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Natator depressusFlatback Turtle

By Emily Thielk

Geographic Range

Flatback sea turtles are native to Australia, and rarely travel far from home. In fact, flatbacks have the smallest geographic range of all their relative sea turtles. They have only been found to live in the northern waters of Australia. However, they will occasionally travel to the Tropic of Capricorn or the coastal waters of Papua New Guinea to feed. ("Flatback Sea Turtle Information & Map", 2003; Ripple, 1996)

Habitat

Flatback turtles prefer the shallow, soft-bottom areas close to the shore, or the inshore waters of bays. They usually will not venture past the continental shelf. In addition, no flatbacks have been found in coral reefs. (Ripple, 1996)

Physical Description

Flatbacks are moderately-sized turtles that usually weigh approximately 70 kgs. In adults, the females are larger than males but males have longer tails. Both males and females have rounded heads which usually match the olive green color of its outer shells or carapaces. The underbelly, or plastron, is an off-white or yellow color. The most distinctive trait of these turtles is their smooth, flat shell which tends to turn up at the edges.

One interesting note about flatbacks' carapaces is that they are much thinner than those of other sea turtles. The keratin layer which protects both the carapace and flippers is so fragile that even small amounts of pressure (such as flapping its flippers against the plastron) can draw blood. This may be one reason the flatback stays away from the rocky terrain of coral reefs. (Ripple, 1996)

  • Sexual Dimorphism
  • female larger
  • Average mass
    70 kg
    154.19 lb
  • Range length
    76 to 96 cm
    29.92 to 37.80 in

Development

Flatback hatchlings are much larger than other sea turtle young, with an average carapace length of 60 mm. Their size not only helps protect them from predators such as crabs and gulls, it also makes them stronger swimmers. Their size and swimming ability makes them less likely to be thrown off course by sea currents or strong waves.

Despite their swimming ability, young flatback turtles generally stay close to shore and do not have a pelagic phase like most other sea turtles. The pelagic phase, also called the "lost year," is when young sea turtles venture out and explore the seas and are almost never seen. No one is really sure where they go during this time.

They reach sexual maturity anywhere between 7 and 50 years of age. (Ripple, 1996)

Reproduction

Mating and nesting takes place in November and December in flatbacks. Female flatbacks dig nests on the slopes of dunes in Australia. The eggs are around 51 mm long and come in clutches of around 50 eggs (other species of sea turtles may have 100 or 150 eggs). The large size of flatback hatchlings provides them with some protection from predators. Due to their size, the sex can usually be determined just by visual identification, however histological examination may be needed for 100% certainty. Flatbacks have no pelagic phase so young hatchlings remain close to shore. (Devaux and Dewetter, 2000; Hewavisenthi and Parmenter, 2001; Ripple, 1996)

  • Breeding season
    November-December
  • Average number of offspring
    50 eggs
  • Average gestation period
    42 days

Lifespan/Longevity

Flatback sea turtles, like most sea turtles, have a relatively long lifespan of up to 100 years. Flatbacks raised in captivity will mature and grow faster because they generally recieve more protien in their diets. Age is measured by the length of the carapace. (Ripple, 1996)

  • Typical lifespan
    Status: wild
    100 (high) years

Behavior

Not much is known about the general behavior of flatback sea turtles. The adults seem to sleep near rocks or under ledges, while young flatbacks sleep on the surface. They can rest of sleep underwater for hours before needing another breath. However, if they are under stress or excited they cannot hold their breath as long.

One area where they have been observed nesting is the island of Mon Repos, located 9 miles northwest of the coastal city of Bundaberg, Queensland. This seems to be a major egg laying site for flatback turtles, however exact numbers have not been collected. It is currently a natural reserve with limited access to tourists. (Devaux and Dewetter, 2000; Ripple, 1996)

Communication and Perception

No information is available on how flatbacks communicate.

Food Habits

Flatback turtles seek out prey such as sea cucumbers, mollusks, jellyfish, prawns, bryzoans and other invertebrates in shallow waters. They are carnivorous, and rarely feed on vegetation, if at all. (Devaux and Dewetter, 2000; Ripple, 1996)

  • Animal Foods
  • mollusks
  • aquatic crustaceans
  • echinoderms
  • cnidarians

Predation

Because flatback turtles are born relatively large (around 51 mm) and are strong swimmers, they are less likely to be caught by predators. Also, flatbacks that hatch during the night have a better chance of survival because the darkness provides them some protection while they adjust to their new surroundings.

Adult flatbacks are protected from most predators by their hard shells and large size. Sharks and killer whales sometimes attack adult flatbacks. (Ripple, 1996)

Ecosystem Roles

These turtles are still being studied and as of now, not much is known about their roles in the ecosystem. They do eat many different kinds of marine invertebrates, so may impact prey populations. Both adults and hatchlings also may be an important food source for various predators.

Economic Importance for Humans: Positive

While at one time flatback eggs were harvested for food, today flatbacks are studied and well protected. They are also seen as a tourist attraction. In addition their meat is not generally desired by humans. This may be because their carnivorous diet has not given it the same taste as other sea turtles. ("Flatback Sea Turtle Information & Map", 2003; Devaux and Dewetter, 2000; Ripple, 1996)

Economic Importance for Humans: Negative

Flatbacks have no known negative impact on humans.

Conservation Status

The flatback is the least endangered of all the sea turtles, due in part to the fact that its meat is not desired by humans for food. Also, because flatbacks tend to stay close to shore, they do not often get caught in fishing nets as many other turtles do. One step that is being taken to prevent accidental turtle catches is the implementation of Turtle Exclusion Devices (TEDS). These are funnels positioned inside nets so that only smaller fish can get caught. The sea turtles are too large to fit in the TEDS and so they are not caught. (Devaux and Dewetter, 2000)

Contributors

Matthew Wund (editor), University of Michigan-Ann Arbor.

Emily Thielk (author), University of Michigan-Ann Arbor, Phil Myers (editor), Museum of Zoology, University of Michigan-Ann Arbor.

Glossary

Australian

Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

World Map

Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

World Map

acoustic

uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

brackish water

areas with salty water, usually in coastal marshes and estuaries.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

ecotourism

humans benefit economically by promoting tourism that focuses on the appreciation of natural areas or animals. Ecotourism implies that there are existing programs that profit from the appreciation of natural areas or animals.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

estuarine

an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.

food

A substance that provides both nutrients and energy to a living thing.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

molluscivore

eats mollusks, members of Phylum Mollusca

motile

having the capacity to move from one place to another.

natatorial

specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

visual

uses sight to communicate

References

Caribbean Conservation Corporation & Sea Turtle Survival League. 2003. "Flatback Sea Turtle Information & Map" (On-line). Accessed 11/09/04 at http://www.cccturtle.org/flatback.htm.

"Great Barrier Reef Marine Park Authority(GBRMPA)" (On-line). Accessed March,22,2002 at www.gbrmpa.gov.au.

Devaux, B., B. Dewetter. 2000. On the Trail of Sea Turtles. Paris, France: Editions Nathan.

Hewavisenthi, S., C. Parmenter. 2001. Influence of Incubation Environment on the Development of the Flatback Sea Turtle. Zoological Record, 3: 668-682.

Ripple, J. 1996. Sea Turtles. Minnesota: Voyaguer Press.

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