Green turtle  (Chelonia mydas)

Green turtles are one of the largest and most widespread of all the marine turtles (5). The oval carapace varies from olive to brown, grey and black with swirls and irregular patters (6), but the common name is derived from the green colour of the fat and connective tissues of this species (2). Two subspecies are currently recognised; the Pacific green turtle (Chelonia mydas agassizii) tends to be smaller than its Atlantic cousin (C. m. mydas) with a narrower carapace that may sometimes be completely black, providing the other common name of 'black turtle' to certain populations (7). The plastron, or undershell, remains a pale yellow or orange throughout life (6). Males are generally smaller than females (11), and green turtles differ in appearance from other marine turtles by the possession of a single pair of scales in front of the eyes and a serrated bottom jaw (2). The tiny black hatchlings are only around 50 millimetres long (6).

Green turtles are found in tropical waters around the globe, particularly associated with the coastline (8).

Adults inhabit shallow feeding grounds that are often seagrass meadows, migrating from these areas to their nesting beaches (6). Once juveniles hatch, they spend an unknown number of years in the open ocean (6).

Green turtles are long-lived and may take up to 59 years to reach sexual maturity (6) Undertaking tremendous feats of navigation, adults return to the same beach to breed each season, part of the population in Brazil astonishingly migrates around 2,250 kilometres across the open ocean to breed on the Ascension Islands (12). Mating tends to occur just offshore of the nesting beaches; using a curved claw on each front flipper and a flat nail at the end of the tail, males are able to grip their mates (2). Females haul out onto the beach at night and dig large nests with their back flippers beyond the high tide mark, they typically lay between 100 and 150 eggs in one nest and then proceed to cover the eggs with sand; the whole process takes around two hours (6). A single female returns to breed only once every two to five years but will lay up to nine nests in that one season (2). Incubation takes between 45 and 70 days, and temperature has been shown to determine of the sex of hatchlings; with females being produced at warmer temperatures (6). Breaking open their eggs with a special hooked 'egg tooth' that will subsequently be lost; hatchlings use their powerful front flippers to reach the surface, and then proceed to the sea (7). The soft-bodied juveniles are particularly vulnerable at this time from a variety of predators, such as ghost crabs and gulls on the beach to sharks and dolphins in the water (7).

Unlike other marine turtles, adult green turtles are almost exclusively herbivorous, grazing on seagrasses and algae (8); it is assumed that juveniles are more omnivorous although the exact composition of their diet is unknown (6)

Green turtles are overharvested in many areas for both their meat and eggs (8). The meat is highly prized and the cartilage underneath the plastron (known as 'calipee') is used in the production of turtle soup (2). In addition, as with other marine turtles, these reptiles are accidentally caught in bycatch of the fishing industry (8). One of the most worrying threats in recent years has been an increase in fibropapillomas; fibrous tumours that can grow on almost any part of a turtles body, impeding movement or sight, and often leading to death (6). Very little is known about the disease, which is believed to be a virus, and its prominence varies amongst different populations, although there may be a link with coastal areas of heavy human use (9).

The green turtle is protected form international trade by its listing on Appendix I of the Convention on International Trade in Endangered Species (CITES) (3). A number of conservation projects around the world work to protect nesting areas from disturbance (6). The increasing prevalence of fibropapillomas is an extremely worrying phenomenon and scientists are working hard to understand this disease.

For more information on green turtles and their conservation see:

• Turtle trax:
  www.turtles.org/
• Marine Turtle Newsletter:
  www.seaturtle.org/mtn/

Authenticated (11/10/02) by Brendan Godley, Marine Turtle Newsletter.
http://www.seaturtle.org/mtn/