The Australian flatback sea turtle (''Natator depressus'') is a

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

of sea turtle in the

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

Cheloniidae Cheloniidae is a family of typically large marine turtles that are characterised by their common traits such as, having a flat streamlined wide and rounded shell and almost paddle-like flippers for their forelimbs. They are the only sea turtles ...

. The species is

endemic Endemism is the state of a species being found only in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also foun ...

to the sandy beaches and shallow coastal waters of the

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

n continental shelf. This turtle gets its

common name In biology, a common name of a taxon or organism (also known as a vernacular name, English name, colloquial name, country name, popular name, or farmer's name) is a name that is based on the normal language of everyday life; and is often con ...

from the fact that its shell has a flattened, lower dome than the other sea turtles. It can be olive green to grey with a cream underside. It averages from in

carapace A carapace is a dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tortoises, the unde ...

length and can weigh from . The hatchlings are larger than other sea turtle hatchlings when they hatch and emerge from their nests. The flatback turtle is listed by the

IUCN Red List of Threatened Species The International Union for Conservation of Nature (IUCN) Red List of Threatened Species, also known as the IUCN Red List or Red Data Book, founded in 1964, is an inventory of the global conservation status and extinction risk of biological spe ...

data deficient A data deficient (DD) species is one which has been categorized by the International Union for Conservation of Nature (IUCN) as offering insufficient information for a proper assessment of conservation status to be made. This does not necessaril ...

, meaning there is insufficient scientific information to determine its conservation status at this time. It was previously listed as vulnerable in 1994. It is not as threatened as other sea turtles due to its small dispersal range. This animal can be 31 to 37 inches long and about 100 kg in weight.

Taxonomy

The flatback sea turtle is the sole species in the ''Natator'' genus, which has no subspecies, but was originally described as ''Chelonia depressa'' in 1880 by American herpetologist

Samuel Garman Samuel Walton Garman (June 5, 1843 – September 30, 1927), or "Garmann" as he sometimes styled himself, was an American naturalist and zoologist. He became noted as an ichthyologist and herpetologist. Biography Garman was born in Indiana Co ...

. The

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

Natator The Australian flatback sea turtle (''Natator depressus'') is a species of sea turtle in the family Cheloniidae. The species is endemic to the sandy beaches and shallow coastal waters of the Australian continental shelf. This turtle gets its co ...

'' (meaning "swimmer") was created in 1908 by Australian ichthyologist

Allan Riverstone McCulloch Allan Riverstone McCulloch (20 June 1885 – 1 September 1925) was a prominent Australian ichthyologist. Born in Sydney, Australia, McCulloch began his scientific career at the age of 13 as an unpaid assistant to Edgar Ravenswood Waite in the ...

, and in the same scientific paper he described what he thought to be a new species, ''Natator tessellatus'', thereby creating a junior synonym. In 1988 Swiss paleontologist Rainer Zangerl assigned the flatback sea turtle to the genus ''Natator'' as the new combination ''Natator depressus''. Because ''Chelonia'' is feminine, and ''Natator'' is masculine, the

specific name Specific name may refer to: * in Database management systems, a system-assigned name that is unique within a particular database In taxonomy, either of these two meanings, each with its own set of rules: * Specific name (botany), the two-part (bino ...

was changed from ''depressa'' to ''depressus''.

Description

The flatback turtle is a sea turtle that can be recognized by its smooth, flat-domed

which has non-overlapping (non-imbricate)

scute A scute () or scutum (Latin: ''scutum''; plural: ''scuta'' "Scutum (shield), shield") is a bony external plate or scale overlaid with horn, as on the shell of a turtle, the skin of crocodilians, and the feet of Bird anatomy#Scales, birds. The ter ...

s and upturned edges along its sides. It has the coloration of olive green or a mixture of grey and green. This matches the coloration of its head. The underside of the shell, also called the

plastron The turtle shell is a shield for the ventral and dorsal parts of turtles (the Order (biology), order Testudines), completely enclosing all the turtle's vital organs and in some cases even the head. It is constructed of modified bony elements such ...

