Chelonia mydas - University of San Francisco

Briana Meyer
Dr. Dever
Herpetology
Research Paper
Chelonia mydas
Chelonia mydas, otherwise known by their common name, the green sea turtle,
are named for the color of their body fat, which is green from the algae or limu that they
eat. The biological classification for this species is as follows:
Kingdom Animalia
Phylum Chordata (Subphylum Vertebrata)
Class Reptilia
Order Chelonia (turtles and tortoises)
Family Cheloniidae (true sea turtles)
Genus Chelonia
Species mydas (subspecies C. m. mydas, C. m. agassizii)
They are believed to have evolved from a terrestrial reptilian ancestor. However,
at some point this species developed an adaptation for marine environments, and
approximately 150 years ago, evolved into Chelonia mydas. Because they are part of
Phylum Chordata they possess specific general characteristics which pertain to all
members in this category. They are deuterostomes and triploblastic. They also possess
the specific characteristics of animals in Class Reptilia which include scales on their skin,
a four-chambered heart which helps to circulate blood throughout their bodies (closed
circulatory system) , and are ectothermic (“Green Sea Turtles,” 2001).
Chelonia mydas are famous for their immense size and lengthy lifespan. Babies
just hatched from the egg weigh about one ounce and have a two inch long carapace.
Subadults or juveniles, ranging from 3-7 years old, have an average weight of 200-350
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pounds with a shell length of at least two and a half feet long. Males are larger than
females, but adults in general can weigh up to 400 pounds, have a shell as long as four
feet, and can live up to 80 years of age (Jansen, 2003).
In the following pages I would like to discuss specifics pertaining to the habitat
range and distribution of the green sea turtle, its feeding practices, physical adaptive
features which help to protect them from predation and other elements in their
environment, and reproductive and mating behaviors practiced. Lastly, however most
importantly, I will investigate several factors which are currently effecting the population
of the green sea turtle, all contributing to the endangered status of this species.
Distribution
Populations of the Chelonia mydas can be found worldwide in warmer waters
with a temperature above 20 degrees Celsius. This includes the United States, off the
east and west coasts of Florida, the Caribbean, off the Pacific coast of Mexico, and other
tropical marine climates. There is also a large population off the coasts of the main
Hawaiian Islands which provide all the essential nutrients needed for this species diet as
well as several popular nesting sites (Jansen, 2003).
Feeding
Adult green sea turtles are herbivores, therefore they possess certain bacterial
microbes in their guts to aid in the digestion of plant material, such as seaweed and algae.
Juvenile green sea turtles are omnivorous, feeding off of plankton, jellyfish and other
invertebrates that may be found in their marine habitat (“Green Sea Turtles,” 2001).
However, like many other species, the green sea turtle may have to make slight
adjustments to the types of organisms they feed off of as well as feeding locations, due to
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changing environmental conditions and other possible interfering factors. Research
performed in the Ogasawara Islands off the coast of Japan gathered data using satellite
telemetry and stable isotope analysis to investigate possible alternative feeding habitats of
Chelonia mydas. Results show that aside from the typical neritic areas (water depths
reach less than 200 meters) where we expect to find sea turtle feeding activity, this
species has also adopted an alternate oceanic feeding habitat where water depths will
often exceed 200 meters (Hatase et al, 2006). This study shows the necessity for a
species to be slightly flexible in adapting to current changes in their habitats. This will
help to increase their likelihood of survival.
Adaptive Features
Due to the marine habitat of the Chelonia mydas they are constantly faced with
many potentially harmful obstacles, such as predation from other carnivorous animals
and pollution of their environment. In order to protect themselves the Chelonia mydas,
like many other species, have specific, physical adaptive features to ensure a better
chance of survival.
All species of turtles have evolved a bony outer shell to protect themselves from
predators. This is known to be the most highly developed protective armor of all
vertebrates. However, the Chelonia mydas has a further adaptation to their shell.
