A Peek Under the Shell: Sea Turtle Secrets Revealed
Story and photos by Marty Snyderman
I don't know if the shadow of the turtle or the shark was the first to catch my eye, but almost instantaneously I recognized that the chase was on. The 8-foot- (2.4-m-) long tiger shark was in pursuit of a 200-pound (90-kg) loggerhead turtle in the shallow waters of the mangroves in the northern Bahamas just east of Walker's Cay. The shark did not rush the turtle, but instead the predator steadily closed the gap over a period of several minutes while the duo swam in midchannel. As the shallow bottom began to slope upward, the turtle sped up and the shark closed the gap from 50 feet to 15 feet (15 m to 4.5 m).
By that time I was in the water with my movie camera rolling and my heart pounding. I wasn't sure what to expect, but I was ready for the shark to make its move. After all, sea turtles are well-documented to be among the favorite prey items of tiger sharks. Seconds later the turtle was over a bottom that was less than 5 feet (1.5 m) deep and the shark was only a few feet away.
Suddenly the turtle turned and headed right at the shark. The turtle bit the shark just behind the head and shook momentarily before the shark pulled free. The shark snapped at the turtle before quickly swimming away. Moments later the shark reappeared only to have the turtle attack and bite again. Once more the shark retreated, but again the shark circled back to continue the fight. This time the turtle bit the shark on the gill slits and tore a chunk of flesh out of the side of the shark's body. The shark pulled free and swam away for good.
Surprised by the ending of this encounter? If so, you are like most divers I know, and you will probably enjoy learning more about the fascinating lives of sea turtles. Worldwide, there are eight species of sea turtles. All inhabit tropical and subtropical seas around the world. Like their land-dwelling cousins, the tortoises, sea turtles are reptiles, cold-blooded vertebrates that breathe air by using lungs. All possess three-chambered hearts and most have scutes that cover their bodies. Unlike tortoises, sea turtles cannot retract their head or limbs inside of their shells for protection.
Sea turtles are described in two families, Dermochelyidae and Cheloniidae. The primary distinguishing feature between the two families is the absence or presence of the hard, scalelike plates known as scutes that cover the body. The number and arrangement of scutes can be used to differentiate between species. Lacking scutes, but possessing a leatherlike carapace, leatherback turtles are the only species described in the family Dermochelyidae. The scute-covered shells of the remaining seven species consist of an upper section known as carapace, and a lower portion known as the plastron.
The ancient land-dwelling ancestors of sea turtles first appeared on Earth about 200 million years ago. These land residents expanded their range some 50 million to 100 million years later as they began to inhabit the world's oceans.
Well-suited for both long distance migrations and sudden bursts of speed, modern-day sea turtles are superb swimmers. Over the course of evolutionary time, the flippers of sea turtles have been modified for swimming rather than crawling. The long powerful fore flippers provide thrust, while the rear flippers assist in stability and directional control much like the rudder of a boat. When encountered by divers, sea turtles are often seen at rest on the sea floor or at the surface, or as they lethargically cruise around a reef. But don't allow their lazy appearance to fool you. Several species have been clocked in excess of 20 mph (32 kmph).
As a group, sea turtles are excellent deep divers as well. Leatherbacks have been documented to repeatedly dive to 1,000 feet (303 m), and specialists believe they routinely venture to 3,900 feet (1,181 m) when seeking prey. Their slow metabolic rates allow leatherbacks to make consecutive deep dives that last 45 minutes each.
When at rest on the sea floor, several species have been known to remain submerged for as long as five hours. This rest should not be confused with the hibernating behavior of green turtles and loggerheads, as these species are known to spend several months buried in mud on the sea floor.
With the exception of leatherbacks, sea turtles tend to inhabit the shallow coastal waters of lagoons, bays and estuaries as well as the waters of the open sea. Leatherbacks are almost always observed in the open ocean except during their nesting season when the females come ashore to lay their eggs.
Many populations of sea turtles migrate hundreds of miles between their nesting and feeding grounds, and as a result of this travel, at one time or another sea turtles are commonly seen in a variety of marine habitats. Some leatherback turtles are known to cover as much as 3,000 miles round trip between nesting and feeding grounds. However, it is rather interesting to note that while some populations of a given species annually migrate over great distances, other populations of the same species do not migrate at all. For example, some populations of green turtles nest, feed and mate in the same geographical area, while other well-studied groups annually migrate as far as 1,500 miles between nesting and feeding grounds. Specialists scratch their heads and theorize, but no one is really certain about the factors that cause some populations to migrate while others do not.
As a group, sea turtles prey on a wide variety of food sources. Some species are herbivores, others carnivores, and some are omnivores, feeding on both plants and animals. The jaws of sea turtles lack teeth, but they are equipped with modified 'beaks' suited to the diet of each species. Finely serrated for cutting, the jaws of adult green sea turtles are perfectly designed for feeding on algae and sea grasses while the pointed jaws and narrow heads of hawksbills allow them to prey on creatures that seek refuge in crevices. In addition to feeding on shrimps and crabs, hawksbills also prey upon sponges, tunicates and squids.
The jaws of loggerheads and Kemp's ridleys are made for crushing and grinding. Both species feed on crustaceans, mollusks, jellies, sea grasses and algae. The beaks of leatherbacks are ideally suited to cut in scissorslike fashion as they feed on jellies, salps and other soft-bodied animals they encounter in the vast expanse of the open sea.
