ea turtles are ancient creatures. They have traveled our planet for more than 200 million years, tracing a highly successful evolutionary path, living in a variety of environments from dry land to the open sea. This makes them fascinating, albeit tough, subjects to study. Scientists have uncovered precious few secrets of the turtles' life in the sea. Nevertheless, what they have already learned provides some valuable clues to understanding these intriguing animals and their important part in both land and marine ecosystems.
The Turtle Family
Of the 16 Orders of reptiles that evolved during the last 310 million years, that of the Testudines, or turtles, is one of only four that remain today (the three other Orders are Crocodilia (alligators, caimans and crocodiles), Squamata (snakes and lizards) and Rhyncocephala (Tuataras).
Turtles, terrapins and tortoises all belong to the Order Testudinata (also called Chelonia), which comprises two Sub-Orders, namely Cryptodira and Pleurodira. The Pleurodires (2 Families) are also known as "side-necks" because they curl their necks into a horizontal S-shape when retracting their heads into their shells. The Sub-Order Cryptodira, or the straight-necked turtles, bend their necks in a vertical S when they pull their heads into the shell. The Cryptodira is the largest Sub-Order, comprising 11 Families, which include 2 Families of living sea turtles. Of the more than 220 species of living turtles, only 7 are marine.
A Closer Look
In most turtles, the shell is made up of large bones, covered on the outside by large scales, known as "scutes". The number, shape and distribution of these scutes are used to differentiate species. The carapace has three basic kinds of scutes. The "vertebrals" run down the middle of the carapace, from head to tail, over the vertebrae (some people also call these "centrals"). Usually there are 5 vertebrals. On either side of the vertebrals (both left and right) there is a row of scutes, the "costals". Many turtles have 4 pairs of costals, or, in other words, 4 costals on the left and 4 on the right (these are also called "laterals"). All around the outside of the carapace, on its margin, are the "marginal" scutes.
The plastron generally has 6 pairs of scutes, occasionally with an additional scute in the center in the front, under the neck, and one in the center in the back, under the tail. Along the side, joining the plastron with the marginals of the carapace, and between the front and back limbs, is the bridge. This is covered with the "inframarginal" scutes.
The head also has scales that are often distinctive from one species to another. For example, just behind the nostrils of sea turtles are the "prefrontal" scales, and their number and form can be used to differentiate the species. The front limbs of sea turtles are long and wing-like, and formed into flippers; the back limbs are paddles (not really flippers) with a membrane joining all five of the toes. Large scales cover the flippers and the top (dorsal surface) of the back limbs, while the skin on the other parts of the limbs has small scales and is very flexible.
In biology, the science of variation and adaptation in living beings, there are usually exceptions to the "rules"! Among sea turtles, the exception is the leatherback turtle, which only has small scales, and no scutes anywhere on its body. Also, it is only the very young leatherbacks that have scales. The bones that form the shell of the leatherback are, with the exception of a few in the plastron, all relatively small.
Is it a he or a she? Marine turtles are heterosexual, with sexual dimorphism being evident in adults. In other words, in marine turtles, males can be distinguished from females only when the animals are adult or nearly adult. Sexual differentiation in hatchlings, juveniles and sub-adults is almost impossible without internal examination.
The adult male has a long, thick tail that extends well beyond the posterior margin of the carapace, often as long as the hind limb. The male carapace tends to be more elongate, tapering at the tail region. The plastron is often more flexible and concave than in the female. Except for the leatherback, the adult male has long, heavy claws which are used to cling to the female during mating. In adult green turtles, the males are less strikingly marked than the females.
Adult females have short tails that do not extend appreciably behind the hind margin of the carapace. They have shorter, thinner claws and a harder, less concave plastron. The female carapace tends to be more highly domed.
Life of a sea turtle (as we know it). Although the first two months of their lives are on land, sea turtles spend most of their lives in the sea. In some very remote islands, sea turtles will rest on the beach, but the general rule is that females come back to the beach only for nesting, and males never return to land. But, no matter how brief, the turtles' sojourns on land have given scientists valuable opportunities to trace and study at least part of the life history of these interesting creatures.
