Sea turtles are ancient reptiles that survived the mass extinction that wiped out dinosaurs at the end of the Mesozoic era. Modern sea turtles arose during the Cretaceous era about 110 million years ago. Today, there are seven species of sea turtles recognized by scientists. They are, in increasing order of size, the olive ridley (Lepidochelys olivacea), Kemp’s ridley (Lepidochelys kempii), the hawksbill (Eretmochelys imbricata), the flatback (Natator depressus), the loggerhead (Caretta caretta), the green (Chelonia mydas), and the leatherback (Dermochelys coriacea).
For the past one hundred million years sea turtles have remained virtually unchanged1. This suggests that their way of life is a very successful one.
1. Distribution and habitat
Sea turtles are found in the warm and temperate seas of the world. They are relatively widespread across the Atlantic, Pacific, and Indian Ocean, except for the flatback turtle that is endemic to Australia and Kemp’s ridley that is only found in the Gulf of Mexico.
Mature turtles live mainly near coastlines and around islands. Adults of most species are frequent in bays or lagoons and in shallow coastal waters with seagrass beds or coral reefs. Often a turtle will have a favourite sleeping location and will return to it night after night2. Juveniles are found in shallow coastal waters and sometimes out at sea as well.
2. Life history
Marine turtles, as are most reptiles, are oviparous. The males spend their lifetime at sea whereas the females must return to land to lay their eggs in the sand. Depending on the species, turtles will lay two to eight clutches of eggs at each breeding season at about two-week intervals. Clutch size usually varies from 50 to 200 eggs, averaging around 120 eggs3. The incubation period of turtle eggs varies from 45 to 75 days depending on incubation temperatures.
After hatching, hatchlings of a same clutch dig towards the surface together, emerge from the nest, instinctively head towards the ocean, and swim out to open waters for several days during what is called the juvenile frenzy. During the following years, dubbed the “lost years” by Archie Carr, the acknowledged founder of sea turtle biology, the juveniles develop in the open ocean, feeding opportunistically on algae, small crustaceans, and invertebrates, and relying on oceanic currents for dispersal4. Sexual maturity is thought to be reached between the age of 20-45 years5, although an age of maturity as short as 15 years may be possible6.
Female sea turtles return to their natal beach to lay their eggs. In order to do so, they may have to migrate several thousands of kilometres from their feeding grounds to their nesting ground7. Of all marine turtles, green sea turtles express the strongest philopatry8. The exact means by which imprinting of the natal beach works remains a mystery. Alongside other mechanisms it is thought that turtles use the magnetic field of the Earth to navigate and find their natal beach9.
Nesting occurs mainly under the cover of darkness when temperatures are cooler and predators are fewer. Inter-nesting interval is of one to four years.
4. Natural threats
Sea turtles are most vulnerable during their early life-stages. Depending on the nesting beach, raccoons, foxes, feral dogs, ghost crabs, and seabirds eat the eggs. Hatchlings are preyed upon by crabs, seabirds, and carnivorous fish. As an evolutionary response to high predation of their offspring, turtles produce a high number of eggs. However, it is estimated that as little as one in a thousand hatchlings may reach breeding age10. On the other hand, adults have very little predators owing to their effective plated armour and are only seldom eaten by tiger sharks.
Furthermore, an infectious tumour-causing disease, fibropapillomatosis, is currently affecting adult green sea turtles11. The cause of the disease is under scrutiny and a treatment has yet to be discovered. Infected individuals develop tumours that affect the individual’s mobility and/or senses and this can lead to death.
5. Anthropogenic threats
The greatest threat to sea turtles is humans. The main factor that contributed to the worldwide decline of marine turtles is the commercial harvest for meat, eggs, shells, and leather. Additionally, the destruction of nesting beaches, oceanic pollution, accidental boat strikes, chemical pollution, photopollution, and habitat loss and degradation are all factors that contribute to the decline of turtle numbers in the wild. An increased source of mortality lies in incidental catches of adults in commercial shrimp trawling. Turtles are caught in the fishing nets and are drowned. Additionally, bycatch of turtles of older age classes by pelagic longlines is a non-negligible threat to the species12. Climate change is also an increasing source of concern for the future of the species13.
6. Temperature-dependent sex detemination
All species of sea turtles exhibit temperature-dependent sex determination (TSD), a process by which the sex of an individual is determined by the temperature at which the eggs incubate. High incubation temperatures produce females whereas low incubation temperatures produce males. The study of TSD generates a wide variety of questions regarding the physiological, ecological, and conservational implications for sea turtle populations worldwide14, particularly in the context of climate change and global warming.
