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Flatback: Australia's Turtle

Australia is a special place, being home to evolutionary unique animals and sometimes just as unique people. The marsupials and birds that inhabit this huge island are well documented but the countries endemism doesn’t end there. Australia is also home to its own species of marine turtle, the Flatback sea turtle (Natator depressus). Characterised by its flattened shell, this turtle is distantly related to green and loggerhead turtles but is genetically quite distinct.

The classification of this turtle caused great confusion among early scientists. It was only declared a separate species by the 1980’s. Compared to the other sea turtles, the flatback differs greatly. The carapace is indeed ‘relatively flat’ but it is also somewhat soft and flexible and tends to turn up at the margins. The skin of the animal is soft and will even begin to bleed if scratched with a fingernail.

Much of our prior knowledge of the species is from anecdotal and unpublished reports from government agencies and academia. However, modern day concern for the conservation of all sea turtle species had led to greater investigation of the flatback. Analysis of digestive systems and stomach contents of flatbacks caught in nets suggest flatbacks are carnivores, feeding on varied diet of invertebrates including sea cucumbers, shrimp, jellyfish, molluscs, sea pens and other organisms living on the soft sea bed.

Perhaps the cause of the previous mystery surrounding this turtle is its small geographic range. Though the species is known to visit the waters in Papua New Guinea to feed, it nests only on beaches in Australia. The largest colony occurs on Crab Island in the North-eastern Gulf of Carpentaria in Queensland. The flatback has the smallest geographic range of the seven sea turtle species. Their distribution is restricted to tropical regions off the continental shelf and coastal waters of Northern Australia. They mostly inhabit water less than 60m deep.

Flatbacks mostly nest on undeveloped beaches usually with sand dune systems. They lay fewer eggs than other species, an average nests of just 54 eggs per clutch but the eggs are very large considering the species body size, eggs been almost the same size as leatherback eggs. Why this turtle lays so few, big eggs is a fascinating question. Which is a better strategy, produce many small eggs or produce fewer, larger eggs?

The answer to this question is most probably linked to another astonishing part of this species ecology. Flatbacks don’t undergo an oceanic stage in their life cycles. When most sea turtles spend their early years growing out in the oceans circularity systems, flatbacks stay near the coast. So perhaps the reason for having larger eggs is because the hatchlings can grow more rapidly in order to avoid certain predators in the shallow coastal waters. Instead of swimming off to the deep ocean, the hatchlings have to grow up in the coastal region which is patrolled by many different predators such as crocodiles, fish and predatory birds. In fact, flatbacks suffer from such high levels of predation that some nests suffer from 60% losses. Sand monitors, herons, pelicans, storks, dingoes and foxes all predate on the eggs and young.

Despite being so close to the coast for all of their lives, we still know surprisingly little about this species of sea turtle. In fact when looking on IUCNs red list the flatback is described as ‘data deficient’. It is estimated that there are around 10,000 flatbacks nesting in Australia each year, with 25,000 – 30,000 mature females thought to exist overall. It is perhaps a safe bet to presume that this species is vulnerable to the same threats as the other marine turtles.

Most sea turtle species are in serious trouble, they are being collected on beaches, caught in gill nets and trawls, hooked on longlines and suffering from heavy development on nesting beaches. However, the flatback could be an exception, because its range is restricted to Australian waters, no international treaties are needed for its conservation. Australia puts pressure on almost all their fisheries to implement the use of TED’s and encourage its citizens to protect the species. Despite this it is very likely that the populations are under pressure from changing habitats, especially through climate change. More studies must take place to better understand this mysterious species. If we don’t fully understand an animal, then how can we hope to give it the full protection in needs?

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