by Joanne Harcke
What is a sea turtle, and how does it differ from a land turtle?
Like all turtles, sea turtles are reptiles, which means they are cold-blooded vertebrates with scaly skin, lungs and a three-chambered heart. And, like most reptiles, sea turtles lay eggs. The turtle's upper shell is called the carapace, and the lower shell is called the plastron. The carapace is covered in hard scales called scutes. Sea turtles do not have teeth, but the jaw is a modified beak. There are no visible ears, but sea turtles do have eardrums that are covered by skin. Sea turtles have good vision underwater, but do not see well out of water.
Sea turtles differ from terrestrial turtles in several ways. Sea turtles have shells streamlined for swimming, and flippers instead of legs. These adaptations allow sea turtles to move effortlessly through the water. They are strong swimmers and deep divers; green turtles can stay underwater for up to five hours.
On the other hand, because the shell is so streamlined, sea turtles cannot retract their heads or flippers to protect themselves from predators. So, although the earliest sea turtles evolved from terrestrial turtles, they are poorly adapted for life on land.
How many species of sea turtles are there?
There are seven types of sea turtles. Their common and Latin names are:
- Flatback (Natator depressus)
- Green (Chelonia mydas)
- Hawksbill (Eretmochelys imbricata)
- Kemp's ridley (Lepidochelys kempii)
- Leatherback (Dermochelys coriacea)
- Loggerhead (Caretta caretta)
- Olive ridley (Lepidochelys olivacea)
Family: Dermochelyidae (for scutless sea turtles, includes only the leatherback) or Cheloniidae (sea turtles with scutes)
A picture of each species can be found in the media section on the right column of this page.
How big are sea turtles? How long do they live? What colors are they?
Even the smallest sea turtles are larger than their land-going counterparts. Green turtles are 30-44 inches long and weight between 150 and 400 pounds. Kemp's and olive ridleys are the smallest sea turtles, with the largest averaging around 30 inches long and about 100 pounds. Leatherbacks are the largest sea turtles, reaching lengths of 4-6 feet and weights from 400 to over 1000 pounds.
Sea turtles can live as long as eighty years, if not more. They range in color from yellow through dark green, brown, and black.
How many sea turtles are there?
Accurate population numbers are hard to determine because turtles seldom come onto land where they can be counted. What is known is that the population of several species of sea turtles is in steep decline. The Kemp's ridley is probably the most endangered; it nests on only one beach in the world, in Mexico. The populations of loggerheads and green turtles also appear to be in trouble.
What do they eat?
Green turtles, and probably flatbacks, are primarily vegetarian. Loggerheads like jellyfish, shrimp, clam, and mollusk, while leatherbacks like soft-bodied animals like jellyfish.
Where are sea turtles found?
Some sea turtle species range in warm oceans world-wide, while others are limited to certain oceans or regions. Greens, leatherbacks, and loggerheads can be found in all oceans, except at the poles. Hawksbill turtles also range world-wide, but are found primarily in tropical reef habitats in the Caribbean and in tropical Australia. The flatback is also found in Australian waters. Kemp's ridley is an Atlantic turtle, preferring the western North Atlantic, while the Olive ridley calls the Pacific Ocean home.
Sea turtles generally prefer shallower waters, like bays, lagoons and estuaries, though some do travel through the open sea.
How do sea turtles reproduce?
Even though sea turtles spend most of their lives in the water, all begin life on the beach. The female sea turtle emerges from to ocean to nest very close to the beach where she was hatched (the ability to return to the place of one's birth is called natal homing). The nesting season in the United States is between April and October. Most females crawl out of the water at night and spend several hours digging a nest, laying eggs, and covering the nest before returning to the ocean. The nest holds about 120 eggs, each about the size of a ping-pong ball. Most females will nest more than once each season, but usually do not nest every year.
The eggs incubate for approximately 55 days. Incubation time is directly related to nest temperature. At colder temperatures the hatchlings take longer to develop. Temperature also determines whether the hatchlings are male or female. Warmer temperatures tend to produce more females.
The hatchlings emerge from the nest at night and follow the moonlight into the ocean. Once in the water, the hatchlings have to avoid many predators before reaching floating Sargasso weeds. Scientists think that small sea turtles spend several years floating in the seaweed, eating and growing. Once they are large enough, the young turtles will return to coastal waters to forage and continuing growing. At fifteen to twenty years of age, sea turtles reach maturity. Mature turtles will gather in mating areas, and females mate with several males before making the journey back to the nesting beach to lay their eggs.
What is typical sea turtle behavior?
Typically sea turtles are solitary animals that spend most of the day feeding and resting. Sea turtles can sleep on the surface of the water, or on the bottom. Scuba divers often see turtles napping under rocks and ledges. Previous tagging and tracking studies have shown that sea turtles can migrate thousands of miles.
Are sea turtles endangered? What threats do they face?
Today only seven species of sea turtle exist world-wide. All seven species are listed as "threatened" or "endangered" under the Endangered Species Act.
Young sea turtles have many natural predators. Raccoons, dogs and ghost crabs raid nests and devour eggs. Hatchlings on the beach are easy targets for crabs and birds. Once in the ocean, sharks and large fish can easily consume a small hatchling.
However, it is primarily interaction with humans that has caused the collapse of sea turtle populations. Humans can interfere with every stage of a sea turtle’s life cycle. Beachfront development, beach nourishment projects, driving on beaches, and artificial lighting all impact sea turtle nesting behavior, and will sometimes prevent females from nesting at all. Boating, fishing, and dredging can harm or even kill swimming sea turtles. Sea turtles can drown when they become entangled in floating garbage, and they can ingest floating debris.
Sea turtles and their eggs are harvested for food and other materials in some parts of the world. However, most harmful interactions between humans and sea turtles are unintentional. The increased human presence in coastal areas results in increased interactions between humans and sea turtles. Decisions that we make every day about living and playing on coastal lands are extremely important for the survival of sea turtles.
Measures to protect sea turtles include restrictions on beachfront development, beach driving, and lighting. Turtle excluder devices (TEDs) are required on all offshore shrimping boats operating in US waters from North Carolina to Texas. These "trapdoors" allow turtles to escape from shrimp nets as they are pulled through the water. The Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) prohibits the international trade of sea turtles.
About the author:
Joanne Harcke is Conservation and Research Coordinator at the North Carolina Aquarium on Roanoke Island.
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