Leatherback Sea Turtle
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| Leatherback Sea Turtle Conservation status: Critical | ||||||||||||||
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| Missing image LeatherbackTurtle.jpg Leatherback Sea Turtle Leatherback Sea Turtle | ||||||||||||||
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| Dermochelys coriacea (Vandelli, 1761) |
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The Leatherback Sea Turtle (Dermochelys coriacea) is the biggest of all turtles alive, reaching a length of 2 metres and a weight of up to 1500 lb (600 kg). One individual male weighed 916 kg, though other turtles of this size are rare. It is found in all tropic and subtropic oceans. It is the only species in the family Dermochelyidae.
This species has many unique features that distinguish it greatly from other sea turtles. Its metabolic rate is approximately 3 times higher than one would expect for a reptile of its size, which, coupled with counter-current heat exchangers and large size, allow it to maintain a body temperature above that of the surrounding water. It also has a shell that lacks the bony scutes of other turtles, being comprised mainly of connective tissue.
Physical Characteristics
The Leatherback is the largest of all the turtles and is very different from other turtles in both appearance and physiology. It has a smooth, blackish carapace with ridges running from head to tail. This shell is not made of bone plates but of soft connective tissue. The carapace does not meet the plastron at a sharp angle like in other turtles, but a gentle curve, giving the animal a semi-cylindrical appearance.
The front flippers of the Leatherback turtle are much larger than those of other turtles, both in proportion and in absolute size. Those of adults can span 270 cm from tip to tip.
The beak of the Leatherback turtle is specially hooked to help it bite jellyfish and its throat has backward-facing barbs to help it swallow them.
Diet
Leatherback turtles subsist on a diet of jellyfish. Because of the translucent nature of its prey, Leatherbacks are often fooled into eating floating plastic debris. Dead leatherbacks have been found with plastic bags, pieces of hard plastic, and monofilament fishing line in their stomachs.
Breeding
Leatherbacks mate at sea; males never leave the water once they enter it as hatchlings. Females mate every three or four years, returning to the beaches where they themselves were hatched to deposit their eggs. One female may lay as many as ten clutches in one breeding season. The interval between laying is about nine days. Mating only occurs after the age of ten years.
After encountering a female (who possibly exudes a pheromone to signal her reproductive status) the leatherback male uses head movements, nuzzling, biting or flipper movements to determine her receptiveness. Marine turtles often face a difficult and sometimes even dangerous task when attempting to reproduce. The male has to mount the female from behind and latch on in order to be able to copulate, but sometimes their shells obstruct this process. Mating can also become dangerous when the male is so desperately overeager to find a mate that he stays underwater for too long, and after encountering the female, he must spend another hour with no air. Fertilization is internal, and multiple males usually mate with a single female. This may be a development evolved to insure against male infertility, eliminate the risk of sperm depletion, allow females to select the highest quality sperm, and increase the genetic variation amongst offspring. However, studies have shown that the process of polyandry in sea turtles actually reduces fertilization success.
Cleavage of the cell begins within hours of fertilization, but development is suspended during the gastrulation period of movements and infoldings of embryonic cells, while the eggs are being laid. Development soon resumes, but the embryos remain extremely susceptible to movement-induced mortality in their nests until the membranes fully develop through the first 20-25 days of incubation, when the structural differentiation of body and organs (organogenesis) soon follows.
Nesting beaches must be comprised of soft sand and have a shallow approach angle from the sea. This is because their soft shells are easily damaged by hard rocks. This is a source of vulnerability for the turtles because these beaches are subject to erosion. Females excavate a nest above the high-tide line with their flippers. They then begin to lay their eggs, producing about 110 ova, 70 of which are large and fertile, the remaining 40 being smaller and sterile. The female carefully back-fills the nest, making sure to disguise it from predators with a scattering of sand.
The eggs hatch in about 60 days. Like some other reptiles, the ambient temperature of the nest determines the gender of the hatchlings. The eggs hatch while still buried under the sand. After nightfall, the hatchlings dig their way to the surface and make their way to the sea. Once the hatchlings reach the ocean they are generally not seen again until maturity. Very few of them survive this mysterious period to become adults. Most hatchlings are eaten by birds or even other reptiles before they have a chance to reach the water. When the lights of a city are visible from a hatching site, Leatherback hatchlings are attracted to the lights and away from the sea. Many of these hatchlings are struck by traffic or otherwise perish.
Atlantic Leatherback Turtles nest between February and July from Georgia in the United States to the U.S. Virgin Islands in the Caribbean.
Range and Habitat
In the summer months, Atlantic Leatherback turtles are most common from the Gulf of Maine in the north to the coast of central Florida in the south. They have also been sighted as far north as the Gulf of St. Lawrence in Canada. Pacific Leatherback turtles are most often seen off the Hawaiian Islands, where they are known to congregate north of the archipelago.
The turtles prefer deep water but are most often spotted within sight of land. In the summer they are frequently seen basking near the surface, particularly in the Long Island Sound, where they have been injured by collisions with boat propellers.
Threats and Conservation
In the United States the Leatherback turtle has been classified as endangered across its range since 1970. It is also listed on Appendix 1 of the Convention on International Trade in Endangered Species of Wild Flora and Fauna (CITES). This makes it illegal to harm or kill the turtles.
Adult Leatherback turtles are large animals that are not particularly vulnerable to natural predators. Eggs and hatchlings are most vulnerable to predation of all kinds. Birds, dogs, or other opportunists are known to dig up nests and consume eggs. New hatchlings are also vulnerable on their journey from nest to sea. Once they enter the water they become prey to many new predators and very few survive to adulthood.
Human activity endangers Leatherback turtles in many ways. Though it is forbidden, eggs are harvested by people in Puerto Rico, the surrounding islands, and possibly in other places. Development of beaches can disturb or destroy the particular kind of habitat that Leatherbacks need to nest, and the lights of development can cause hatchlings to move away from the sea rather than toward it. Human use of beaches can crush nests and hatchlings or bury eggs too deep for the hatchlings to emerge. Finally humans may disturb nesting females out of curiosity.
While adults are at sea their major threats are all from humans. Ingesting plastic, rubber, tar, oil, and other synthetic materials can kill an adult Leatherback or severly injure it. Many have been injured by colliding with boats, especially in shallow water. The equipment associated with commercial fishing, including lines, nets, ropes, and cables can entangle adult turtles and cause them to drown. Though "Turtle Exclusion Devices" are mandated on nets, they often fail to allow an animal the size of an adult Leatherback to escape. The NOAA estimates that about 640 adult Leatherback turtles are killed each year by commercial fishing enterprise.
Nets are purposely set for other kinds of sea turtles in some areas of Puerto Rico. Though they are not intended for Leatherbacks, some are occasionally caught. Despite its illegality, the practice continues.
Sources and External Links
- The Turtles.org home page (http://www.turtles.org/leatherd.htm)
- The Leatherback Trust (http://www.leatherback.org/)
- The Oceanic Resource Foundation (http://www.orf.org/turtles_leatherback.html)
- NOAA Office of Protected Resources (http://www.nmfs.noaa.gov/prot_res/species/turtles/leatherback.html)
- The Physics factbook (http://hypertextbook.com/facts/1999/RachelShweky.shtml)
- Leatherback Sea Turtle at CRESLI (http://www.cresli.org/cresli/turtles/leaback.html)
Leatherback turtle at Animal Diversity Web (http://animaldiversity.ummz.umich.edu/site/accounts/information/Dermochelys_coriacea.html)