, has a much lighter coloration of a pale yellow. The flatback sea turtle has an average carapace length ranging from , and weighs from on average. Very large specimens are reported to weigh up to . 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. Features of this sea turtle which help contribute to its recognition are the single pair of

prefrontal scales The prefrontal scales on snakes and other reptiles are the scales adjacent and anterior to the frontal, towards the tip of the snout, which are also in contact with the internasals. See also * Snake scales * Scale (zoology) In zoology, a sc ...

on the head, and the four pairs of costal scutes on the carapace. Another unique feature of this species of sea turtle is the fact that its carapace is found to be much thinner than other sea turtle carapaces, causing its shell to crack under small pressures. The skull superficially resembles that of the

olive ridley The olive ridley sea turtle (''Lepidochelys olivacea''), also known commonly as the Pacific ridley sea turtle, is a species of turtle in the family Cheloniidae. The species is the second-smallest and most abundant of all sea turtles found in t ...

but details of the braincase most closely resemble those found in the

green sea turtle The green sea turtle (''Chelonia mydas''), also known as the green turtle, black (sea) turtle or Pacific green turtle, is a species of large sea turtle of the family Cheloniidae. It is the only species in the genus ''Chelonia''. Its range exte ...

Distribution and habitat

The flatback sea turtle has the smallest range of the seven sea turtles. It is found in the continental shelf and coastal waters of tropic regions. It does not travel long distances in the open ocean for migrations like other sea turtles. It can typically be found in waters of or less in depth. It does not have a global distribution like the other sea turtles. The flatback sea turtle can be found along the coastal waters of Northern Australia, the

Tropic of Capricorn The Tropic of Capricorn (or the Southern Tropic) is the circle of latitude that contains the subsolar point at the December (or southern) solstice. It is thus the southernmost latitude where the Sun can be seen directly overhead. It also reach ...

, and the coastal areas of

Papua New Guinea Papua New Guinea, officially the Independent State of Papua New Guinea, is an island country in Oceania that comprises the eastern half of the island of New Guinea and offshore islands in Melanesia, a region of the southwestern Pacific Ocean n ...

and

Indonesia Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania, between the Indian Ocean, Indian and Pacific Ocean, Pacific oceans. Comprising over List of islands of Indonesia, 17,000 islands, including Sumatra, ...

. Its distribution within Australia is in the areas of eastern

Queensland Queensland ( , commonly abbreviated as Qld) is a States and territories of Australia, state in northeastern Australia, and is the second-largest and third-most populous state in Australia. It is bordered by the Northern Territory, South Austr ...

Torres Strait The Torres Strait (), also known as Zenadh Kes ( Kalaw Lagaw Ya#Phonology 2, �zen̪ad̪ kes, is a strait between Australia and the Melanesian island of New Guinea. It is wide at its narrowest extent. To the south is Cape York Peninsula, ...

and

Gulf of Carpentaria The Gulf of Carpentaria is a sea off the northern coast of Australia. It is enclosed on three sides by northern Australia and bounded on the north by the eastern Arafura Sea, which separates Australia and New Guinea. The northern boundary ...

Northern Territory The Northern Territory (abbreviated as NT; known formally as the Northern Territory of Australia and informally as the Territory) is an states and territories of Australia, Australian internal territory in the central and central-northern regi ...

, and

Western Australia Western Australia (WA) is the westernmost state of Australia. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to the south, the Northern Territory to the north-east, and South Australia to the south-east. Western Aust ...

. 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. This turtle sticks 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

habitat In ecology, habitat refers to the array of resources, biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species' habitat can be seen as the physical manifestation of its ...

s that females prefer for nesting sites are sandy beaches in tropical and subtropical areas, avoiding beaches fronted by

intertidal The intertidal zone or foreshore is the area above water level at low tide and underwater at high tide; in other words, it is the part of the littoral zone within the tidal range. This area can include several types of habitats with various sp ...

coral reef flats. They prefer beaches where the sand temperature can be in the range of at nest depth. These temperatures help determine the hatchling's sex, and can lead to female-biased hatchling ratios in nesting sites with darker sand that absorbs more heat.

Life history

Early life

Flatback hatchlings have grey backs with dark outlined scutes, a white belly, and blue eyes. Different nesting sites have different seasons, which can vary year-by-year. In eastern Queensland,

clutches A clutch is a mechanical device that allows an output shaft to be disconnected from a rotating input shaft. The clutch's input shaft is typically attached to a motor, while the clutch's output shaft is connected to the mechanism that does the ...

begin to hatch during the beginning of December until late March, with a peak in hatchling emergence during February. A flatback sea turtle hatchling is larger than other sea turtle hatchlings with its carapace length averaging around . Its large size helps protect it from some of the predators after hatching, and allows it to also be a stronger swimmer. The hatchlings tend to stay close to shore and lack the

pelagic The pelagic zone consists of the water column of the open ocean and can be further divided into regions by depth. The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or water column between the sur ...

phase of other sea turtles. They absorb their egg's yolk sac in the nest and continue to be nourished by it for a few days, while emerging and entering the sea, until they begin feeding on the

macroplankton Plankton are the diverse collection of organisms that drift in water (or air) but are unable to actively propel themselves against currents (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they pr ...

present in their surface-dwelling environment. Once they are large enough, they begin feeding on soft-bodied invertebrates, such as soft corals and jellyfish, and occupying soft-bottomed

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...

habitats.

