An Introduction to Sea Turtles with an Emphasis on Leatherbacks

This topic submitted by Gina Hayes ( hayesgl@miamioh.edu) at 10:41 PM on 5/17/05.

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An Introduction to Sea Turtles with an Emphasis on Leatherbacks

Sea turtle species have survived for over 200 million years and are now on the verge of extinction. Maybe looking into their past will give us information about their future: Reptiles came on the scene during the Pennsylvanian Period about 300 million years ago. The first turtle fossil was found in 220 million years old sediments from the Triassic Period. During this time period scientists are still unsure of how turtles evolved because there is no record of their ancestors in the geologic record. Proganochelys was the first fossilized turtle, it was a freshwater species resembling today’s snapping turtles. But, how can a completely different animal simply appear in the fossil record? Scientists believe that the ancestors of water going turtles were terrestrial, and turtle evolution was extremely fast, thus allowing for a lack of ancestral evolution records. (Spotilla, 2004) Recently, scientists have found that it only takes a minute change in genetic composition to lose a function or add a different function to an organism, such as the plastron and carapace found on turtles, thus allowing for an evolutionary change in a short amount of time. This controlling function was a tremendous success that is still a part of modern day turtle species. (Spotilla, 2004)

Turtles were at the height of their success in the Mesozoic Era, during this time turtles came and went from the ocean to land a number of times. Eventually, these turtles evolved into modern day sea turtles with the evolutionary advantages that have allowed them to be so successful for the last 200 million years. Such evolutionary advantages are their paddle-like front flippers, lack of “fingers” or claws, and the ability to handle changes in the environment. Sea turtles survived the extinction of the dinosaurs at the end of the Mesozoic Era with relative ease. They continued to live, grow and reproduce like there was no disturbance taking place. Two of the four families of sea turtle living during the Cretaceous period have become extinct since the dinosaurs plight, the Toxochelyidae and the Protostegidae (which includes the giant sea turtle Archelon) families. The other two families are Cheloniidae, which includes six of the seven species present on Earth today; and Dermochelyidae, which includes the great leatherback. Cheloniidae once had as many as twenty different genera swimming in the oceans, but as time went on only four genera survived to date: Chelonia, Eretmochely, Lepidochelys and Caretta. (Spotilla, 2004) Dermochelyidae reached its peak during the Eocene period about 50 million years ago, “at least six different species of leatherbacks lived in the oceans worldwide, their fossils found in Africa, Europe, North America, and New Zealand.” (Spotilla, 2004) Today, only one species of the great leatherbacks has survived: Dermochelys coriacea.

The leatherback is the largest and most amazing of all the sea turtle species. The adults can weigh between 500 and 2000 pounds and grow from 4 to 8 feet on average. (FWS, 2004) Leatherbacks have the most widespread distribution of all modern day sea turtle species. They have been seen around the globe including tropical environments and on the edge of the coldest oceans on Earth. The leatherback is a cold blooded reptile just like its ancestors, although it has a fat layer that adds insulation, thus they have the ability to swim in cooler waters and dive deeper than any other reptile. Leatherbacks are known to dive 4000 feet and deeper, that is as deep as a whale. (Spotilla, 2004) The ability to dive to such depths may lie in the biology of this type of turtle. Leatherbacks have a special type of shell that is covered with a thick, rubbery, “leathery” layer; this might make deep diving easier than with a hard shell which might not be able to withstand the pressures. “Other distinguishing characteristics of the leatherback include: specializes in eating jellyfish, lays the largest eggs of all the turtles, produces the heaviest clutches of eggs, produces the largest hatchlings, migrates the greatest distances, grows faster than any other reptile, and is among the widest ranging vertebrates on the planet.” (Spotilla, 2004)

Leatherbacks reach maturity between 6 and 10 years of age (FWS, 2004); at this time reproduction can begin. Every 2 to 3 years after maturity a female will return to her natal beach and lay 1 to 11 clutches of 50 to 100 eggs. (Spotilla, 2004) On average, the nesting female will return to that beach every 9 to 10 days to lay each clutch. (FWS, 2004) The nesting period for the United States and Caribbean leatherbacks is between March and June, and on the Pacific coasts of Mexico and Costa Rica between October and March. (Spotilla, 2004) This process takes about 2 hours from the time each female emerges from the ocean. She will crawl up the beach to dry sand, preferably close to deep water, and start digging a hole approximately 3 feet deep with her rear flippers and a shallow body pit with her front flippers. Then, she will deposit her clutch in the hole, cover it with sand making sure it is firmly packed over her eggs. Lastly, she makes her way back out to sea. Unfortunately, this arduous task is not increasing the leatherback population. There are many factors that play against the hatchlings survival, many of them never even get to the water after their 55 to 75 day incubation period (FWS, 2004), and often they never even hatch. These threats will be discussed later.

The migration patterns of the leatherback, as stated previously, are widespread. There doesn’t seem to be an exact pattern for their migration routes. This may be one reason why there is only one species of leatherback turtles (Spotilla, 2004), perhaps it’s because the species of certain areas migrate and reproduce so widely, thus not allowing for speciation.

The risks that sea turtles are facing are tremendous. All seven surviving species are endangered and/or threatened throughout most of their ranges. The leatherback was put on the endangered species list in 1970, and is still endangered throughout its range in the Atlantic and Pacific oceans. (NOAA, 2004) Most of the risks facing these majestic creatures are human induced such as results of human population growth, habitat loss, toxins, bycatch, pollution, predation and other environmental risks. Human population growth is destroying sea turtle habitats, both feeding and nesting grounds. Coastal areas in the US and around the world are important places for sea turtle nesting, but “over-developed coastal areas have reduced natural nesting habitats”. (Broward County, 2005) Therefore, coastal development and other factors such as erosion of beaches are resulting in habitat loss for all sea turtle species.