Because of their marine habitat they are constantly in water and must travel long
distances and stay afloat, therefore the shell of the sea turtle actually weighs less and is
more streamlined than that of a terrestrial turtle, which makes traveling through water
much easier. All four of their limbs (front and rear) have evolved into flippers, making
them very efficient swimmers, covering long distances in short periods of time. Sea
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turtles have been known to move through the water as fast as 35 mph. Also, sea turtles
are capable of containing higher concentrations of carbon dioxide in their blood
compared to other animals. Therefore they can use the oxygen they consume more
efficiently. They are able to store oxygen in their blood as well as in their muscles, in
large quantities. They will usually swim to the surface to breathe every few minutes,
however when sleeping they can go for as long as two hours before having to take a
breath (“Green Sea Turtles,” 2001).
Another adaptation to their environment would be the salt gland they have, one
located behind each eye. In order to maintain homeostasis, their controlled internal
environment, they must somehow excrete all the excess salts they are taking in from the
salt water in which they live. The salt gland helps them to excrete salt in the form of a
“tear” from their eyes. This helps to maintain the salt levels in their blood (“Green Sea
Turtles,” 2001).
Reproductive and Development
Chelonia mydas have very specific and unique mating practices. They are an
oviparous species, meaning they reproduce by laying eggs. However, because they
develop at such a slow rate (taking approximately 25 years to reach sexual maturity) they
do not begin to reproduce until they are adults. Once they have reached adulthood and
fully matured they are able to reproduce. Typically, mating will begin in March and
occurs in waters adjacent to the nesting beaches where the females will eventually lay
eggs. Adults must first migrate to these nesting beaches from their foraging grounds,
which can often be a long voyage. Males will make this trip every year, while females
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only have to migrate every 2-4 years. Once near these grounds fertilization occurs
internally (Broderick, Godly, & Hays, 2001).
Next, nesting will occur from late April to September. Nesting occurs at night
and the female must come ashore to lay her eggs on the beaches. This may occur as
many as five times at 11 to 18 day intervals. However, researchers at Kyoto University in
Japan have discovered that in areas where tropical climates allow for the sand
temperature to remain within the thermal tolerance range (TTR), optimal temperature for
egg incubation, mating practices will proceed year round. This suggests behavioral
evolution of the species Chelonia mydas, for they will nest during periods when the sand
temperature is most favorable, avoiding the months when nest temperatures could
possibly drop below the TTR (Yasuda et. al, 2006).
The female will find a safe place and dig a pit in the sand to deposit a bundle of
100-120 ping-pong ball sized eggs before gently covering up the hole with her flippers to
protect the eggs from the external environment. According to Annette Broderick, head of
the Marine Turtle Research Group at the University of Wales, as stated in the Journal of
Physiological and Biochemical Zoology, “In all nest-building species the placements,
both temporally and spatially, of a clutch [of eggs] is crucial to the survival of the
offspring, especially in oviparous species” (Broderick et. al, 2001). The location of the
nest is essential to the survivorship of the green sea turtle. If the female does not take
care to find a secure place to lay her eggs there is little chance that the eggs would
survive a two month incubation period. Some extra conscience females have even been
known to dig “decoy” nests near the real nest in hopes of tricking predators and
distracting them from the true nest sight (Broderick et. al, 2001).
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After making sure the nest is securely covered with sand the mother will return to
the water leaving the eggs to incubate for approximately two months before they hatch.
The little hatchlings will then use a structure called the egg tooth, which is a protrusion
from their beaks, to dig their way out of the nest, in a group effort. Once free, they must
find their way safely to the shore without any parental care and forage for food in the
ocean.
There are many theories as to how the hatchlings determine whether it is safe to
emerge from the nesting site. An article published in the Journal of Experimental Marine
Biology and Ecology investigates potential stimuli. After collecting and analyzing their
data from two separate beaches on Ascension Island in the South Atlantic, researchers
concluded that a change in the temperature of the sand would indeed have an effect on
the temporal pattern of hatchling emergence. It is very important for hatchlings to be
able to detect this drop in temperature, which is indicative of nighttime. This provides the
best opportunity for hatchlings to forage across the beaches to the water, hopefully
avoiding predation (Glen et. al, 2006).
