GREEN SEA TURTLES
Green Sea Turtle on beach at French Frigate Shores
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:
Phylum Chordata (vertebrates)
Class Reptilia (reptiles)
Order Chelonia (turtles and tortoises)
Family Cheloniidae (true sea turtles)
sub-species aggazizi (Hawaiian population)
As reptiles, green sea turtles, like all other species of sea turtles, possess
the following traits:
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.
- 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
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.
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 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.
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
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.
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