Increased exposure to toxins and pathogens are also a growing concern for sea turtle populations. A recent study shows that there are diseases emerging from wildlife populations similar to that of humans. This relates to the emerging disease fibropapillomatosis. The disease was once rare, but now, “occurs around the globe and in one recent sample from the Hawaiian Islands more than 90% of green turtles showed symptoms of the illness”. (Jones, 2004) The source of this tumor causing disease has not yet been identified, sea turtles, “can be added to a fairly long list of species that have become susceptible to various types of pathogen as a consequence of environmental change-almost all of which are human-induced”. (Jones, 2004)

Bycatch is another problem for sea turtles and has been for a very long time. Bycatch is “the incidental take of undesirable size or age classes of the target species, or to the incidental take of other non-target species. Individuals caught as bycatch can be unharmed, released with injuries, or killed”. (Lewison et al., 2004) Sea turtles are only caught as bycatch because they are not a target species. Typically they are caught in trawl nets used by the shrimp catching industries. Bycatch is an issue that has caused, and is causing, sea turtle populations to decline. This issue has been somewhat mitigated due to the implementation of Turtle Excluder Device’s (TED’s).

Two types of pollution are affecting turtle populations, regular everyday pollution such as litter, and ecological light pollution. Pollution affects sea turtles just as it does any other organism, one specific example is as follows: Leatherbacks eat many jellyfish, in fact, juveniles must eat their weight in jellies everyday to fulfill their needs, and often plastic bags are discarded into the oceans resembling leatherbacks favorite food. Once the turtle ingests the plastic it causes major problems and even death. Ecological light pollution is a problem for all turtle species. All of the coastal development on nesting beaches interferes with turtle hatchlings natural ability to make their way to out to sea. When hatchlings emerge from their nests they go toward the lit area, which is typically the ocean because of the moons’ reflection, but the coastal development is now the “lit” area and hatchlings are losing their orientation.

Sea turtle predation has lessened in recent years due to conservation efforts. There aren’t as many poachers for the turtle eggs as there have been in the past nor are the turtles being slaughtered for food as they once were. But, natural predators still exist, not really for adult turtles that can typically out swim their one marine predator, the shark, but for the turtle eggs and hatchlings. Animals such as raccoons and dogs will find nests and consume the eggs, also if the hatchlings leave the nest at the wrong time they run into hungry birds. Although, it is important to remember that humans are effecting sea turtles more than anything else, because, after all, the sea turtles have been around for over 200 million years and are now on the brink of extinction for one reason: us.

Environmental risks are also a contributing factor for these populations. One example, climate change and potential impacts could significantly effect sea turtle populations because of temperature dependent sex determination. Sex of sea turtles is dependent upon the temperature at the time. For example, the eggs near the center of the nest typically develop at a higher temperature, thus they tend to be female, whereas those near the outside develop at a lower temperature, and thus they tend to be male. Males develop at lower temperatures around 82 degrees and females develop at higher temperatures around 86 degrees. (Spotilla, 2004)

There are many conservation efforts taking place to aid in the protection of these magical creatures. Different leatherback populations are declining more rapidly than others, such as the Pacific leatherbacks that are almost completely extinct. There are protection measures happening in different areas of the world, some of which are in Costa Rica. One important sea turtle conservation site is near Talamanca. The ANAI association is a not for profit NGO that has been pursuing sustainable development in the Talamancan region since 1978. (ANAI, 2004) Part of this effort has focused on sea turtle conservation and the protection of one of the main nesting beaches utilized by leatherback turtles. By involving communities in the act of conservation there has been a tremendous success. No longer are the eggs being poached by the people, but instead are being protected by them. There are many other conservation organizations and projects in place to save the last of the great leatherbacks and other sea turtles. If more people don’t become aware and involved with the conservation efforts sea turtles could face the same fate as their distant reptilian friends, the dinosaurs.

References
1. ANAI. (n.d.) About ANAI. Retrieved May 15, 2005 from http://www.anaicr.org/anai/en/about_anai.html#tala

2. Broward County Florida-Biological Resources Division. (n.d.)
Sea turtle conservation program. Retrieved February 9, 2005 from http://www.co.broward.fl.us/bri00600.htm

3. Jones, A. (2004) Sea turtles: old viruses and new tricks. Current Biology, 14, R842-R843.

4. Lewison, R., Crowder, L., Read, A., & Freeman, S. (2004) Understanding impacts of fisheries bycatch on marine megafauna. TRENDS in Ecology and Evolution, 19 (11), 598-604.

5. National Oceanic and Atmospheric Administration-Office of Protected Resources. (n.d.) Marine Turtles. Retrieved February 2, 2005 from http://www.nmfs.noaa.gov/pr/species/turtles

6. Orenstein, Ronald. Turtles, Tortoises and Terrapins: Survivors in Armor. Firefly Books. Buffalo, New York. 2001.

7. Spotilla, James. Sea Turtles: A complete guide to their biology, behavior and conservation. The John Hopkins Community Press. Baltimore, Maryland. 2004.

8. United States Fish and Wildlife Service-North Florida Field Office. (n.d.) Sea Turtle Quick Facts. Retrieved May 15, 2005 from http://northflorida.fws.gov/SeaTurtles/turtle-facts-index.htm

9. United States Fish and Wildlife Service. (n.d.) Endangered Species Act of 1973. Retrieved February 12, 2005 from http://endangered.fws.gov/esa.html#Lnk09


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