fly maggots. Nests can be killed off by bacterial infection or by fungi. Nests are also lost due to erosion and damage due to bad weather.

In the Mediterranean, the main predators of eggs are foxes, crows and other day diurnal birds scavenge eggs from disrupted nests. When the hatchlings emerge, they are very vulnerable and are preyed upon by birds and fish. The main predators of hatchlings in the Mediterranean are ghost crabs and large pelagic fish such as Coryphaena hippurus. Crows and other birds will feed off any hatchlings which are on the beach in the daytime.

Sharks are the biggest predators to mature green turtles throughout the world, however dolphin fish and groupers prey upon even medium sized juvenile turtles.

Broderick & Godley (1995)

Commensals and disease:

Unlike loggerhead turtles, the green turtle has very few commensals. Little is known about specific diseases suffered by this species. However, at sea, invertebrates such as leaches (Ozobranchius branchiatus and Ozobranchius marggoi), invade the epithelial areas of the body, especially near to the cloacal opening, eyes, axils, etc. causing necrosis, and it is reported that heavy infestations produce a kind of papillomatosis.

Since 1989, the incidence of green turtle disease (first observed over 50 years ago) - fibropapillomatosis, has sharply increased and now poses a real threat to the survival of many green turtle populations ( Barrett - Endangered Species Bulletin Vol.XXI No.2). There is a suggested link between the disease and an increase in contaminants but this has yet to be proven. In Hawaii, 49-92% of turtles captured have the disease. The cause of the disease is unknown but a viral infection is suspected. The disease is characterised by one or more non-cancerous fibrous tumours commonly located on areas of soft skin. The disease may cause an increased susceptability to parasites, obstruct swimming and feeding, disorientate, impair vision and in some cases, even prove fatal. The disease also occurs in other species of sea turtle.

Feeding:

Mature green turtles feeds during the day time in the sea grass beds that grow in shallow waters. They graze these sea grasses, such as Zoostera, Thallasia and Vadalia, which are then digested with the aid of micro-organisms in a special part of the turtle's intestine. Juvenile green turtles, and hatchlings are omnivorous. It is thought that this allows them to grow faster as they have a higher protein intake. The serrated jaw or tomium allows for efficient grazing of sea grasses.

Threats to survival:

Although the green turtle has a large world-wide population compared to many other species of turtle, its survival is still threatened. Causes are:

1. Loss of Habitat-

due to commercialisation, including tourism and industrialisation

2. Accidental capture by fishing activities-

due to drift-nets, long line fishing, pursening and shrimping

3. Pollution-

due to industrial effluent, plastics, oil spillages etc.

4. Human predation-

due to egg collecting which are used as an aphrodisiac in many countries and sell for high price. Thousands of eggs taken from nests each year.

Meat eating: regarded as the most palatable of all the turtle meat, hundreds of wild Green Turtles are killed each year for their flesh and manufacture of turtle soup. Modern turtle "ranching" has possibly reduced this number.

Turtle-shell: by product of killing for flesh, but low demand.

This section is dealt with in a later section under threats.