I. BIOLOGY AND NATURAL HISTORY
Green sea turtles are reptiles whose ancestors evolved on land and returned to the sea to live about 150 million years ago. They are one of the few species so ancient that they watched the dinosaurs evolve and become extinct. The biological classification of the green sea turtle is listed below:
Kingdom Anamalia Phylum Chordata (vertebrates) Class Reptilia (reptiles) Order Chelonia (turtles and tortoises) Family Cheloniidae (true sea turtles) Genus Chelonia Species mydas sub-species aggazizi (Hawaiian population)
As reptiles, green sea turtles, like all other species of sea turtles, possess the following traits:
- 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 calledscutes. 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 asalt 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.
II. A CLOSER LOOK AT HONU
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 asbasking 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.
III. FACTORS AFFECTING GREEN SEA TURTLE POPULATIONS:
There were once several million green sea turtles worldwide. Today, fewer than 200,000 nesting females are thought to remain. In Hawaii, scientists currently estimate that only 100 to 350 females nest each year, predominantly at French Frigate Shoals in the Northwest Hawaiian chain. Listed below are some of the factors believed to have contributed to the decline of the green sea turtle, as well as other sea turtle species:
Sea turtles have long been hunted for a variety of uses. Their shells have been used to make jewelry and ornaments, their skin to make small leather goods, their meat and eggs for food, and their fat for oil. In modern times, the number of sea turtles taken has increased dramatically due to the opportunity for profits they provide through commercial trade.
Ancient Hawaiians used the meat of the green sea turtle for food. Green sea turtles are also recognized as being the main ingredient in turtle soup. Before protective laws such as the Federal Endangered Species Act of 1973 were passed, green sea turtles were killed in large numbers to feed fishing crews in the Northwestern Hawaiian Islands and to provide meat for restaurants. Hawaiian populations experienced dramatic declines as a result. Because sea turtles take so many years to reach sexual maturity, it has taken 20 years since the passing of the Endangered species Act to see evidence of a population recovery.
Their natural habits also make sea turtles vulnerable to hunters. Because they lay their eggs in such a predictable way and are defenseless on land, poachers continue to kill hundreds of sea turtles each year for their eggs, shells and meat, despite laws prohibiting these activities in many countries. Egg clutches are especially easy to spot. After laying her eggs, the female turtle must struggle back to the ocean leaving a “tell-tale” trail behind in the sand.
Some native Pacific Islanders, as well as groups of native peoples in other parts of the world, continue to hunt depleted sea turtle populations for food. Continued subsistence takes under such conditions seriously risk both the survival of the species and the availability of this food source in the future.
B. Effects of Some Fisheries
Another important cause of sea turtle death is incidental (or non-deliberate) catch in fishing gear. Commercial shrimp fishers use nets that trap and drown more than 10,000 sea turtles each year. Many sea turtles could be saved if the shrimpers would use devices, called turtle excluder devices (TEDS), that keep turtles out of the nets. Laws exist that require shrimpers to install and use such devices, yet many shrimpers do not abide by them. In addition, thousands of sea turtles become entangled in longlines, driftnets, coastal gill nets and other discarded fishing gear each year.
C. Marine Debris
Litter and other marine debris can prove deadly to sea turtles when they entangle the turtles or are mistaken for food and ingested. Plastics are particularly harmful as they are not easily digested and remain in the turtle’s stomachs for long periods of time, releasing toxic substances. Ingested plastics also can clog the turtle’s digestive system. blocking the proper passage of food. Thus, sea turtles may actually starve from ingesting plastic debris. Balls of oil and tar have also been found in the throats and stomachs of deceased sea turtles indicating that oil spills may pose another cause for concern.
D. Coastal Development and Habitat Degradation
Sea turtle nesting beaches are lost each year to coastal development, leaving the females without a familiar place to lay their eggs. Noise, lights and beach obstructions are disruptive to nesting areas and threaten this critical part of the sea turtle’s life cycle. Some turtles may chose to nest on less developed beaches nearby, while others may not nest at all. Pollution and degradation of their marine habitat also threaten the turtle’s survival.
A fairly recent phenomenon recorded in Hawaii’s population of green sea turtles as well as in populations off the coast of Florida is the presence of a disease called fibropapilloma. Fibropapilloma causes the growth of large bulbous tumors predominantly on the soft tissues of the turtles. Once turtles are stricken with the disease they do not appear to recover. The tumors often spread to many parts of the body, ultimately killing the turtles. While the exact cause of the disease is not known, scientists suspect that a virus, parasite or the effects of marine pollution may be involved. A survey conducted in Kaneohe Bay on the island of Oahu in 1991 indicated that more than 50% of the green sea turtles in the Bay are affected and 36% off the island of Molokai.
IV. PROTECTIVE MEASURES
A. Federal Protection
Green sea turtles, as well as other sea turtles in Hawaii, are fully protected under both the federal Endangered Species Act (see Appendix 2) and under Hawaii state law. These laws prohibit hunting, injuring or harassing sea turtles or holding them in captivity without first obtaining a special permit for research or educational purposes. Swimmers and divers should be aware that riding sea turtles is illegal as it puts the animals under unnecessary stress. Fines for violating these laws protecting turtles can be as high as $100,000 and may even include some time in prison.
Under provisions in the Endangered Species Act, the National Marine Fisheries Service, the U.S. Fish and Wildlife Service and the State of Hawaii’s Department of Land and Natural Resources have recently formed a recovery team to help restore Hawaii’s green sea turtle population to previous levels. The goals of the recovery team are to identify research, management and enforcement needs for effective sea turtle conservation in the islands as well as promoting sea turtle protection through public education programs.
B. International Protection
International trade in sea turtle parts of products is also illegal under an agreement known as the Convention for International Trade of Endangered Species of wild fauna and flora, or CITES (see Appendix 2). Unfortunately, trade in sea turtles and their products continues at an alarming rate even though it is against the law. International trade currently focuses on two major markets: tortoise shell, which is used to make jewelry, eyeglass frames and ornaments, and small leather goods. Two species of sea turtles other than the green sea turtle are hunted primarily for these markets. They are the Hawksbill and the Olive Ridley, both sighted in Hawaiian waters. When returning from a foreign country, it is illegal under CITES for United States citizens to bring any sea turtle products into the country. Violators may be fined up to $20,000 and be sentenced up to one year in prison.
SUGGESTED ACTIVITIES FOR TEACHERS
– Discuss how sea turtles differ from their terrestrial ancestors. Discuss how they differ from mammals that live in the sea. Both whales and sea turtles evolved from animals that originated on land. In returning to the sea, what problems did each have to overcome in order to live successfully in a marine environment?
– Lead a discussion comparing the type of a survivorship curves exhibited by humpback whales and green sea turtles. Discuss the advantages/disadvantages of both types of life history strategies.
– List all the dangers facing sea turtles today. Make another list of all the laws that currently protect sea turtles. Note which laws are being violated. As a class, discuss why this is so, and make suggestions on how to make compliance with the existing laws more effective.
– Over the course of the unit, ask students to be “on the lookout” for sea turtles when they go the beach. Have students record all observations of sea turtles, including the date, time of day, location and if they notice any tumors on the turtles they spot. Near the end of the unit, compile a class report of all data collected. Send your report to biologists at National Marine Fisheries Service. They will appreciate your help! (You could also have a contest to see who can spot the most turtles)