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

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...

of sea turtle in the

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

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 t ...

. The species is

endemic Endemism is the state of a species being found 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 found else ...

to the sandy beaches and shallow coastal waters of the

Australia Australia, officially the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands. With an area of , Australia is the largest country by ...

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 contrast ...

from the fact that its shell has a flattened or lower dome than the other sea turtles. It can be olive green to grey with a cream underside. It averages from 76 to 96 cm (30 to 38 inches) 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 70 to 90 kg (154 to 198 lb). 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 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 the world's most comprehensive inventory of the global conservation status of biologi ...

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 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 a natural history, naturalist/zoologist from Pennsylvania. He became noted as an ichthyologist and herpetologist. Biography Garman was ...

. The

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

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 ...

'' (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 ...

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

Description

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

, 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 its head. The underside, also called the plastron, has a much lighter coloration of a pale yellow. The flatback sea turtle has an average carapace length ranging from 76 to 96 cm (30 to 38 in), and weighs from 70 to 90 kg (about 155 to 200 lb). 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 connected to the frontals, towards the tip of the snout, which are in contact with the internasals. See also * Snake scales * Scale (zoology) In most biological nomenclature, a ...

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. This feature causes the shell to crack under the smallest 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 ...

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 exten ...

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 reac ...

, and the coastal areas of

Papua New Guinea Papua New Guinea (abbreviated PNG; , ; tpi, Papua Niugini; ho, Papua Niu Gini), officially the Independent State of Papua New Guinea ( tpi, Independen Stet bilong Papua Niugini; ho, Independen Stet bilong Papua Niu Gini), is a country i ...

. Its distribution within Australia is in the areas of eastern

Queensland ) , nickname = Sunshine State , image_map = Queensland in Australia.svg , map_caption = Location of Queensland in Australia , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , establishe ...

Torres Strait The Torres Strait (), also known as Zenadh 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, the northernmost extremity of the Australian mai ...

and

Gulf of Carpentaria The Gulf of Carpentaria (, ) is a large, shallow sea enclosed on three sides by northern Australia and bounded on the north by the eastern Arafura Sea (the body of water that lies between Australia and New Guinea). The northern boundary i ...

Northern Territory The Northern Territory (commonly abbreviated as NT; formally the Northern Territory of Australia) is an Australian territory in the central and central northern regions of Australia. The Northern Territory shares its borders with Western Aust ...

, and

Western Australia Western Australia (commonly abbreviated as WA) is a state of Australia occupying the western percent of the land area of Australia excluding external territories. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to t ...

. 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, the term habitat summarises the array of resources, physical and 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 ...

s 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 (84 to 91 °F) at nest depth, which are the temperatures that help determine the hatchling's sex.

Life history

Early life

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 its carapace length averaging . Its large size helps protect it from some of the predators after hatching, and allows it to also be a 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. Flatback hatchling

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 within the slopes of the dunes. 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. Depending on the area of the nesting site, the nesting season can go from November to January or can last the entire year. Females 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 turtle's clutch are fewer than other sea turtles. It 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 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 (84 °F), 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 marine animals with a leathery skin and an elongated body containing a single, branched gonad. Sea cucumbers are found on the sea floor worldwide. The number of holothurian ...

shrimp Shrimp are crustaceans (a form of shellfish) with elongated bodies and a primarily swimming mode of locomotion – most commonly Caridea and Dendrobranchiata of the decapod order, although some crustaceans outside of this order are ref ...

jellyfish Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbre ...

mollusks Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...

, and other

invertebrates Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordat ...

. It will also occasionally feed on seagrasses, even though it rarely feeds on vegetation. Red Fox (Vulpes vulpes) (1)

Predators

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

dingo The dingo (''Canis familiaris'', ''Canis familiaris dingo'', ''Canis dingo'', or ''Canis lupus dingo'') is an ancient ( basal) lineage of dog found in Australia. Its taxonomic classification is debated as indicated by the variety of scienti ...

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 (biology), order Carnivora, being present across the entire Northern Hemisphere including most of North America, Europe ...

es, feral dogs, and

feral pig The feral pig is a domestic pig which has gone feral, meaning it lives in the wild. They are found mostly in the Americas and Australia. Razorback and wild hog are Americanisms applied to feral pigs or boar-pig hybrids. Definition A feral ...

s. The aquatic predators to this turtle are

shark Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...

s. The hatchlings also face predation from

crab Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen) ( el, βραχύς , translit=brachys = short, / = tail), usually hidden entirely under the thorax. They live in all th ...

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

s, and juvenille

saltwater crocodile The saltwater crocodile (''Crocodylus porosus'') is a crocodilian native to saltwater habitats and brackish wetlands from India's east coast across Southeast Asia and the Sundaic region to northern Australia and Micronesia. It has been l ...

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

fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...

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

Gillnets in storage at unidentified cannery, Nushagak, Alaska, 1917 (COBB 173)

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,

by-catch 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 a 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 different spec ...

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. 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 entangle fish, dol ...

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. Historically climate change was thought to be an influential factor affecting the success of flatback sea turtle development, however, through recent research, performed on a select group of turtles, this was found to be untrue. Researchers have studied whether increased nest temperatures would be detrimental to embryos, whether through

embryo death Embryo loss (also known as embryo death or embryo resorption) is the death of an embryo at any stage of its development which in humans, is between the second through eighth week after fertilization. Failed development of an embryo often results i ...

or negatively adapted phenotypes. However, the increased nest temperature did not reduce the success of the hatchlings, the hatchling body size, but it did accelerate the development of the embryo. It was also discovered that there was a high 'pivotal sex-determining temperature' in the specific flatback population, which shows that some populations may had adapted to maintain large number of hatchlings of both sexes even under the

effects of climate change The effects of climate change impact the physical environment, ecosystems and human societies. The environmental effects of climate change are broad and far-reaching. They affect the water cycle, oceans, sea and land ice ( glaciers), sea le ...

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 Indigenous peoples are culturally distinct ethnic groups whose members are directly descended from the earliest known inhabitants of a particular geographic region and, to some extent, maintain the language and culture of those original people ...

. 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 liv ...

, 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 Avoid Island is one of the Flat Isles, within the island chain called the Northumberland Islands, around off the eastern coast of Queensland, Australia, approximately south of Mackay. Owned and managed by the Queensland Trust for Nature (QT ...

, 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 roughly between the latitudes 21°S and 22 ...

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