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Green Sea Turtle
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. In Hawaii, the green sea turtle is known as "Honu" (pronounced hoe-new).
Green Sea Turtle - Physical Description:
Green sea turtle hatchlings weigh about one ounce and have a carapace (shell) length of 2 inches. Subadults average about 200-350 pounds, with a carapace length of at least 2 1/2 feet long. Adults can grow to 4 feet and weigh up to 400 pounds. The carapace is a mottled dark brown on top and creamy white below. The carapace often will be covered in green algal growth. It is very difficult to distinguish between the sexes, except as mature adults, when males have a longer thicker tail than females. Males will also develop a single mating claw on the trailing edge of their fore flippers.
Green Sea Turtle - Distribution and Range:
Green sea turtles are found throughout the world’s oceans, with major populations in the United Statues found off Florida’s east and west coasts, in the Caribbean, and off the Pacific coast of Mexico. The Hawaiian population of green turtles appears to be genetically isolated from other populations in the Pacific, as they remain within Hawaiian waters throughout their lives. The main Hawaiian Islands serve as a feeding area for subadult and adult green sea turtles; 90% of this population migrate to the Northwestern Hawaiian Islands for mating and nesting.
Green Sea Turtle - Feeding:
Green sea turtles get their name from the color of their body fat, which is green from the algae or limu that they eat. Adult green sea turtles are primarily herbivorous, feeding on nearshore limu pastures around the main Hawaiian islands. However, as juveniles, green sea turtles are omnivorous, feeding on plankton, jellyfish and fish eggs floating near the surface of the open ocean. This juvenile period is termed the “lost years” and lasts about 3-7 years. From the time that the hatchlings enter the water to the time that the turtles show up in the main Hawaiian Islands, it is unknown exactly where the turtles go and what they’re feeding upon.
Green Sea Turtle - Symbiosis:
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.
Green Sea Turtle - Reproduction:
Hawaiian green sea turtles do not reach sexual maturity until they are about 25 years old, sometimes taking up to 50 years! Once sexually mature, adults migrate from the foraging grounds to the nesting grounds, primarily at French Frigate Shoals, located 800 miles northwest of the main islands. Males appear to migrate every year, arriving ahead of the females. Females only migrate every 2-4 years. It is believed that females return to the same birth where they were born. Mating starts in March and occurs in the waters adjacent to the nesting beaches. Nesting occurs from late April through September, with a peak in June and July.
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 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.
Green Sea Turtle - Lifespan:
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. It is believed that green sea turtle can live up to 80 years of age. Unfortunately, because humans have hunted sea turtles for so long, we have not yet allowed populations to recover fully to track their natural lifespan.
Green Sea Turtle - Predators:
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. Hatchlings may fall prey to sea fish, birds, sea crabs,or perhaps cats or dogs. Of course, humans are the main non-natural predator of sea turtles, killing them for their eggs, meat, oil, and shell. Because sea turtles are slow-growing and reproduce at such a high age, many populations were decimated to the point of near-extinction.