A view from the air--Where have the forests gone? (Sierpe River Area, SW Costa Rica)
An Introduction to Sea Turtles: Evolution and Comparison of Leatherbacks, Loggerheads, Greens and Hawksbills
Modern sea turtle have only been around for about the last 120 million years, so what was going on before that? How did they become what they are today? How do the remaining species compare to each other?
History and Evolution
Reptiles became part of the evolutionary scene approximately 300 million years ago at the end of the Paleozoic Era. Whereas, the first turtle fossil wasn’t discovered until about 80 million years later during the Triassic period, which was the beginning of the Age of Reptiles. The first fossil was that of Proganochelys, a freshwater, much larger version of modern day snapping turtles. There is quite a mystery surrounding turtle evolution; it is thought that it happened in a relatively short time due to minimal fossil evidence leading up to the find of Proganochelys. It is thought that a minute change in genetic composition could lead to the creation of a carapace and plastron, thus a small genetic change could have lead to a huge evolutionary advantage. (Spotilla, 2004) During the Mesozoic Era turtles went back and forth between land and sea many times, and it wasn’t until about 120 million years ago during the Cretaceous period that modern sea turtles evolved. Thus, flippers and other sorts of evolutionary advantages didn’t result until that time.
At the end of the Mesozoic there was a huge mass extinction, possibly the most well-known of all mass extinctions due to the loss of the dinosaurs. After the mass extinction approximately 66 million years ago, the Age of Reptiles gave way to the Age of Mammals. The reptiles that faired the best during and after the extinction were perhaps the sea going turtles. Four families of sea turtles survived, Toxochelyidae, Protostegidae, Cheloniidae and Dermochelyidae. The former two families have since become extinct, Toxochelyidae during the Eocene epoch and Protostegidae during the Oligocene epoch. There are currently seven species of sea turtles swimming in the oceans, six of which are in the family Cheloniidae, the other is the solitary member of the Dermochelyidae family. This paper will compare four of the seven sea turtle species, leatherbacks, loggerheads, greens and hawksbills. All are from the Cheloniidae family with the exception of the leatherback which is from Dermochelyidae.
The major differences between the species are the shape of their heads and the placement of their scutes or scales. (Spotilla, 2004) The shell size and weight distribution also plays a small role. The following graph (Figure 1) shows the shell size distribution between the four focus species:
Figure 1: Shell size distribution between leatherbacks, loggerheads, greens and hawksbills.
The leatherbacks have the largest shell range of between 52 and 70 inches, whereas the hawksbills have the smallest range of between 30 and 35 inches. The loggerheads and greens have very similar distribution ranging from 32 to 49 inches. The mass distribution (Figure 2) of the same four species of turtle show similar trends.
Figure 2: Mass distribution of leatherbacks, loggerheads, greens and hawksbills.
Again, the leatherbacks are the largest with a mass between 550 and 2000 pounds and the smaller hawksbills mass is between 95 and 165 pounds. The loggerheads and greens have a very similar mass, although the green turtle is often said to be larger. (Spotilla, 2004)
Dermochelys coriacea: ~34,000 nesting females (CCC, 2005)
Leatherback sea turtles are classified as endangered on the US Federal Endangered Species List, and as critically endangered internationally. The leatherback turtle is named for its’ “leathery” type skin covering the carapace. It is the only sea turtle that doesn’t have a “hard” shell; perhaps this helps them dive deeper than any other species of sea turtle. The carapace is covered with rubbery type skin with seven distinct ridges running its full length. Leatherbacks have very delicate scissor-like jaws which are quite useful when feeding on their main source of food, jellyfish. Leatherbacks are the most widely distributed of all sea turtles (Figure 3); they are known to migrate around the globe from the tropics to the edge of the coldest oceans on Earth. (CCC, 2005)
Figure 3: Leatherback range and nesting sites worldwide. (CCC, 2005)
Caretta caretta: ~60,000 nesting females
Loggerhead sea turtles are classified as threatened on the US Endangered Species List, and as endangered internationally. They are typically carnivorous reptiles feeding on mollusks and other sorts of shellfish in shallow water along continental shelves. (CCC, 2005; Spotilla, 2004) They get their name from their powerful jaws, which aid in their food consumption. Their head is very large and their carapace can vary from red to brown to yellow. Loggerheads are found in both tropical and temperate waters worldwide (Figure 4).
