By John A. Crawford
Sea turtles are reptiles related to land turtles, lizards and snakes. Sea turtles and their ancestors have graced the seas for over 200 million years. They managed to survive whatever catastrophe ended the age of dinosaurs. The ice ages came and went, but the turtles are still swimming the oceans. These ancient nomadic reptiles are truly survivors.
Sea turtles are still found in all the warm waters of the earth. Ocean life has given them all some common characteristics. All species have legs modified into efficient swimming flippers. These are clumsy on land but make sea turtles very fast and graceful in the water. The front flippers are used much like underwater wings, the rear ones like rudders. Turtles, as air breathing, egg laying reptiles, must return to the surface for air and to the shore for nesting.
The salinity of the seas presents problems to marine reptiles. Sea turtles excrete excess salt through salt glands in the eye socket. Out of water, this secretion appears much like tears.
The five species of sea turtles are found along the eastern and gulf coasts of North America are: the leatherback, green turtle, hawksbill, Kemp's ridley and loggerhead.
The largest of all living reptiles, the leatherback, Dermochelys coriacea, is an annual visitor and rare nester on the southeastern coast of United States. April and early May seem to be the period of the leatherback migration into Georgia waters. This concentration of turtles coincides with the occurrence of vast numbers of the cannonball jellyfish. The diet of these huge turtles is made up almost entirely of jellyfish.
Interestingly, the body temperature of this reptile is warmer than its surroundings. This makes the leatherback the only known "warm blooded" reptile.
The largest leatherback ever measured washed ashore dead in 1988 on a beach in Wales. This male specimen was measured by scientists at 2019 pounds.
The Atlantic green turtle, Chelonia mydas, has been called the world's most valuable reptile. This refers to its history as a food source. Unfortunately, millions of these animals have ended up in the soup pot. The very name green turtle refers to its greenish fat which is used to make the famous green turtle soup.
Vegetarians as adults, green turtles graze on sea grass pastures of the tropics. Juvenile green turtles are summer visitors along the southeast coast where they feed on algae, sea grasses and crustaceans. Some adult green turtles nest in Florida.
The smallest of the sea turtles, the Kemp's ridley, Lepidochelys kempi, is also the most endangered. In the past 40 years, the mass nestings or arribadas of this turtle have dropped from tens of thousands of individuals to less than 400 females per year.
Known to nest only on a few beaches near Rancho Nuevo, Mexico, the Kemp's ridley has the most limited nesting range of any sea turtle. This concentration of nesting females has made the species very easy for humans to exploit. Great numbers were killed for meat and leather. Hogs and dogs ate huge quantities of eggs. Humans also gathered eggs and shipped them to distant cities. Today, the beaches of Rancho Nuevo are patrolled by Mexican Marines and scientists to protect the remaining ridleys.
Juvenile Kemp's ridleys frequent the waters of the southeast U.S. coast. Until recently, hundreds drowned every year in shrimpers' nets. However, new regulations call for shrimpers to pull turtle excluder devices (TEDs) in their nets, which have since saved the lives of many turtles. In spite of the introduction of TED's, many scientists fear that the Kemp's ridley is doomed to extinction.
The hawksbill, Eretmochelys imbricata, rarely strays north from the tropics where it lives near coral reefs. Sponges provide some of its diet. This turtle is not often killed for food, but it is hunted extensively for its shell. The marbled plastic-like scutes, which cover its shell, are removed and become the source of "tortoise shell". Japan, the leading user of "tortoise shell," has recently banned its import.
By far, the most common sea turtle inhabiting our southeastern coastal waters is the loggerhead, Caretta caretta. Thousands of nesting females crawl ashore each year on the beaches of Florida, Georgia, and the Carolinas. The loggerhead is primarily a carnivore. Its diet includes crabs, shrimp, whelks, conches, fish, clams, jellyfish, sea urchins and even sponges.
Probably reaching a maximum weight of over 600 lb.., the normal mature specimen is between 200 and 350 lb.. The largest shell size on record is 45 inches long.