Reproduction in sea turtles is not completely understood. It is widely believed that the age of sexual maturity varies considerably within a given species. Studies have demonstrated that some hawksbills and green turtles reach sexual maturity in only three years while other members of their species require as long as 13 years.
When courting, two or more males generally pursue a single female. Fertilization is internal and copulation occurs in the water. Anywhere between a few days and a few weeks after mating, females head to the shore to dig their nests and deposit their eggs. Nesting behavior is similar in all species. Females typically come ashore during, or close to, high tide at night during the warmest times of the year. Once the ladies have crawled above the high tide line, they dig a hole in the sand known as a 'body pit.' This depression is slightly larger and deeper than their body. At one end of the pit, the female excavates a smaller hole known as the egg cavity where she will lay her eggs, usually between 80 and 150 per nesting session. Slightly larger than pingpong balls, the eggs are round and white. Many females nest as often as five times a year.
The length of incubation varies from 50 to 70 days. Juveniles almost always hatch at night and quickly head for the sea. But specialists report that less than 10 percent of the hatchlings survive as long as one year as they are heavily preyed upon by a variety of birds, crabs, sharks and other large fishes. However, it is also believed that those who manage to avoid the perils of the early years can live as long as 30, and perhaps even 50, years in the wild.
The sex of sea turtles is not determined at the time of fertilization. Instead, the sex of hatchlings is fated by the temperature of the sand surrounding the nest. The deeper the sand, the cooler the eggs are during incubation. Turtles hatching from eggs that are deep in a nest tend to develop into males. The eggs that were deposited toward the top of the same nest tend to develop into females. Depending upon the temperature of the sand it is also documented that all of the hatchlings can be the same sex, either males or females.
To distinguish between males and females of a given species, you need to compare the length of the tails. Typically the tails of the males are considerably longer. However, as a sport diver this method only works well when both a male and female can be observed simultaneously.
It is easy to conclude that sea turtles are rugged animals when you first see their hard shells and tough-looking heads. There is no question that their armament provides these reptiles with some protection. However, this does not mean that sea turtles are built for divers to grab, handle or ride. Like many marine creatures, sea turtles can be easily stressed beyond their ability to cope, and when stressed they are prone to being drowned. The bottom line: Be thoughtful when diving around turtles. Allow them to come to you if they care to do so, but don't grab or chase.
Who's Who In The World Of Sea Turtles
Leatherback turtle (Dermochelys coriacea)
The largest and most widely distributed of all sea turtles. Routinely attain proportions of 6 feet (2 m) and 1,300 pounds (585 kg). Shell appears soft and leathery. Prey almost exclusively on jellies.
Green sea turtle (Chelonia mydas)
Best-known sea turtle due to its value as a food source and extensive research about its natural history. Named for the green color of their body fat, not the shell color as is commonly believed. Carapace of adults tends to be close to 3 feet (1 m) long with animals that size weighing between 200 and 300 pounds (90 and 135 kg).
Black sea turtle (Chelonia agassizi)
Some specialists question whether black sea turtles should be considered as a subspecies of green sea turtle. Only sea turtles known to nest on Ecuador's Galapagos Islands.
Hawksbill turtle (Eretmochelys imbricata)
Small- to medium-sized turtles that are relatively easy to identify due to the series of jagged scutes at the tail and the overlapping plates on their shell. For many years their shells were quite popular as use in jewelry, combs, boxes and d/cor. Prey upon sponges, shrimps, tunicates and smaller squids.
Loggerhead (Caretta caretta)
Reddish-brown markings are a striking feature of many individuals. Encountered in wide variety of habitats ranging from extremely turbid, shallows of muddy bays, estuaries and bayous to clear water. Hatchlings believed to live early years in rafts of sargassum and debris. Primarily prey on bottom-dwelling invertebrates as adults.
Olive Ridley (Lepidochelys olivacea)
Reaching a maximum length of 30 inches (76 cm) while weighing less than 100 pounds (45 kg), this is the smallest of all sea turtles. Common name derived from olive green, heart-shaped carapace. Dive as deep as 500 feet (151 m) to feed upon myriad shrimps, crabs, snails, tunicates, sea urchins and other invertebrates.
Kemp's Ridley (Lepidochelys Kempi)
The second-smallest of the sea turtles and unfortunately the most endangered. Olive-gray shells are oval-shaped. Range from the Gulf of Mexico north to Cape Cod, Massachusetts. Females tend to nest during the day. Prey primarily upon crabs, mussels and snails.
Australian Flatback (Natator depressa)
Named for characteristic flat backs. Occur only in waters off Australia. Attain proportions of 40 inches (102 cm) and 200 pounds (90 kg).
Survivors In A Changing World
Over the eons, sea turtles have adapted to a multitude of environmental events that have dramatically altered their living conditions. However, in the last several hundred years all species of sea turtles have been threatened to the point of near extinction by the deeds of humans. Nesting grounds have become inhabited and commercialized. Fisheries apply pressure and pollutants threaten the health of ecosystems.
But there is reason for hope as feeding and nesting grounds are being protected in many areas around the world, and regulations imposed on fisheries are reducing the take of sea turtles both as primarily targetedspecies and as incidental kill.