Coastal Animals: Sea Turtles, Dolphins, and Seals

Sea Turtles

From a taxonomic perspective, sea turtles belong to the class Reptilia. Their order is Testudines, and the suborder Cryptodira, which also includes freshwater turtles. Sea turtles are divided into two families based on their carapaces: bony turtles covered with horny scutes, and leatherback turtles. There are eight recognized species: green (Chelonia mydas), black (Chelonia agassizii), loggerhead (Caretta caretta), Kemp's ridley (Lepidochelys kempii), olive ridley (Lepidochelys olivacea), hawksbill (Eretmochelys imbricata), leatherback (Dermochelys coriacea), and flatback (Natator depressus). The earliest turtle records date from the Triassic era (SeaWorld, 2004).

The habitat of sea turtles is mostly temperate waters. They are often found in the shallower waters of lagoons, coastal bays, and nearshore zones. Migration, however, can involve journeys of thousands of miles. Females frequently undertake these migratory trips to lay eggs on specific beaches. When the eggs hatch, the hatchlings make their way out to open oceans. Migratory habits differ across species and even within the same species depending on location. Population studies of sea turtles are challenging because counts can only be based on nesting females and hatchlings moving toward the ocean. Additional complications arise because females may return regularly to the same beach, travel to different beaches, or nest more than once in a single season.

Sea turtle species sizes are measured across the edges of the carapace and by weight. The largest are green turtles — the largest recorded specimen had a carapace diameter of five feet and weighed more than 850 pounds. The ridleys are the smallest, with a carapace less than a foot across and a weight of roughly fifty kilograms. Species are identified and classified partly by color. Their heads and flippers are not retractable, unlike those of land turtles. The fore flippers are used for paddling, while the hind limbs serve for balance and directional changes in the water. On land, sea turtle locomotion is clumsy and slow.

Sea turtles do not have teeth; their jaws are hardened to a degree, which explains their diet — primarily jellyfish. Shrimp is another favorite food, which sea turtles ferret out from the mud.

Until maturity, it is difficult to distinguish males from females. Males have longer and thicker tails, which house the male reproductive organs. The fore flippers of males also have better developed claws to help grasp females during mating. Sea turtles hear primarily at lower frequencies. They see well underwater and are sensitive to ultraviolet electromagnetic radiation. Their sense of smell is acute and works better in water.

Sea turtles are highly adapted to swimming. They swim at approximately one mile per hour, though leatherback turtles are known to reach speeds up to four miles per hour faster than other species. Sea turtles are accomplished divers, capable of reaching depths of up to 1,000 feet. Their slow metabolisms allow them to swim without breathing for extended periods. Green sea turtles can remain underwater for several hours and can slow their heart rates to as few as one beat every nine minutes. Sea turtles are generally non-sociable and remain solitary, though some males assist in the nesting process. Hatchlings, upon emerging from their papery or leathery shells with the aid of a temporary egg tooth, are immediately on their own. Clutch sizes range from two to nine eggs. Incubation takes between forty-five and seventy days.

Sea turtles are vulnerable to shark attacks at sea. On land, they face predation from animals such as monitor lizards and foxes. Increased use of beaches has reduced available nesting areas. Turtle eggs have historically been collected for food or purported medicinal uses. In open waters, turtles are frequently caught in shrimp trawler nets. Water pollution leads sea turtles to ingest plastic debris mistaken for jellyfish. Newly hatched sea turtles are naturally attracted to the open horizon and ambient light, and artificial lighting from human-inhabited areas often disorients hatchlings.

Bottlenose Dolphins

The bottlenose dolphin is the most familiar and most studied of all cetaceans, largely because it frequents coastal waters and adapts relatively well to captivity. A second ecotype of bottlenose dolphin ventures farther out into open oceans. Bottlenose dolphins generally avoid waters deeper than 30 meters, though they are capable of diving to depths of up to 300 feet (SeaWorld, 2002).

Bottlenose dolphins are fairly sociable and interact freely with other cetaceans, including other dolphin species and even humpback whales. There are reports of dolphins cavorting with fish and seals. They have been given various common names — grey, black, Atlantic, and Pacific bottlenose dolphins — and are also called cowfish and bottlenose porpoises.

Bottlenose dolphins are classified in the suborder Odontoceti (toothed whales). They have between 20 and 26 teeth on each side of either jaw, and their binomial classification is Tursiops truncatus. Their distinctive bottle-shaped beak gives them their English name. Bottlenose dolphins typically live for about twenty-five years, though individuals as old as fifty years have been reported. They grow to between eight and twelve feet in length. Their coloration is mostly slate gray with a lighter underside; the beak is white, and the fins and tail flukes are darker. The dorsal fin is relatively tall and curved backward. Unlike many cetaceans, the cervical vertebrae of bottlenose dolphins are not fused, giving them greater flexibility in the neck region — a characteristic they share with beluga whales. They can weigh up to 1,500 pounds or more.

Males are significantly larger than females. Males reach sexual maturity between 10 and 13 years of age; females between five and twelve years. Gestation lasts approximately one year. The calf remains with the mother for three to six years. Occasionally a mother will use a surrogate to help care for her calf. Tossing a calf into the air has been observed and is thought to be a form of discipline. Bottlenose dolphins live in social pods of around twenty individuals, though larger aggregations of up to 100 animals have been observed.

Bottlenose dolphins have been found in both hemispheres. Their temperature-based habitat ranges from tropical to temperate waters, and they are rarely if ever found in polar regions. They are considered abundant in the North Pacific. Male bottlenose dolphins often bear scars from encounters with killer whales, other dolphins, and sharks. Short-term migrations are observed when habitat quality is reduced by excess fishing pressure or by the presence of sharks and killer whales. These dolphins are not selective feeders; they consume most small fish such as herring and cod, as well as shrimp, crabs, and shellfish. Their beak-like snouts help them feed on burrowed marine animals. During feeding, individuals have even been observed surfing onto sand to strand fish. They also use their tail fins to create shock waves that immobilize prey. Occasionally, a pod will herd an entire shoal of several million fish into ever-tightening balls; deprived of oxygen, the fish become sluggish and are then easy prey. This cooperative feeding technique is typical of smaller cetaceans.

Bottlenose dolphins swim at a leisurely pace of about five to seven miles per hour but can accelerate to 30 miles per hour when threatened. Human encroachment is probably the greatest danger they face. Whalers sometimes hunt them accidentally, and in some cases they are deliberately harpooned. They are also caught in shrimp and shark nets. Chemical pollution in coastal waters has created health problems for bottlenose dolphins. In 1987, large numbers of these animals died along the East Coast of the United States due to a viral infection.