When the breeding season starts, sexually mature males and females migrate hundreds or thousands of kilometers from their feeding grounds to breeding grounds, coastal waters near their nesting sites. In general, the males arrive earlier in the season at the breeding grounds and they also leave earlier than the females to return to the feeding grounds. The female stays near coastal waters between nestings in the "internesting habitat", resting in preparation for her next nesting. Right after nesting, she ovulates, shedding over a hundred eggs from her ovaries into her oviducts, where they are fertilized and covered with egg white (albumin) and shells. The entire process from ovulation to oviposition (egg laying) takes about two weeks. Where many turtles concentrate to nest, the area is often called a "turtle rookery."
A nesting begins when the female turtle emerges from the sea onto the beach at night. She climbs to the top of the beach, well above the high tide line, and digs a large depression in the surface of the beach using a swimming movement of her front flippers, creating the "body pit". After ten minutes or more, actively throwing sand behind her, she begins digging with her hind limbs, excavating the egg chamber. In green turtles, this vertical chamber may be about 80 cm deep.
When she can no longer dig any deeper, with hardly a pause, the nester lays more than a hundred leathery-shelled eggs, each about the size of a ping-pong ball. This may take nearly half an hour. Inside, each egg is a pinhead-sized embryo that has developed to the stage of mid-gastrulation. Development is stopped at this stage a couple of days before the nester comes ashore to lay the eggs. While the eggs are being deposited into the egg chamber, they can tolerate bouncing, rolling, tumbling or handling, but about two hours after being laid, the embryo will resume development, and may be killed by a simple roll of the egg.
After nesting, the female goes back to the internesting habitat to rest and complete the next clutch of eggs. She can be expected to lay several clutches of eggs at approximately two-week intervals before finally migrating back to her feeding ground. During the breeding migration, courtship and residency within the internesting habitat, the adult turtles eat almost no food, depending mainly on stored fat reserves.
The season and frequency of nesting among females depend on individual populations and species of turtles. Worldwide, remigration (when turtles that have nested in earlier seasons return to nest in subsequent seasons) and renesting intervals have been determined through results of recapturing turtles that have been tagged. At the Baguan Island Marine Turtle Sanctuary, Turtle Islands, Tawi-Tawi, the renesting interval is 11.08 days and remigration interval (period between two nesting seasons) is 3 years. Both recruits (turtles nesting for the first time) and remigrants (those that have nested in a previous season) nest 3-4 times in a season.
Female sea turtles do not exercise parental care. The eggs incubate in the sandy beach to become "hatchlings" (baby turtles) in about 2 months. The sex of the hatchling and the rate of development are determined by temperature. Higher temperatures produce more females and result in shorter incubation periods.
After hatching, the young turtles dig their way up out of the nest and scurry to the ocean, usually in a group. It is thought that during incubation, or immediately following emergence from the nest, the hatchling is imprinted on to the earth's magnetic field at the rookery. This "imprinting" guides them back to the same site when they are sexually mature.
Once they enter the water, the hatchlings spend at least several months or even more time - probably several years - dispersing in oceanic currents. Sargassum rafts floating on the surface of the ocean often provide a refuge for hatchling green turtles and loggerheads. These mats of brown algae harbor a diverse, specialized fauna, including many kinds of little fishes, crustaceans, worms, mollusks, tunicates (sea squirts) and coelenterates (jellyfishes, sea anemones, etc.) that appear to be suitable forage for the little turtles. Seaweed mats and drift lines may act as fish nurseries providing food and shelter. Besides the concentration of potential prey and the concealment the rafts offer to baby turtles, there is a tendency for the mats to occur off high-energy beaches. This enhances the probability that, during their seaward journey, the little turtles leaving a nesting beach will intercept these mats. Floating logs, coconuts, and other jetsam may attract small turtles where floating seaweed is absent.
