Green Sea Turtle
Chelonia mydas

Habitat and Biology:

illustration: M. Demma © ICRAM

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 benthic 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 in other parts of the world.
• In the Mediterranean, the main nesting sites are in South Eastern Turkey and Cyprus.
• 339–360 female green turtles are estimated to nest annually in the region. These population estimates are likely to be optimistic (Broderick et al, 2002).
• 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

illustration: M. Demma © ICRAM

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 depends 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)

 

illustration: M. Demma © ICRAM

Size and weight of Hatchlings:   Hatchlings   min. weight (g) max weight (g) SE Africa 18.4 - Hawaii - 35 Comoro Island 21.6 -   min. size (mm) max. size (mm) Yemen 44 - NE Australia - 54 The resultant size and weight of the hatchlings depends on the size of the egg.

 

illustration: M. Demma © ICRAM

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, sea turtles on a high protein diet can reach sexual maturity in as little as 10 years.

illustration: M. Demma © ICRAM

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 a 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:

Seagrass

illustration: M. Demma © ICRAM

• Mature green turtles feed 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.