Once in the water, being able to locate and eat prey is essential to the survival of
these vulnerable young. Research shows that the incubation temperature has been proven
to have an affect on the body size and swimming capabilities of the hatchlings. Thus,
some are more likely than others to succeed in feeding and escaping predators. A report
in Coral Reefs states that eggs incubated at 26 degrees Celsius produced hatchlings that
were larger in size than hatchlings from eggs that were incubated at 28 and 30 degrees
Celsius. Also, hatchlings from eggs incubated at 26 degrees Celsius had a lower stroke
rate frequency and lower force output than those incubated at 28-30 degrees Celsius.
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From this data researchers were able to conclude that hatchlings that were incubated at
cooler temperature’s experienced muscle fatigue at a faster rate. This experiment further
proves the paramount role temperature plays in the survival of the offspring of this
species. If the offspring cannot survive to full development, there is a good chance the
species will eventually become extinct (Burgess et al, 2006).
Factors Affecting Chelonia mydas Populations
The green sea turtle has been classified as endangered on the IUCN Red list of
threatened species since 1982, meaning that this species is on a trajectory towards
extinction and drastic measures need to be taken to protect Chelonia mydas populations.
There were once several million however, today there are less than 200,000 reproducing
female green sea turtles in Hawaii. Also, scientists have estimated as few as 100 to 350
nesting females. The nesting female population on Aves Island of the coast of Venezuela
has declined by more than 90% since 1879 (Broderick et. al, 2006). This massive
decline in populations of green sea turtles can be attributed to several factors such as
commercial over-harvesting for eggs and food, fisheries by-catch, dredging, collision
with boats, loss of nesting sites, ingestion of marine debris, and pollution. A few specific
examples are discussed below (Lam et. al, 2006).
Hunting of the Chelonia mydas is one of the initial reasons they became
endangered. Before the federal government passed the Endangered Species Act of 1973,
green sea turtles were killed in large numbers to feed fishing crews and to provide meat
for restaurants in the Northwestern Hawaiian Islands, causing a large decrease in their
population. They are also hunted for their shells to make jewelry, their skin is harvested
to make small leather goods, their fat for oil, and even their eggs and meat pose as
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traditional meals in some Southeast Asian countries. Also, because of their predictable
nesting patterns and their vulnerability once on land, it is easy for poachers to hunt them
and make a profit, despite the laws prohibiting these actions (Toeeng & Rankin, 2005).
Predation has also become a problem for this species. Scavengers will destroy
nests and eat the eggs along beaches. Young hatchlings are very vulnerable to attack by
predators as well as environmental elements once they have surfaced above the sand. A
study done on Jupiter Island off the east coast of Florida by Hobe Sound National
Wildlife Refuge (HSNWR) showed that by removing primary predators, such as
armadillos and raccoons, from nesting sites, offspring managed to safely hatch and travel
to the nearby ocean for feeding. By 2002 predation was reduced to less than 10% at this
site. Unfortunately, funding for the project ran out by 2004, and with only two months of
the hatching period to go, predation surged back in even greater numbers, wiping out a
great portion of the Chelonia mydas offspring population (Engeman, R. et al, 2006).
Another factor affecting the green sea turtle population is pollution. Litter and
other debris can be especially harmful because the turtles can get entangled in this
material or may mistake it for food and ingest it. Plastics are often a problem because
they are very abundant amongst litter and are difficult to digest. These materials could
block the passage way for ingestion preventing them from eating. If ingestion is
successful, once digested, these foreign materials may turn out to be harmful, releasing
toxic chemicals in the turtles gut during digestion (“Green Sea Turtles,” 2001).
An article published in Environmental Pollution discusses the results of a study
investigating the exposure of green sea turtles to certain harmful contaminants in their
environment. After collecting egg samples from islands off the coasts of Hong Kong
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esearchers discovered many widely used synthetic chemicals, including polychlorinated
biphenyls (PCBs), and organochlorine pesticides (OCPs). Due to the long lifespan of
Chelonia mydas, and their migratory practices, they are at great risk of being exposed to
many of these toxic and often when accumulated in large quantities, deadly, chemicals.
