Sea Turtle Biology
Evolution and adaptation
records lead to the conclusion that sea turtles are contemporaries
of the dinosaurs. Although dinosaurs disappeared, turtles
continue to survive to the present date.
Those early turtles lived in marshes. Later
some of them began to live on land while others spent the
majority of their lives in the water. Throughout the evolutionary
process sea turtles have retained the following features:
- Like all reptiles, they are poikilotherms,
which means that they use the heat of the environment as
the main source of maintaining body heat
- They resemble the primitive amphibians and
birds in having only single small bone in the ear to conduct
sound. Sea turtles are especially sensitive to low frequencies
like ground vibrations and surf
- They have lungs and breathe air
- They lay their eggs on land
- Their heart is divided into two auricles
and one ventricle, resulting in incomplete double circulation,
which means they can tolerate a fairly high level of carbon
dioxide in their blood
- Their body is protected in a horny shell.
One exception is the leatherback sea turtle (Dermochelys
coriacea) with its leathery skin shell covering.
shell. The shell is the most noticeable feature of
all turtles. Comprised of a number of bones, it is an armoured
enclosure for soft vital organs. It consists of mainly
two parts: the upper shell or carapace and the underside,
or plastron. Tortoises and terrapins often have dome-shaped
carapaces which allow them to withdraw their head and limbs
in their shell in case of danger. Sea turtles do not have
this capability. The evolutionary process has led them
into having more hydrodynamically shaped carapaces.
Propulsion. Over time the stumpy legs
of land turtles evolved into the flattened flippers of sea
turtles. The front flippers are used for propulsion while
the rear flippers act as rudders.
Respiration. Sea turtles breathe with
lungs. This forces them to regularly surface in order to
take in air. As mentioned earlier, sea turtles are able to
sustain larger concentrations of carbon dioxide in their
blood than most other air-breathing animals. Furthermore,
both blood and muscle tissue can store oxygen in large quantities.
This allows them to spend a lot of time under water. Sea
turtles have been observed spending up to six hours under
Essentially, sea turtles live in the marine
environment where they mate, feed, migrate and hibernate.
Females return to land to dig nests and lay eggs. Males will
almost never return to land.
Little is known about the sea turtle's juvenile
years. After the eggs hatch, the hatchlings crawl to the
sea. For the first few days they swim frantically towards
the sea. Once in the open sea, they let themselves drift
with the currents. Little is known about what happens next
until it is time for them to return to their breeding habitats
to nest. It is believed that during their pelagic stage they
feed first on plankton and then snails, turnicates, gooseneck
barnacles and other organisms of the high seas. After this
pelagic state, sea turtles tend to live in coastal waters,
except the olive ridley (Lepidochelys olivacea)
and the leatherback that remain pelagic animals for their
Sea turtles do not have the speed and agility
to catch fast-moving prey. Therefore, most feed on slow moving
or sedentary animals like jellyfish, molluscs, sea urchins,
horseshoe crabs, sponges, and on sea grasses. Turtles have
been found to have a well-developed sense of smell that may
help them to locate food.
the time comes for reproduction, adult sea turtles migrate
toward their nesting areas. Mating has been observed during
these migrations, as well as in the offshore waters adjacent
to the nesting beaches. Soon after mating, the females approach
the nesting beaches where they emerge to dig nests and lay
their eggs. Most species nest at night, with the exception
of the ridleys that also nest during the day. The most spectacular
nesting is the "arribada" (Spanish for "arrival") of the
ridleys, when thousands of turtles come out to nest simultaneously
over a few days.
Mating: Adult males
have longer, thicker tails with the cloacal vent further
back than in females. During mating the male mounts the female
holding on to her carapace with its fore limbs. His longer
tail is bent downwards, pressing his cloacal opening against
the female's cloaca. There is evidence that females can store
sperm for long periods of time for future fertilisation of
Construction: Leaving the water, the female
sea turtle crawls up the beach to find a suitable nesting
spot. Females of most species will return to the sea
without laying eggs if lights or noise on the beach disturbs
them. Once the female has found a nesting site, she hollows
out a body- pit sweeping away the dry top sand with her
flippers. Then using alternate movements of her rear
flippers she digs a pitcher-shaped hole, the egg chamber.
When the female has finished digging, she drops her eggs
into the egg chamber one or two at a time. While laying her
eyes excrete "tears" which is a liquid from a special gland
in the eyes. This liquid carries off excess salt, and also
serves to keep her eyes moist and free from sand.