Reproduction

A flatback sea turtle is sexually mature anywhere between 7–50 years of age, and an adult female will nest every two to three years. 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, on soft sandy beaches without

coral reefs. A female will return to the same beach for her subsequent clutches within the same nesting season. She will return for other nesting seasons, as well. The nesting season depends on the area of the site and can range from October to January, such as in southern Queensland, or can last the entire year, such as in the northern and northwestern territories. Females lay, on average, approximately three clutches throughout the nesting season, with intervals of 15 days between nestings. 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 turtle's clutch are fewer than other sea turtles. It will have an average of approximately 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 large within these clutches, at about long. 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 , the hatchling will be a male, and if the temperature is above this 29 °C it will be female.

Ecology

Diet

The flatback sea turtle is an omnivorous species, but predominantly eats a carnivorous diet. It feeds mostly on the prey found within the shallow waters where it swims. It has been found to feed on soft corals,

sea cucumbers Sea cucumbers are echinoderms from the class Holothuroidea ( ). They are benthic marine animals found on the sea floor worldwide, and the number of known holothuroid species worldwide is about 1,786, with the greatest number being in the Asi ...

shrimp A shrimp (: shrimp (American English, US) or shrimps (British English, UK)) is a crustacean with an elongated body and a primarily Aquatic locomotion, swimming mode of locomotion – typically Decapods belonging to the Caridea or Dendrobranchi ...

jellyfish Jellyfish, also known as sea jellies or simply jellies, are the #Life cycle, medusa-phase of certain gelatinous members of the subphylum Medusozoa, which is a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animal ...

mollusks Mollusca is a phylum of protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant species of molluscs are recognized, making it the second-largest animal phylum after Arthropoda. The num ...

, and other

invertebrates Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordate subphylum ...

. It may also occasionally feed on seagrasses, though it rarely feeds on vegetation.

Predators

The flatback sea turtle is preyed upon by both terrestrial and aquatic organisms. The terrestrial predators it must face are

dingo The dingo (either included in the species ''Canis familiaris'', or considered one of the following independent taxa: ''Canis familiaris dingo'', ''Canis dingo'', or ''Canis lupus dingo'') is an ancient (basal (phylogenetics), basal) lineage ...

s, invasive

red fox The red fox (''Vulpes vulpes'') is the largest of the true foxes and one of the most widely distributed members of the order Carnivora, being present across the entire Northern Hemisphere including most of North America, Europe and Asia, plus ...

es,

feral dog A free-ranging dog is a dog that is not confined to a yard or house. Free-ranging dogs include street dogs, village dogs, stray dogs, feral dogs, etc., and may be owned or unowned. The global dog population is estimated to be 900 million, of wh ...

s, and

feral pig A feral pig is a domestic pig The pig (''Sus domesticus''), also called swine (: swine) or hog, is an omnivorous, domesticated, even-toed, hoofed mammal. It is named the domestic pig when distinguishing it from other members of the g ...

s. Known predators of adults of this species are

shark Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the ...

s and

saltwater crocodile The saltwater crocodile (''Crocodylus porosus'') is a crocodilian native to saltwater habitats, brackish wetlands and freshwater rivers from India's east coast across Southeast Asia and the Sundaland to northern Australia and Micronesia. It ha ...

s. The hatchlings also face predation from

crab Crabs are decapod crustaceans of the infraorder Brachyura (meaning "short tailed" in Greek language, Greek), which typically have a very short projecting tail-like abdomen#Arthropoda, abdomen, usually hidden entirely under the Thorax (arthropo ...

sea bird Seabirds (also known as marine birds) are birds that are adaptation, adapted to life within the marine ecosystem, marine environment. While seabirds vary greatly in lifestyle, behaviour and physiology, they often exhibit striking convergent ...

s, and juvenile

s on their journey to the waters. Once in the water, the hatchlings can be preyed upon by big

fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...

and even sharks. Due to their large size when they are born and their strong swimming skills, the likelihood of capture is lowered.