This is why it is so important for humans to be aware of the waste they produce. Much of
the by-product resulting from human activity is disposed of in another species habitat and
will often cause harm (Lam et. al, 2006).
Coastal development and habitat degradation are also main reasons endangering
the green sea turtle population. Nesting beaches are transformed each year due to coastal
development. Noise, lights and beach obstructions are disruptive and can make usually
safe nesting areas unfamiliar and dangerous to females when they are trying to lay their
eggs (Troeeng & Rankin, 2005).
Lastly, a disease known as fibropapilloma has had a negative effect on the
Chelonia mydas population. This disease has been discovered in Hawaii’s green sea
turtle population as well as off the coast of Florida. Fibropapilloma causes large tumors
to develop on the turtle’s soft tissue. These tumors will often spread to other parts of the
body and will ultimately end up killing the infected turtle. The cause of this disease is
currently unknown, however scientists suspect that a virus, parasite or perhaps the effects
of marine pollution could be associated with development of fibropapilloma (Troeeng &
Rankin, 2005).
Discussion
Due to the length of time it takes for the Chelonia mydas to reach sexual maturity
(approximately 25 years) it takes them many years to recover from substantial population
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declines. This is why actions need to be taken to ensure that this species does not become
extinct. Currently the green sea turtle is fully protected by the federal Endangered
Species Act and by Hawaiian state law. These laws prohibit hunting, injuring or
harassing them, as well as holding them in captivity without obtaining a special permit
for research or educational purposes. Fines will also be placed on swimmers and divers if
they attempt to ride on the back of a green sea turtle because it will put them under
unnecessary stress. These fines can be as high as $100,000 (Hays et al., 2002).
Also, a recovery team has recently been formed to help restore the Hawaii’s green
sea turtle population. To ensure the survival of this species it is important that laws are
obeyed and their habitat is respected. Without the consideration and awareness of the
public, the chances of survival of the Chelonia mydas are minimal.
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References
Broderick, A., Godly, B., & Hays, G. 2001. Metabolic heating and the prediction of sex
ratios for green turtles (Chelonia mydas). Physiological and Biochemical Zoology,
74(2): 161-170.
Broderick, A. et al. 2006. Are green turtles globally endangered? Global Ecology and
Biogeography, 15: 21-26.
Burgess, E., Booth, D., Lanyon, J. 2006. Swimming performance of hatchling green
turtles is affected by incubation temperature. Coral Reefs, 25: 341-349.
Earthtrust. 2001. Green sea turtles. Retrieved November 8, 2006, from website:
http://www.earthtrust.org/wlcurric/turtles.html
Engeman R., et al. 2006. Impact on predation of sea turtle nests when predator control
was removed midway through the nesting season. Wildlife Research, 33: 187-192.
Glen, F. et al. 2006. Thermal control of hatchlings emergence patterns in marine turtles.
Journal of Experimental Marine Biology and Ecology, 334: 31-42.
Hatase, H. et al. 2006. Individual variation in feeding habitat use by adult female green
sea turtles (Chelonia mydas): are they obligately neritic herbivores? Population
Ecology, 149: 52-64.
Hays, G. 2002. The case of the green turtle (Chelonia mydas) at Ascension Island.
Canadian Journal of Zoology, 80: 3-5.
Jansen, Katherine. 2003. Green sea turtle. Retrieved November 8, 2006, from the
University of San Francisco Library:
http://proquest.umi.com.ignacio.usfca.edu/pqdwebindex.htm
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Lam, J., et al. 2006. Levels of trace elements in green turtle eggs collected from Hong
Kong. Environmental Pollution, 144: 790-801.
Troeeng, S. 2005. Long-term conservation efforts contribute to positive green turtle
Chelonia mydas nesting trend. Biological Conservation, 121: 35-37.
Yasuda, T., et al. 2006. Do female green turtles (Chelonia mydas) exhibit reproductive
seasonality in a year-round nesting rookery? Journal of Zoology, 269: 451-457.
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