Loggerhead behavior patterns such as nighttime nesting and the production of large numbers of eggs are typical of sea turtles in general. Crawling from the surf after dark, the female selects a nest site above the reach of high tides (Fig. 4). Some females make several "false crawls" before finding suitable nesting sites. Loggerheads have been seen pushing their noses to the sand while looking for a nesting place. It is not known what they are looking for--perhaps a certain odor or moisture level in the sand.
After selecting her nest site the female excavates a "body pit." She creates this large depression using all four flippers, then begins digging alternately with her scoop-like rear flippers. This digging process continues for about 20 minutes and results in a light bulb-shaped hole 20 to 25 inches deep. When the mother turtle is satisfied with her nest cavity she places her rear flippers on each side of the narrow opening and lays eggs for about 20 minutes. The average number of eggs is about 120 but one nest under observation contained 198. The eggs have flexible shells and are the shape and approximate size of ping-pong balls. .She completes the process by covering the nest first with her rear flippers then scattering sand with her front flippers. The nest appears to be an effective disguise to human eyes, but does not always deceive animal predators.
Disguise and location alone do not insure a successful nest. Predators with a taste for turtle eggs include raccoons, ghost crabs, dogs, wild hogs and humans. Birds, commonly thought to be major predators, are little or no threat to the nest. Storm tides, rain and even plant roots can destroy the nest.
Eggs that survive in the nest take approximately two months to hatch. It takes several days of concerted effort for the hatchlings to dig through the sand covering the egg chamber (Fig. 5). They almost always wait for the top layers of sand to cool before making their nighttime break for the sea. Emerging from the nest at night protects the hatchlings from most bird predators.
The odyssey of these young turtles will carry them well offshore. They apparently seek out rafts of the brown algae, Sargassum. Off our shores, that would put them in or near the Gulf Stream. Floating, and feeding on the myriad invertebrates living in the Sargassum, these young turtles probably ride around the fringes of the Sargasso Sea.
Biologists have long sought the nursery for the "missing year stage" of the loggerheads' life cycle. Fishermen from the Azores knew all along that they were drifting with ocean currents southward toward the trade-wind-driven westward currents above the equator for a free ride back "home." The "missing year" actually seems to be three or four years. This is when we first see them again off our eastern seaboard.
Unfortunately, most of the juvenile sea turtles we see are dead, the victims of human activities and natural mortality. It takes many thousands of eggs to produce just one sexually mature adult.
If a young female loggerhead survives all the hazards of her early pelagic life, she returns to the waters off nesting beaches. It is thought that loggerheads reach sexual maturity after twenty years of age. The males never return to shore but gather with the mature females to mate.
Some scientists believe that loggerheads return to the beach of their birth for nesting, but this has never been proven. However, it is known that the Kemp's ridley does, due to the limited nesting sites of this species.
The nesting time for sea turtles is usually on a two-or three- year cycle. They may nest five or more times, on a thirteen day rotation, in one season. Individual turtles may return to the same region to make their nests, but we don't know if they return to the same nesting beaches.
Adult sea turtles have few natural enemies. Large sharks have been found with turtles and turtle parts in their stomachs, but man is the greatest enemy. The killing of adults and taking of eggs for food has all but stopped on our coast, but trawl fishing and habitat destruction take a great toll. Lights on beachfront buildings and parking lots have lured countless hatchlings to their deaths under the wheels of cars. Normally attracted to the brighter horizon of the open sea, the hatchling babies are confused by the artificial lighting.
Hope does exist. The protection of nests from predators is ongoing in many areas. Some coastal communities are shading or turning off beachside lights during hatching season. Shrimpers are using Turtle Excluder Devices, or TED's. These devices have been shown to be effective in preventing net drowning of sea turtles.
Time will tell if these ancient creatures can survive the present age of mammals. Unless we make adjustments in our use of coastal and offshore areas, one of earth's oldest rituals, the nesting of these huge and gentle reptiles, may soon be a memory.