Natator depressus

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.

• Biogeographic Regions
• australian australian
  • native native
• pacific ocean pacific ocean
  • native native

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.

• Habitat Regions
• temperate temperate
• tropical tropical
• saltwater or marine saltwater or marine

• Aquatic Biomes
• coastal coastal
• brackish water brackish water

• Other Habitat Features
• estuarine estuarine

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.

• Other Physical Features
• ectothermic ectothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• female larger

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.

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.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• oviparous oviparous

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.

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.

• Key Behaviors
• natatorial natatorial
• motile motile

Communication and Perception

No information is available on how flatbacks communicate.

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical

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.

• Primary Diet
• carnivore carnivore
  • eats non-insect arthropods
  • molluscivore molluscivore
  • eats other marine invertebrates

• 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.

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.

• Positive Impacts
• food food
• ecotourism ecotourism

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.

Additional Links

Encyclopedia of Life

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.

References

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.

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 .