* They are cold-blooded, meaning that they get their body heat from the environment rather than making their own.
* They breathe air
* Their skin has scales
In addition to these reptilian traits, all species of turtles have evolved a bony outer shell which protects them from predators, as turtles are not known for their speed. The shell covers both the dorsal (back) and ventral (belly) surfaces and is considered the most highly developed protective armor of any vertebrate species to have ever lived. The dorsal portion of the shell is known as the carapace and is covered with large scale-like structures called scutes. The ventral portion of the shell is known as the plastron. The carapace and plastron are connected at the sides by hard-shelled plates known as lateral bridges. Openings exist between the carapace and plastron for the head, tail, and limbs. While most species of land turtles and tortoises are able to retract their heads into their shells for added protection, sea turtles are not able to do so, and their heads remain out at all times.
The sea turtle's body is wonderfully adapted to life in the ocean. Their shells are lighter and more streamlined than those of their terrestrial counterparts, and their front and rear limbs have evolved into flippers making them efficient and graceful swimmers, capable of swimming long distances in a relatively short period of time. Sea turtles have been known to move through the water as fast as 35 mph. When active, sea turtles swim to the surface every few minutes in order to breathe. When sleeping or resting, which usually occurs at night, adult sea turtles can remain underwater for more than 2 hours without breathing. This is due to the fact that turtles are capable of containing higher concentrations of carbon dioxide in their blood than most other air-breathing animals, enabling them to use their oxygen very efficiently. Both muscles and blood are able to store oxygen in large quantities, allowing sea turtles to remain underwater for such long periods of time. Juvenile sea turtles have not developed this ability as well as adults and must sleep afloat at the water's surface.
In addition to solving the problems of swimming and breathing, sea turtles have also come up with an ingenious way to rid their bodies of the salts they accumulate from the seawater in which they live. Just behind each eye is a salt gland. The salt glands help sea turtles to maintain a healthy water balance by shedding large "tears" of excess salt. If a sea turtle appears to be "crying" it is usually not cause for alarm, as the turtles are merely keeping their physiology in check. It is not because they are upset or sad.
Four of the seven existing species of sea turtles can be found in Hawaiian waters. They are the green sea turtle, the hawksbill, the leatherback and the olive ridley. Of these, by far the most common is the green sea turtle, or honu (pronounced hoe'-new), as it is known in Hawaiian.
Green sea turtles, Chelonia mydas, get their name from the color of their body fat, which is green from the algae or limu they eat. Adult green sea turtles are herbivores, meaning that they eat only plants, and therefore do not pose a threat to any other marine animals. Similar to cows, green sea turtles depend on bacteria in their guts for digestion of plant material. Juvenile green sea turtles on the other hand are carnivorous. Their diet consists of jellyfish and other invertebrates. Adult green sea turtles can weigh up to 500 pounds and are often found living near coral reefs and rocky shorelines where limu is plentiful. Although the carapaces of green sea turtles are mostly dark brown in color, they can be covered with patches of algae on which fishes in turn feed. This type of feeding arrangement is an example of symbiosis. Symbiosis occurs when a relationship forms between individuals of two different species for an extended period of time. This particular relationship of the fish eating algae off the turtle's shell would be considered a form of mutualism, a type of symbiosis in which both species benefit from their association. Here, the fish get a free meal, and the sea turtle gets a clean shell.
The life span of sea turtles in not known. Hawaiian green sea turtles seem to grow very slowly in the wild, usually taking between 10 and 50 years to reach sexual maturity - 25 years is the average. Their long period of maturation helps to explain why it takes sea turtles so many years to recover from a substantial population decline. Male and female green sea turtles look virtually alike until they mature. Then, the two sexes are easy to tell apart: the males have long, thick tails, while the females have short, stubby ones. This is an example of sexual dimorphism, or, the ability to differentiate between the sexes of a particular species on the basis of external body characteristics.
Although green sea turtles live most of their lives in the ocean, adult females must return to land in order to lay their eggs. Biologists believe that nesting female turtles return to the same beach where they were born. This beach is referred to as a natal beach. Often sea turtles must travel long distances from their feeding grounds to their natal beach. Just how sea turtles find their natal beaches is not known. Hawaii's green sea turtles migrate as far as 800 miles from their feeding areas along the coasts of the main Hawaiian islands to their nesting beaches in the Northwestern Hawaiian islands. Males accompany the females during the migration, which usually occurs in the late spring, and mate with them off the shores of the nesting beaches. The most popular nesting beaches are on French Frigate Shoals, where an estimated 90% of the Hawaiian population of green sea turtles mate and lay their eggs. Females do not mate every year, but when they do, they come ashore often- as many as five times every 15 days to make nests in the sand and lay eggs.