Conservation

Status

On the International Union for the Conservation of Nature or the IUCN's official website the flatback sea turtle is listed as data deficient. However, the flatback sea turtle is listed as vulnerable nationally in Australia. It is the least endangered of all of the sea turtles. Unlike other sea turtles, there is not a big human demand for the meat of the flatback sea turtle. It does not swim far from shore; thus, it does not get caught in nets as often as other sea turtles. These reasons can contribute to why it is not in more danger of extinction.

Threats

All marine turtles are faced with threats such as habitat loss, the wildlife trade, collection of eggs, collection of meat, fishery

bycatch Bycatch (or by-catch), in the fishing industry, is a fish or other marine species that is caught unintentionally while fishing for specific species or sizes of wildlife. Bycatch is either the wrong species, the wrong sex, or is undersized or juve ...

, pollution, and climate change. The flatback sea turtle is 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 is the destruction of 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. The flatback sea turtle also falls prey to incidental capture. It is caught by fishermen, particularly by

trawling Trawling is an industrial method of fishing that involves pulling a fishing net through the water behind one or more boats. The net used for trawling is called a trawl. This principle requires netting bags which are towed through water to catch di ...

gillnet Gillnetting is a fishing method that uses gillnets: vertical panels of netting that hang from a line with regularly spaced floaters that hold the line on the surface of the water. The floats are sometimes called "corks" and the line with corks is ...

fishing,

ghost net Ghost nets are fishing nets that have been abandoned, lost, or otherwise discarded in the ocean, lakes, and rivers. These nets, often nearly invisible in the dim light, can be left tangled on a rocky reef or drifting in the open sea. They can ...

s, and crab pots. Lastly, pollution is a concern for this creature. Pollution can affect the timing of egg laying, how it chooses its nesting site, how hatchlings find the sea after emerging, and how adult turtles find the beaches. Climate change was historically thought to be a threat to successful hatchling development, but a lab experiment on a Carpentarian population has found this to be untrue. Researchers have studied whether increased nest temperatures would be detrimental to embryos, whether through embryo death or negatively adapted phenotypes. However, the increased temperatures did not reduce the hatchling body size or their success, but it did accelerate the development of the embryo. This study found a higher pivotal temperature (the temperature that produces an equal number of males and females) than had previously been described, which suggests that some populations may have adapted to maintain large number of hatchlings of both sexes even under the

effects of climate change Effects of climate change are well documented and growing for Earth's natural environment and human societies. Changes to the climate system include an Instrumental temperature record, overall warming trend, Effects of climate change on the ...

. Instead, climate change may threaten flatback turtles by increasing the frequency or intensity of tropical storms and raising sea levels, which may result in the loss of nests or entire nesting sites. Since this species is very loyal to its nesting sites and has a long development time and lifespan, they are less able to adapt to changes in the environment within a short time.

Conservation methods

In 2003, a recovery plan was set in place nationally to help this species along with other sea turtles. This plan aims to reduce mortality rates through actions within commercial fisheries and to maintain a sustainable harvest by

Indigenous people There is no generally accepted definition of Indigenous peoples, although in the 21st century the focus has been on self-identification, cultural difference from other groups in a state, a special relationship with their traditional territ ...

. Monitoring programs are being developed and integrated, along with managing factors that affect the reproductive success of this species. In

Kakadu National Park Kakadu National Park is a protected area in the Northern Territory of Australia, southeast of Darwin. It is a World Heritage Site. Kakadu is also gazetted as a locality, covering the same area as the national park, with 313 people recorded l ...

, a monitoring program has already been set up for this species. This species' critical habitat is being identified for protection. There are also efforts to enhance the spread of information about the flatback sea turtle as well as cooperation and actions internationally. There is an important turtle rookery on Avoid Island, in the

Flat Isles The Northumberland Islands are a scattered island chain off the eastern coast of Queensland, Australia. Geography and history The Northumberland Islands are to the south-east of the city of Mackay, Queensland, Mackay roughly between the lat ...

of the

Northumberland Islands The Northumberland Islands are a scattered island chain off the eastern coast of Queensland, Australia. Geography and history The Northumberland Islands are to the south-east of the city of Mackay roughly between the latitudes 21°S and 22 ...

group, which is a nature refuge owned by the Queensland Trust for Nature since 2006. There has been a monitoring program in place since 2013, and researchers use the facilities to collect data for various projects about the species.

References

External links

*
Great Barrier Reef Marine Park Authority: Flatback turtles