Download Turtle booklet 1 - Australian Outback Spectacular

Turtles
Turtles
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1
Turtles
CLASSIFICATION
Turtles are vertebrates - animals with backbones. They belong to the order Testudines which
include the tortoises and turtles. (There are no true tortoises found in Australia).
a.
b.
c.
Figure 2: Examples of: a. sea turtle, b. fresh water turtle and c. land tortoise
About 210 known species survive today. Only seven species are sea turtles. These are the:
Green Turtle
Chelonia mydas
Flatback Turtle
Chelonia depressa
Loggerhead Turtle
Caretta caretta
Hawksbill Turtle
Eretmochelys imbricata
Pacific Ridley Turtle
Lepidochelys olivacea
Kemps Ridley Turtle
Lepidochelys kempi
Leatherback Turtle
Dermochelys coriacea
FEATURES
General
Sea turtles are found throughout the world in tropical and sub-tropical oceans and are a
dominate life-form in littoral and coastal ecosystems. With the exception of Kemps Ridley
which is only found in the Atlantic Ocean, all can be found in Australian waters.
Sea turtles live their life in the sea with the females returning to land only to lay their eggs.
They do not need to drink fresh water and are able to excrete excess salt through special
glands in the eye. Their eyes have adapted to life in the water which may make them short
sighted on land.
Like other reptiles they are poikilothermic (po-kee-low-thermic) or ectothermic which means
that their body temperature takes on that of the surrounding environment.
External
Marine turtles have strong, paddle shaped forelimbs. Most have one or more claws on the
leading edge of the flippers, a non-retractible head and a completely roofed over skull. Their
limbs pass out from the body sideways and are also non-retractable. Except for the
Leatherback Turtle, the shell, which covers their body providing protection against predators,
is composed of bone. It is then covered by a layer of overlapping keratinised plates known as
scutes.
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Turtles
Central scutes
Carapace
Eye
Plastron
Head
Tail
Mouth
Fore flipper
Hind flipper
Lateral scutes
Marginal scutes
Figure 3: The external features common to all hard shelled sea turtles
Turtles vary in size according to species. Until maturity is reached it is difficult to distinguish
between male and female turtles unless they are examined internally. To do this, a small
incision is made in the hind flipper region and the stem of a laproscope is inserted. Through
the use of fibre optics the internal organs and gonads can be viewed. This allows the
researcher to determine the maturity, sex and sexual development of the turtle.
Figure 4: Dr Colin Limpus, volunteers and Sea World staff determine the maturity, weight and size of
the turtles
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Turtles
Skeleton
The turtle's shell is part of the skeleton. The backbone and ribs are attached to bony plates
which form the outer shell hiding the hips and shoulders. The dorsal (top part) of the shell is
called the carapace and the ventral is called the plastron. They have several bones making
up the lower jaw and one bone in the middle ear which assists vibrations to the inner ear.
Figure 5: The skeletal structure of the sea turtle
The shell (except for the Leatherback) is covered with hard scales made of keratin. Although
it looks and feels very hard, the turtle can feel the pressure of your touch through its shell.
The bones that once formed the turtle's toes have become extremely long and have formed a
flipper.
Feeding
Sea turtles do not have teeth. Instead, they have jaws which crush, tear or bite depending on
their diet which varies between species. There is little competition between the species for
food:
Species
Green
Loggerhead
Olive Ridley
Kemps Ridley
Hawksbill
Leatherback
Flatback
Distribution
tropical and sub-tropical
tropical and sub-tropical
tropical
tropical to sub-tropical seas of North Atlantic
tropical reefs
tropical to cool temperate
tropical
Diving and Locomotion
The turtle's shell has flattened, making it very
streamlined. There are five fingers and toes in their
four strong, flat flippers. The front flippers propel the
turtle through the water, while the back ones act as a
rudder. They are able to dive for long periods because
of their ability to fully ventilate their lungs. They can
also go into oxygen debt which means that they stop the
blood flow into areas except for the heart, lungs and
brain so they are able to remain underwater for longer
periods.
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Diet
seagrass, algae
molluscs, crabs
small crustaceans, molluscs
crustaceans, molluscs
mainly sponges
mainly seajelly
soft corals, sea pens
Figure 6: Flipper movement
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Turtles
Breeding
Like most reptiles, the sea turtle lays eggs. The breeding seasons are usually in
spring/summer but do differ between species. Nesting behaviours also differ. Mating
generally occurs at the ocean surface after the female has laid the clutch from the mating
completed the season before. Fertilisation is internal and it is common that a number of
males will mate with one female.
Generally the female returns to the same nesting
area each time she breeds which often involves long
migrations.
Some may travel up to 6000 km,
following migration routes. The mother scoops out a
deep hole in the sand using her hind flippers and
then lays her eggs. She then covers the eggs with
the sand to keep them warm and damp before
returning to the sea. She will lay at least two and up
to several clutches of eggs within a season, which
can total between 200 and 1,000 eggs depending on
species and size of female.
The reason for
producing so many eggs is because the death rate
of the young turtles is so high.
Figure 7: The female turtle digs a hole in
the sand to lay her eggs
Growth and Development
The temperature of the nest will determine the sex of many of the young turtles. If the nest
temperature is 25 - 26 degrees the majority will be male. If 31 - 32 they will generally be
female.
Depending on the species and weather conditions, turtle eggs hatch after an incubation
period of about eight weeks. The baby turtle within the egg has a sharp "egg tooth" on the
end of its nose which is used to slice its way out of the leathery shell. This "egg tooth" falls off
shortly after hatching. Once out of the egg, the hatchling begins to dig its way up to the
surface. They wait for the right tide and time before leaving the sandy nest and head toward
the ocean swimming many kilometres out to sea where they drift on the surface for many
years, feeding on planktonic life.
Predation
Lizards, pigs, dingoes and other feral and domestic animals raid the eggs while they are still
in the nest. Indigenous people rob hundreds of nests of all their eggs. Birds are added to the
list of predators as the turtles head towards the water's edge. When they reach the water,
large fish, crocodiles, octopus, and sharks wait for them. Whilst floating close to the surface
with the currents, they are again preyed on by other fish, sharks and Killer whales.
Mature turtles are usually large enough to be only bothered by large sharks and Killer whales.
A sea turtle is able to partially protect its flippers by folding them under the plastron. Still,
there are many turtles seen with one or more of their flippers missing or partially missing but
still survive quite well.
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