Figure 4: Loggerhead range and nesting sites worldwide. (CCC, 2005)
Chelonia mydas: ~203,000 nesting females
Green sea turtles are classified as endangered on the US Federal Endangered Species List, and endangered internationally. Greens begin life as omnivores eating both grasses and small crustaceans (CCC, 2005), but as they reach maturity they feed exclusively on grasses and algae; thus acquiring a slightly green hue from which they receive their name. Green turtles prefer to stay near food sources, such as sea grass beds and shallower coastline areas. Similar to loggerheads, green turtles are also found in both temperate and tropical waters worldwide. (Figure 5) (CCC, 2005)
Figure 5: Green turtle range and nesting sites worldwide. (CCC, 2005)
Eretmochelys imbricata: ~34,000 nesting females
Hawksbill sea turtles are listed as endangered on the US Federal Endangered Species List, and as critically endangered internationally. Hawksbills are, perhaps, the most beautiful turtles with a gorgeous “tortoise shell” carapace that is typically orange and brown. It is one of the smaller sea turtles that feed almost exclusively on sponges on reefs and in rocky, shallow waters. (Spotilla, 2004; CCC, 2005) The hawksbill prefers tropical waters and is typically only found within such a range. (Figure 6)
Figure 6: Hawksbill range and nesting sites worldwide. (CCC, 2005)
Threats to sea turtle populations
The risks facing sea turtles today are tremendous. All seven surviving species are endangered and/or threatened throughout most of their ranges. For example, 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 impacts of human population growth, habitat loss, toxins, bycatch, pollution, predation and other environmental risks. Human population growth is destroying sea turtle habitats, both their 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 diseases emerging from wildlife populations are similar to that of humans. This relates to the emerging disease fibropapillomatosis. The disease was once rare, but now, occurs in turtles around the globe. 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 now be added to a fairly long list of species that have become susceptible to various types of pathogens as a consequence of environmental change-almost all of which are human-induced. (Jones, 2004)
Bycatch is another problem for sea turtle populations, 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 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 Devices (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 leatherback’s favorite food. Once the turtle ingests the plastic it causes major problems and even death. (Spotilla, 2004) Ecological light pollution is another problem affecting all turtle species. All coastal development on nesting beaches interferes with turtle hatchlings’ natural ability to make their way out to sea. When hatchlings emerge from their nests they go toward the lighter area, which is typically the ocean because of the moons reflection, but coastal development has made the coast brighter than the sea resulting in hatchlings’ loss of orientation.
Sea turtle predation has lessened in recent years due to conservation efforts. There aren’t as many poachers for turtle eggs as there have been in the past nor are 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 turtle eggs and hatchlings. Animals such as raccoons and dogs will find nests and consume the turtle eggs, and if hatchlings leave the nest at the wrong time they run into hungry, predatory birds. Although, it is important to remember that humans are affecting sea turtles more than anything else, because, after all, sea turtles have been around for over 200 million years and are now on the brink of extinction for one reason: humans.
Environmental risks are also a contributing factor for these populations. For instance, climate change and potential impacts could significantly affect sea turtle populations because of temperature dependent sex determination. Sex of sea turtle hatchlings is dependent upon the temperature. 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 of the nest 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)
Other risks include such incidents as boat collisions and affects from all terrain vehicles (ATV’s) on nesting beaches. Both of these actions increase sea turtle mortality by propeller incisions from boats, and sand compaction and ruts from ATV’s that can negatively impact sea turtle nests.
There are many conservation efforts taking place to aid in the protection of these magical creatures. One specific example is the Caribbean Conservation Corporation (CCC), co-founded by Archie Carr, who was a very important advocate for sea turtle protection worldwide and also a leading scientist. This organization’s goal is “to ensure survival of sea turtles within the Wider Caribbean basin and Atlantic through research, education, training, advocacy and the protection of the natural habitats upon which they depend.” (CCC, 2005) CCC selected sea turtles as their conservation focus because “these ancient creatures are among the most important indicators of the health of the world’s marine and coastal ecosystems. CCC believes whether sea turtles ultimately vanish from the planet or whether they remain a wild and thriving part of the natural world, will speak volumes about both the general health of the planet and the ability of humans to sustainably coexist with the diversity of life on Earth.” (CCC, 2005)
1. 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
2. Caribbean Conservation Corporation and Sea Turtle Survival League. (2005) Species Fact Sheets. Retreived June 5, 2005 from http://www.cccturtle.org/contents.html
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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