Flatback sea turtle
|Flatback sea turtle|
|Nesting on Great Keppel Island off Queensland Coast, Australia|
|Distribution map and nesting beaches of flatback sea turtle|
The Australian flatback sea turtle (Natator depressus) is a sea turtle located along the sandy beaches and shallow coastal waters of Australia. They can only be found in the waters around the Australian continental shelf. It belongs to the family Cheloniidae, along with other sea turtles. This turtle gets its name from the fact that their shell has a flattened or lower dome than the other sea turtles. They can be olive green to grey with a cream underside. These turtles average from 76 cm to 96 cm in length and can weigh between 70 kg to 90 kg. The hatchlings, when emerging from nests, are larger than other sea turtle hatchlings when they hatch. The flatback turtle is listed by the IUCN Red List of Threatened Species as Data Deficient, meaning there is insufficient scientific information to determine its conservation status at this time. It was previously listed as Vulnerable in 1994. They are not Threatened like other sea turtles due to their small dispersal range.
The flatback turtle is a sea turtle that can be recognized by its smooth flat-domed shell, or carapace, which has upturned edges along the sides. It has the coloration of olive green or a mixture of grey and green. This matches the coloration of their heads. The underside, also called the plastron, has a much lighter coloration of a pale yellow. The flatback sea turtle is of an average length, ranging from 76 cm to 96 cm, and weighs from 70 kg to 90 kg. The females of this species are larger than the males in adulthood and also have been found to have longer tails than their male counterparts.
A feature of the carapace of this sea turtle that helps contribute to its recognition are the single pair of prefrontal scales that are up by the head and the four pairs of coastal scutes. Another unique feature of this species of sea turtle is the fact that their carapaces are found to be much thinner than other sea turtle carapaces. This feature causes the shell to crack under the smallest pressures.
The flatback sea turtle has the smallest range of the seven sea turtles. They are found in the continental shelf and coastal waters of tropic regions. These turtles do not travel long distances in the open ocean for migrations like other sea turtles. They can be typically be found in waters of 60 m or less in depth. They do not have a global distribution like the other sea turtles. Flatback sea turtles can be found along the coastal waters of Northern Australia, the Tropic of Capricorn, and the coastal areas of Papua New Guinea. Their distribution within Australia is in the areas of Eastern Queensland, Torres Strait and Gulf of Carpentaria, Northern Territory, and Western Australia.
The distribution of nesting sites can be found across Queensland, the Northern Territory, and Western Australia, with the greatest concentration found in Queensland, in the Gulf of Carpentaria. Within Queensland, the nesting sites can be found from the south in Bundaberg to the Torres Strait in the north. The main nesting sites in this range are the southern Great Barrier Reef, Wild Duck, and Curtis Island. The Torres Strait contains the major nesting sites for these turtles. Within the Northern Territory, nesting sites are more widely dispersed in this area with a wide variety of beach types on this coastline. In the Western Australia area, the important nesting sites found have been the Kimberley Region, Cape Dommett, and the Lacrosse Island.
The flatback sea turtle lives in the shallow, soft bottomed tropical and subtropical waters. These turtles stick to the continental shelf of Australia and can be found in grassy areas, bays, lagoons, estuaries, and any place with a soft-bottomed sea bed. The habitats that females prefer for nesting sites are sandy beaches in tropical and subtropical areas. They prefer beaches where the sand temperature can be in the range of 29 °C to 33 °C at nest depth, which are the temperatures that help determine the hatchling's sex.
The hatchlings begin to leave the nests during the beginning of December, and the clutches will continue to hatch until late March. The peak of hatchling emergence can be seen during February. A flatback sea turtle hatchling is larger than other sea turtle hatchlings with their carapace length averaging 60mm. Their large size helps protect them from some of the predators after hatching, and allows them to also be stronger swimmers. The hatchlings tend to stay close to shore and lack the pelagic phase of other sea turtles. The hatchlings will feed on the macroplankton present in their surface-dwelling environment.