Green sea turtles nest only at night. The female must pull herself out of the water and all the way to the dry sand of the upper beach using only her front flippers. This is a difficult task as her front limbs have been modified into highly effective swimming flippers, and do not support the bulk of her weight in the sand. Reaching the upper portion of the beach, she uses her front flippers to dig a broad pit in the sand and her rear flippers to delicately carve out a bottle-shaped burrow. She then lays her clutch, which consists of approximately 100 leathery-skinned eggs, in the burrow and covers them carefully with sand. Lastly, she buries the pit entirely to disguise the location of her nest. Her parenting job completed, she returns to the sea, leaving her young to fend for themselves.
Green sea turtle eggs take about two months to incubate. Studies indicate that the temperature of the eggs during incubation influences the sex of baby sea turtles. Lower temperatures tend to produce males, while higher temperatures tend to produce females. The baby turtles are able to break through the eggshell and hatch by chipping away at the shell with a structure called an egg tooth, a temporary hard protuberance on their beaks. After hatching, the tiny one-ounce turtles take a number of days to dig their way out of their nest. Emerging from the nest must be a group effort as one hatching would not be able to escape by itself. Working together, the hatchings scrape away the roof of the nest until they reach about an inch away from the surface of the beach. The hatchlings nearest to the surface stop their digging if the sand feels hot, indicating that it may be daytime. They wait to resume digging until the sand feels cool, indicating that it is night, and safer to emerge by avoiding the harsh rays of the sun and possibly, predatory birds. Once out of the nest, the hatchlings find their way to the ocean, by heading towards the brightest horizon. Thus, artificial lights on nesting beaches can mean death to the young turtles as they may confuse them and cause the them to lose their way. When they find their way to the ocean, the hatchlings must swim continuously for the next day and a half to two days. The young turtles remain at sea and do not come inshore until at least one year later.
Unfortunately, not all of the hatchlings reach the ocean. Many are snatched up by hungry crabs and other predators along the way or become lost and die. In addition, some are eaten by sharks and other carnivorous fishes while at sea. Only a few baby turtles from each nest will survive into adulthood. This type of a life history strategy exhibited by the green sea turtle can perhaps be more easily understood by comparing it to other patterns that we see in nature.
Over time, all populations of living organisms show characteristic patterns of survivorship. These patterns, called survivorship curves, can be graphically illustrated by plotting the average number of individuals of a particular species alive at each age against time. There are three different types of survivorship curves that characterize most living organisms. They are: convex, concave and constant.
Populations with convex survivorship curves have relatively low levels of mortality of offspring. Most individuals survive to old age. Species with convex survivorship curves produce relatively few offspring per reproductive effort and tend to invest a great deal of energy in the parental care of each individual. Convex survivorship curves are characteristic many large animals, including humans.
A concave survivorship curve is characteristic of organisms that produce large numbers of offspring per reproductive effort and employ little to no parental care, leaving the offspring to compete for their own resources. Mortality is very high among offspring, but those that do reach adulthood have a good chance of surviving to old age. This type of survivorship curve is exhibited by the green sea turtle, all other species of sea turtles, as well as most species of plants, invertebrates and fishes.
Species with constant survivorship curves have an equal chance of dying at any time during their life span. Such species include the marine hydra and many species of microorganisms that reproduce asexually.
Because of their efficient mobility in the water and their size, adult green sea turtles have only two known predators: sharks and people. Tiger sharks are believed to feed regularly on green sea turtles. Near their nesting grounds in the Northwestern Hawaiian Islands, where tiger sharks are more plentiful, adult male and female turtles can often be seen crawling up on the beaches and laying motionless in the sun for hours. This phenomenon known as basking is believed to help the turtles avoid predation by tiger sharks and also serves to increase their body temperature and speed up their metabolism, as sea turtles are cold-blooded.
Green sea turtles are found throughout the world's oceans. Like the other six species of sea turtles, green sea turtle populations are considered either endangered or threatened. Hawaii's population of green sea turtles is listed as threatened under the federal Endangered Species Act, indicating that they may become endangered in the near future. Some populations of green sea turtles in other parts of the world are not as lucky; the populations off the coast of Florida and the Pacific coast of Mexico are already listed as endangered. While tiger sharks are known to feed upon the green sea turtles, man it seems, may pose a greater threat to the animal's survival.