7. Protection and conservation
All seven species of sea turtles are present on the International Union for the Conservation of Nature (IUCN) Red List of Threatened Species. Both Kemp’s ridley and the hawksbill are categorized as “critically endangered”. The green and the loggerhead are “endangered”, whereas the olive ridley and leatherback are “vulnerable”. This means that these species are all facing a risk of extinction in the wild with varying degrees of urgency. The flatback is listed as “data deficient”15.
In addition, all species are listed on Appendix I of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). This means that the international trade of marine turtle specimens (except when the purpose of import is not commercial, for instance for scientific research) is prohibited16.
Marine turtles play an important ecological role in both the coastal and the marine ecosystems. It is therefore crucial to protect and conserve turtles. For example, the list of ecological services that green sea turtles provide include maintaining healthy seagrass communities, supplying concentrated sources of high-quality nutrients to nesting beaches via unhatched eggs, maintaining a balanced food-web, and providing habitats for other species17. In the same way, the other species of sea turtles affect their respective habitats in a positive way. If marine turtles were to disappear, diversity and functionality of their habitats would be negatively affected and the structure and function of entire ecosystems would be changed17.
For more infomation about sea turtles and their study, I would highly recommend the following books:
So Excellent a Fishe: a Natural History of Sea Turtles. Archie F. Carr.
Conserving Sea Turtles. Nicholas Mrosovsky.
Sea Turtles: A Complete Guide to Their Biology, Behavior, and Conservation. James R. Spotila.
Saving Sea Turtles: Extraordinary Stories from the Battle Against Extinction. James R. Spotila.
The Biology of Sea Turtles, Volume III. Editors: Jeanette Wyneken, Keneth J. Lohmann, and John A. Musick.
And finally, here are three resourceful websites:
1 Bustard R. H. Sea Turtles: Natural History and Conservation. Taplinger Publishing Company (1973).
2 Spotila J. R. Sea Turtles: A Complete Guide to Their Biology, Behavior, and Conservation. John Hopkins University Press (2004).
3 Sea Turtle Conservancy. Information about sea turtles: frequently asked questions. World Wide Web electronic publication <http://www.conserveturtles.org> last accessed on 21/08/2014.
4 Witham R. The “lost year” question in young sea turtles. American Zoologist 20:525-530 (1980).
5 Scott R., Marsh R. & Hays G. C. Life in the really slow lane: loggerhead sea turtles mature late relative to other reptiles. Functional Ecology 26:227-235 (2012).
6 Bell C. D. L., Parsons J., Austin T. J., Broderick A. C., Ebanks-Petrie G. & Godley B. J. Some of them came home: the Cayman Turtle Farm headstarting project for the Green Turtle Chelonia mydas. Oryx 39(2):137-148 (2005).
7 Hays G. C., Mortimer J. A., Ierodiaconou D. & Esteban N. Use of long-distance migration patterns of an endangered species to inform conservation planning for the world’s largest Marine Protected Area. Conservation Biology (in press) doi: 10.1111/cobi.12325 (2014).
8 Pritchard P. C. H. The conservation of sea turtles: practices and problems. American Zoologist 20:609-617 (1980).
9 Luschi P., Benhamou S., Girard C., Ciccione S., Roos D., Sudre J. & Benvenuti S. Marine turtles use geomagnetic cues during open-sea homing. Current Biology 17(2):126-33 (2007).
10 Wickramasinghhe R. H. Sea turtle of Sri Lankan waters. Vidurava 13(2):20-22 (1991).
11 Alonso Aguirre A. & Lutz P. L. Marine turtles as sentinels of ecosystem health: is fibropapillomatosis an indicator? EcoHealth 1(3):275-283 (2004).
12 Lewison R. L. & Crowder L. B. Putting longline bycatch of sea turtles into perspective. Conservation Biology 21(1):79-86 (2006).
13 Hays G. C., Broderick A. C., Glen F. & Godley B. J. Climate change and sea turtles: a 150-year reconstruction of incubation temperatures at a major marine turtle rookery. Global Change Biology 9:642-646 (2003).
14 Lutz P. L., Musick J. A. & Wyneken J. (editors). The Biology of Sea Turtles, Volume II. CRC Press (2003).
15 IUCN. IUCN Red List of Threatened Species, version 2014.2. World Wide Web electronic publication <http://www.iucnredlist.org> last accessed on 21/08/2014.
16 CITES. Appendices I, II and III. World Wide Web electronic publication <http://cites.org/eng> last accessed on 21/08/2014.
17 Wilson E. G., Miller K. L., Allison D. & Magliocca M. Why healthy oceans need sea turtles: the importance of sea turtles to marine ecosystems. World Wide Web electronic publication <http://na.oceana.org> last accessed on 21/08/2014.