A flatback sea turtle is sexual mature anywhere between 7–50 years of age and they will nest every two to three years. The mating occurs while the male and female are out at sea; therefore, the males will never return to shore after they hatch. The flatback nesting sites can only be found along the coast of Australia within the slopes of the dunes. A female will return to the same beach for their subsequent clutches within the same nesting season. They will return for other nesting seasons, as well. Depending on the area of the nesting site, the nesting season can go from November to January or can last the entire year. They are able to lay up to four times throughout the nesting season, and the intervals between nesting can be 13–18 days. While using her front flippers to dig, the female will clear away the dry sand located at the top. After she clears the sand, the female will create an egg chamber using her back flippers. After she has laid her eggs, she will then cover the nest again using her back flippers, while also tossing sand back with her front flippers.
The number of eggs in a flatback sea turtles' clutch are fewer than other sea turtles. They will have an average of 50 eggs laid each time in a clutch, while other sea turtles may lay up to 100-150 eggs in a clutch. The eggs are about 55mm long within these clutches. The sex of the flatback turtle hatchling is determined by the temperature of the sand that the egg is in. If the temperature is below 29 °C then the hatchling will be a male, and if the temperature is above this 29 °C it will be female.
The flatback sea turtle is an omnivorous species, but predominately eats a carnivorous diet. They feed mostly on the prey found within the shallow waters where they swim. They have been found to feed on soft corals, sea cucumbers, shrimp, jellyfish, mollusks, and other invertebrates. They will also occasionally feed on seagrasses, even though they rarely feed on vegetation.
Flatback sea turtles are predated upon by both terrestrial organisms and aquatic organisms. The terrestrial predators they must face are foxes, feral dogs, and pigs. The aquatic predators to these turtles are sharks. The hatchlings also face predation from crabs, birds, and small crocodiles on their journey to the waters. Once in the water, the hatchlings can be predated upon by fish and even sharks. Due to their large size when they are born and their strong swimming skills, the likelihood of capture is lowered.
The flatback sea turtle is listed as Vulnerable nationally in Australia. These sea turtles are the least endangered out of all of the sea turtles. Unlike other turtles, there is not a big human demand for their meat. They do not swim far from the shores; thus, they do not get caught in nets as often as other sea turtles. These reasons can contribute to why they are not in more danger.
All marine turtles are faced with threats such as habitat loss, the wildlife trade, collection of eggs, collection of meat, by-catch, pollution, and climate change. Flatback turtles are specifically threatened by the direct harvest of eggs and meat by the indigenous people of Australia for traditional hunting. These people are given the right to harvest by the government, but only if for non-commercial purposes. Another threat they are struggling with is the destruction of their nesting beaches due to coastal development and the destruction of feeding sites at coral reefs and the shallow areas near the shore. Camping on these beaches compacts the sand and contributes to dune erosion, and the wheel ruts caused by vehicles driving on the beaches can trap the hatchlings on their journey to the sea. Coastal development contributes to barriers that make it difficult or impossible for adult turtles to reach nesting and feeding sites. These turtles also fall prey to incidental capture. They are caught by fisherman, particularly by trawling, gillnet fishing, ghost nets, and crab pots. Lastly, pollution is a concern for these creatures. Pollution can affect the timing of their egg laying, how they choose their nesting site, how hatchlings find the sea after emerging, and how adult turtles find the beaches.
In 2003, a recovery plan was set in place nationally to help this species along with other sea turtles. This plan aims to reduce the mortality rates through actions within the commercial fisheries and to maintain a sustainable harvest with the indigenous people. Monitoring programs are being developed and integrated, along with managing factors that affect the reproductive success of this species. In Kakadu National Park, a monitoring program has already been set up for this species. This species' critical habitat is being identified for protection. People are also trying to enhance the communication about information on the flatback sea turtle, and enhance the cooperation and actions internationally.
- Colin J. Limpus, The Flatback Turtle, Chelonia depressa Garman in Southeast Queensland, Australia, Herpetologica, Vol. 27, No. 4 (Dec., 1971), pp. 431-446
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