Among the reptiles, only a small number of species have become adapted for life in the sea. Of these, just a few are fulltime reef residents. Reptiles frequently found around reefs include the sea turtles and sea snakes.
Sea turtles move through the water with the grace of ballerinas. These large reptiles, related to the more familiar but generally smaller land turtles, are superbly adapted for sea life. Their limbs are modified as strong flippers that effortlessly push their streamlined bodies through the water.
Turtles are such expert swimmers and so perfectly designed for their lifestyles that it would be easy to think of them as big fish; however, unlike fish, turtles must swim to the surface to breathe. These animals have lungs instead of gills, and they breathe through their noses. Even though active turtles need to surface every few minutes, they are capable of staying under water for long periods of time by holding their breath. When resting in caves or on ledges, sea turtles may remain submerged up to two and one-half hours.
As true marine animals, sea turtles rarely leave the ocean. The only reason they visit the shore is to lay eggs, and then only the females emerge. Nesting females always return to the beaches where they were born to lay their eggs (see Figure below). Males accompany them only as far as the shallow water.
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| Sea turtles dig nests on sandy beaches where they lay their eggs. |
Several species of sea turtles occasionally visit the coral reef; however, two species are reef residents: the Atlantic hawksbill sea turtle (Eretmochelys imbricata) and the green sea turtle (Chelonia mydas). From time to time they may be joined by other species of marine turtles that take time out of their migratory travels to rest and eat in luxurious reef accommodations. The green sea turtle is an impressive animal that grows up to 3.5 feet (1.1m) long and can weigh 400 pounds (181 kg). As seen in the upper color insert on page C-8, its carapace, the upper shell, is mottled in shades of dark brown on top and creamy white below. This type of dark-on-top, light-onthe-bottom coloration, called countershading, makes turtles hard to see in the water. From above, their carapace looks like the seafloor, and from below the plastron, the lower shell, blends in with the sky. Such camouflaging helps turtles get close to their prey before striking. It also helps them avoid sharks, their only predators.
Every two or three years, sexually mature green sea turtles make long journeys to mate and lay their eggs. They leave their feeding grounds and swim 600 miles or more, returning to the beaches where the females were born. During the months of March and April, mating occurs in offshore waters, then the females go ashore to lay their eggs. Green sea turtles have the ability to retain viable sperm for months after mating. The eggs laid at one mating were fertilized much earlier.
No longer supported by the water’s buoyancy, female green sea turtles drag themselves across the beach to sandy spots above the tide line. While ashore, they shed sticky tears that keep their eyes moist and free of sand. Using powerful hind legs, each female digs an egg chamber, a task that may take the entire night. When the chamber is finally finished, the female deposits 100–200 eggs, each about the size of a Ping-Pong ball, then covers the nest and returns to the sea.
Hatching begins after 60 days of incubation, usually early in July. Working together, the hatchlings scrape sand off the roof of the nest and pack it into the nest floor, a strategy that builds up the nest until it is almost even with the beach.
Using moonlight reflected in the ocean water as a beacon, all of the hatchlings scramble from the nest one evening and race toward the water. Some are picked off by birds and crabs on the beach, and others are grabbed by fish waiting for them in the shallow water.
Those that survive strike out on their own, swimming nonstop for the next 36 to 48 hours. When they get out far enough, the baby turtles are picked up by currents and carried into the open ocean. Green sea turtles remain at sea for several years, feeding on jellyfish and other invertebrates. When they are juveniles, they return to the reef areas where adults are living. There, young turtles join the colony, grazing on algae among corals and rocks.
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| A hawksbill sea turtle swims around the coral reef. |
The Atlantic hawksbill sea turtle, pictured in Figure 6.2, is another reef resident. With an orange, brown, or yellow carapace that measures up to 35.8 inches (91 cm) long, the hawksbill can weigh 100–150 pounds (40–60 kg). The head of the turtle is narrow, with two pairs of scales in front of the eyes and a hawklike jaw that accounts for its name. The shape of this turtle’s mouth is ideal for reaching into the cracks and crevices of coral reef where it finds sponges, octopuses, and shrimps. The hawksbill also eats squid that are swimming in the water column. After eating, it rests on the ledges and caves of the reef.
Nest building and egg laying occur every two to three years, preceded by mating in areas of shallow water. Male hawksbill turtles can be distinguished from females by their long tails. A female nests two to four times during each egglaying season, depositing about 160 eggs in each clutch.
Renesting females will usually return to the same part of the beach, often building her second and third nests within sight of the first one. After 50 to 70 days of incubation, hatchlings climb out of the nest and make their way to the sea. Mortality rates are very high, but the ones who survive swim out to sea. Like the green sea turtles, they are not usually seen again until they return to the area as juveniles.
Sea snakes make up 86 percent of the marine reptiles, and they are primarily found in tropical waters. Some species, including the olive sea snake (Aipysurus laevis), live around reefs. With a stout, round body that averages about 3.9 feet (1.2 m) in length, the snake varies in color, ranging from dark brown to purplish brown on the dorsal side, fading to light brown on the ventral surface. The flattened tail is creamy white and has a brown ridge on the dorsal side.
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| Aipysurus laevis |
The olive sea snake prefers reef waters that are 16.4 to 147.6 feet (5 to 45 m) deep where they can prey on fish, fish eggs, cuttlefish, and crabs yet surface quickly for air. During the day, the snake feeds by weaving among coral structures in search of animals that are at rest. When prey is located, the snake uses constriction to hold the victim while it injects venom with its fangs. The olive sea snake’s venom contains enzymes that begin digesting the prey from the inside.
After courtship in the open water, olive sea snake mating takes place on the reef floor. Competition for mates is fierce and several males may vie for a single female. The young snakes, which are born alive, grow quickly, maturing in four to five years.
Even though not as numerous as the olive sea snake, the turtle-headed sea snake makes it home on many reefs. Preferring lagoons to energetic parts of the reef, this snake is often found in large aggregates. The turtle-headed sea snake is a daytime feeder that slowly moves through the reef, seeking out small fish and crustaceans that it immobilizes with venom. It also feeds on the eggs of fish, such as gobies and blennies, that spawn in the lagoon, scooping them up from the reef floor with its hard pointed snout, the feature that most resembles a turtle’s head. The snake is a voracious eater and feeds every two or three hours.