The most extraordinary specimen of all shelled marine reptiles, the leatherback (Dermochelys coriacea) was introduced to the scientific world in 1554 by French naturalist Guilaume Rondelet. Before Rondelet, these turtles were placed in the same category as mythical sea serpents. He described the beast as been huge in size and possessing the hide of an ox, tough and black. The leatherback is a true relic of an ancient marine world, being the only surviving member of the Dermochelyidae turtle linage.
Leatherback turtles are a natural wonder, breaking many records. The species is the largest turtle in the world and one of the largest living reptiles anywhere. They are the deepest diving reptile, getting down to depths of 4000 feet, the same as a whale. It is the fastest swimming turtle, streamlined shells create very little drag. Leatherbacks are also the fastest growing reptile, they can reach adult size in 7-13 years, reaching around 140-160 cm in length. Their large body size and thick, fatty insulation allow them to control their body temperature, maintain a warm constant in the frigid North Atlantic, not following the rules of a supposedly cold blooded animal.
Perhaps the most striking feature of this species, apart from its size, is the strange texture of its shell. As the name suggests the leatherback does not have the hard shell seen in other turtles, its shell is covered in a leathery skin. Their shells are composed of numerous small ‘dermal bones’ that lie above the distinct ribs and just beneath the skin surface. It is thought the leatherback evolved this soft shell to compress at the great depths they dive to, a hard shell might snap under the immense pressure.
Furthermore, leatherbacks have a unique diet. They are specialised jellyfish eaters, having to eat their weight in jellyfish each day to support their rapid growth rates. Jellyfish are mainly water but do contain some protein, small amounts of fat, iron and a variety of vitamins and minerals, still water content is so high it is a wonder how such a large creature can extract enough energy from this diet. This can be explained due to the evolution of an extraordinarily long oesophagus. From the mouth it extends all the way to the rear of the body before looping back up the side to enter the stomach. It can expand to hold a tremendous amount of food, it can store large amounts of food while slowly digesting the nutrients.
Amazingly leatherbacks seem to be able to control their body temperature, just like a warm blooded mammal. High metabolic rates in the muscles allow them to keep warm. For example in Canada leatherbacks have a body temperature of 15-25c when the ocean is just 5c. Adults have a thick layer of fat under their shells and fat under the skin of their shoulders and necks, offering excellent insulation. But what about when they migrate to tropical waters? When leatherbacks get hot they can pump blood from deep within their body parts past the layer of fat, this allows heat to be dissipated away.
Preying on jellyfish not only spurs rapid growth but also fuels the turtle’s long-distance migrations and high reproductive output. Their migrations appear to follow or anticipate the arrival of large schools of jellyfish. Like big whales the leatherback swims from near arctic to the tropics and as far south as New Zealand and the Southern Ocean. However, the turtle exceeds the distribution of whales by climbing out of the sea and heaving itself across the beach to nest. Wide dispersal leads to high gene flow within the species giving little genetic differences between populations. Leatherbacks nesting on the pacific coast of Mexico and Costa Rica migrate south to feeding grounds as far down as the coasts of Peru and Chile.
The largest nesting colony of leatherbacks is off the coast of French Guiana. More than 7000 females laid as many as 50,000 nests there in 1988 and 1992. Each season female leatherbacks will lay 1-10 clutches of 50-100 large eggs, individuals will return to nest again after 2-7 years. Hatchling success in undisturbed nests is only about 50% and some beaches suffer from high nest predation. The low success is a mystery. Some suggest eggs may be laden with toxins that increase egg mortality. Those hatchlings that do make it enter the ocean and swim vigorously for at least six nights and days. They can orientate in the earth’s magnetic field to remember where they were born before been carried off by the prevailing currents to convergence zones away from shore.
Moving on, population numbers are hard to gather but those observed are declining at an alarming rate. In 1980 there was an estimated 115,000 nesting females in the world, by 1994 this figure had reduced to fewer than 50,000 worldwide. In Terengganu, Malaysia nesting beaches used to boast up to 10,000 nesting leatherbacks, now they are locally extinct due to mass egg harvesting from locals. It is estimated there could now be around 30,000 leatherbacks less, but in truth this figure could be wildly inaccurate.
What has happened to the world’s largest turtle? Firstly years of egg harvest from many beaches have limited supply of new turtles into the populations. Killing of nesting beaches of Mexico, panama and other countries completely eliminated certain populations. Plastic pollution also heavily effects the leatherbacks who cannot distinguish between a jellyfish meal and a plastic shopping bag which then get entangled in their digestive system.
Perhaps the most significant cause of decline is the slaughter in the oceans. In the Pacific around the 1980’s, giant drift nets stretched for hundreds of miles and hundreds of feet deep, decimating turtle populations. These huge nets are now illegal but leatherbacks still have to evade coastal gill nets, often focused nearby turtle feeding grounds. Chile’s swordfish gill net fishery killed thousands of leatherbacks in the 1990’s. Nowadays longline fishing has become the most fearful obstacle. Fishing fleets from 40 nations now set 3.8 million hooks each day, 1.4 billion hooks a year. Some scientists suggest around 7 loggerhead and 1 leatherback is killed per 1000 hooks. If you do some quick maths that is about 230,000 loggerheads and 55,000 leatherbacks a year, mortality in the oceans is terrifying.
However there is hope. Conservation efforts in South Africa have built a nesting population from around 20 females to more than 100 nesting females each year. Similar programs from around the Caribbean do offer some hope that leatherback populations can bounce back. The IUCN had put leatherbacks under the highest protection as critically endangered, but now uncertainties on population numbers have moved the status down to vulnerable. This is a mistake, leatherbacks need the upmost protection if we are to keep them alive.
Development on nesting beaches must be heavily controlled. There are many beaches you can build a hotel but there are not many beaches where a leatherback can nest. Longline and gill net fishing must be heavily controlled. TEDs need to be modified to accommodate the larger leatherbacks. Egg collection needs to be made illegal worldwide. We need to put pressure on governments to make these things happen. Focus needs to be put on saving the Pacific populations that are most at threat, if not we could lose genetic diversity within the species. We either need to take the necessary steps today or we can lose this incredible species forever.