Immediately after the female is finished laying her eggs,
she starts covering the egg chamber. When this is complete
the turtle starts to press firmly the loose sand above the
eggs with "kneading" movements of her rear flippers. She
then conceals the nest site with sweeping movements of the
front flippers, throwing a considerable amount of sand. The
female then leaves the site and returns to the sea.
The eggs: The eggshells
are pliable, parchment-like in texture. The number of eggs
a female lays varies from species to species (about 50 eggs
for flatbacks Natator depressus and 80 to 120 for the other
species). There are even variations within a species. During
one nesting season a female lays several clutches. Generally,
sea turtles nest every two to four years.
eggs hatch in seven to ten weeks, depending on the temperature
of the sand. As hatchlings break out of the eggs they struggle
towards the surface. Because oxygen is scarce, this upward
movement may take from 2-4 days. As they reach the upper
levels of sand during the day, they stop any further movements
as high temperatures make them torpid. They wait just below
the surface until the sand has cooled at night or in the
early morning, then they emerge en masse and race toward
the sea. The reflected starlight guides them to the water.
With such large number of eggs, nests usually hatch in batches.
For instance, in Greece, a successful nest of 110 eggs will
quite likely produce 35 to 50 hatchlings when it first "breaks",
then for another 2 to 10 days produce two to three smaller
batches of 5 to 20 hatchlings. Some of the eggs are probably
unfertilised or contain unhatched embryos.
Threats to survival
The World Conservation Union (IUCN) has declared
6 species of sea turtles as endangered or critically endangered.
The threats for sea turtles are both natural and man-induced.
Threats: There are of course natural threats
to the survival of sea turtles, their hatchlings and
eggs. Weather is a prime factor. Wind, rain and cold
and unnaturally high tides, all take their toll on turtles
at all stages of their lives. Temperatures below 14°C
stun even adult turtles. Beach erosion destroys nesting
beaches. The natural enemies of eggs and hatchlings vary
depending on geographical location. In Greece, foxes,
dogs and occasionally jackals may dig up the eggs. Hatchlings
racing for the sea are sometimes eaten by those animals
as well as by martens and rats, or by birds like crows,
gulls and cormorants. Once the hatchlings have reached
the sea they may be devoured by large fish such as sharks.
Threats from Man
- Capture by fishermen for exploitation purposes
- Poaching of eggs for consumption
- By-catch in fishing gear resulting in drowning
or being killed by fishermen.
- Nesting beaches are dramatically shrinking
because of development, especially tourist development.
a) Lights shining on the beaches disorient both nesting
females and hatchlings trying to get to the sea
b) Sand compaction due to vehicular traffic may disturb
the balance of gases and their absorption by the eggs
c) Beach furniture on the nesting areas often make an impenetrable
wall that denies access to the back of the beach for nest
d) Heavily trafficked beach paths, planting shade trees,
or setting up umbrellas result in lower sand temperatures
which has an impact on the incubation of the nest
e) Human presence on the nesting beaches at night scares
sea turtles trying to nest
f) Sand castles and vehicle tracks may trap hatchlings
in their race to the sea.
- Pollution of the seas is yet a further threat.
Turtles often mistake discarded plastic bags for jellyfish,
tar balls or a chunk of polyethylene for something to eat.
If they consume these foreign substances their intestines
may become clogged and they may die.
Sea turtles in Greece
of the seven species in the world, only three can be regularly
found in the Mediterranean (loggerheads - Caretta caretta,
green turtles - Chelonia mydas and leatherbacks
- Dermochelys coriacea). Of these three only the
loggerhead sea turtle nests in Greece. The most important
nesting beaches are in Zakynthos (Bay of Laganas), Peloponnesus
(Bays of Kyparissia, Lakonikos and Koroni) and on Crete (Rethymno,
Bay of Chania and Bay of Messara). The beaches of Zakynthos
have an extremely high nesting density. The density on one
beach (Sekania) may reach about 1500 nests per km, one of
the highest in the world.
The loggerheads' carapace is heart-shaped and has five pairs
of costal scutes. The colour of the carapace is reddish brown
but can be obscured by a covering of green algae. The plastron
is pale yellow.
Loggerheads tagged in Greece have been recovered
over a wide area, up to 1500 km away, in Italian, Tunisian,
Libyan waters suggesting purposeful non-random movement.
Most of the recoveries are from the Gulf of Gabes in Tunisia
and the northern part of the Adriatic Sea which probably
indicate that these are the preferred wintering areas for
The most serious threats to loggerheads in the Mediterranean
are tourist development of nesting beaches and incidental
catch in fishing gear.