Except for the leatherback, which remains entirely pelagic (living in the open seas), young turtles move into shallow coastal waters (in green turtles, this occurs when the carapace is about 30 cm long). Here they feed principally on benthic (bottom-living) organisms. Each individual is thought to remain associated with a restricted feeding area for many years but may shift to different feeding areas as it matures.
There is considerable variation in growth rates, but green and loggerhead sea turtles may require as long as 30-50 years to reach maturity.
It is thought that, when turtles reach sexual maturity, both males and females migrate to the area where they hatched. There they breed and complete the reproductive cycle, and at the end of the breeding season, they return to the feeding area from which they began their migrations. This cycle is repeated with each breeding season. In most species, the energy demands of long migrations and egg-laying dictate that most females do not nest annually but, on the average, every 2-5 years. The males of at least some populations breed more frequently (every 1 or 2 years).
Five sea turtle species are known to occur in the Philippines. These are Chelonia mydas (green turtle), Eretmochelys imbricata (hawksbill turtle), Lepidochelys olivacea (olive ridley turtle), Caretta caretta (loggerhead turtle), and Dermochelys coriacea (leatherback turtle). These species are described below.
The green turtle is
one of the most tropical of marine turtles. It is widely distributed in
tropical and subtropical waters, near continental coasts and around islands;
they are less common in temperate waters. Baby green turtles that live
in the open ocean feed on small animals found on the surface of the sea,
while juveniles and adults feed mainly on seagrasses and algae.
Local names in the Philippines: Pawikan (most Filipino dialects); bildog (Isabela); talisayon, magdarahit (Bicol); darawanan, wara-cawa (Samar); kutuan (Cuyo, Palawan); tortuga (Zamboanga and Basilan); pudno (Tausug and Samal); payukan (Mapun)
Hawksbills are most
common where living coral reef formations are present, in clear, shallow
waters of mainland and island shelves, including lagoon and bays, feeding
mainly on sponges and soft corals. The most tropical of all sea turtles,
they have very few major nesting places remaining.
Local names in Philippines: Pawikan (most Filipino dialects); ulaniban, kinarahan (Samar); karahan (Bicol); sisikan (Mapun and Tausug); payukan (Sulu)
Lepidochelys olivacea (Olive ridley turtle)
Olive ridleys are
found in continental coastal waters, principally in the eastern Pacific,
Indian Ocean and south Atlantic where the water does not drop below 20°C.
Occasionally, they are carried by warm currents into subtropical areas.
Hatchlings and smaller turtles feed on fish egg masses and jellyfish in
the open ocean. In coastal waters, they feed mainly on crabs and shrimps.
Local names in the
Philippines: Pawikan (most Filipino dialects); mukoy (Bicol)
Caretta caretta (Loggerhead turtle)
Loggerheads are widely
distributed in coastal tropical and subtropical waters (16-20°C) around
the world, and can nest successfully outside of the tropics. This species
commonly occurs in temperate waters and in boundaries of warm currents.
It is capable of living for a relatively long time in a variety of environments,
such as brackish waters of coastal lagoons and river mouths. Once they
have matured to the benthic stage, loggerheads are equipped with powerful
jaws than can crush crabs and mollusks.
Local names in the Philippines: Pawikan (in most Filipino dialects); bulawon (Bicol)
Dermochelys coriacea (Leatherback turtle)
The leatherback is the most widely distributed of all sea turtles. Adult leatherbacks are adapted to colder water than other sea turtles, a capability that is due to their protective thick and oily dermis, counter-current heat exchangers in the limbs, and other physiological adaptations. They can occur far from tropical and subtropical nesting grounds, where water temperatures are between 10° and 20°C. Leatherbacks feed on soft-bodied invertebrates such as jellyfish, comb jellies and scalps.
Leatherbacks are known to inhabit feeding grounds in Philippine waters, but no nesting has been documented. It is not known in the Turtle Islands.
Local names in Philippines: Pawikan (in most Filipino dialects); benereran, binalimbing (Bicol dialects); kantuhan (Cuyo, Palawan); kulod, ratong (Samar)
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