General information on Marine turtles
History
Sea turtles inhabit warm waters of our planet’s oceans, bays and estuaries. They are similar to
their terrestrial (land) cousins, the tortoises, and to freshwater turtles, except that their legs
have been modified into flippers to aid them in swimming. Their shape has taken on a
flattened, more streamlined appearance – tapering off in the rear to allow for less water
resistance during swimming. All sea turtles except the leatherback turtle have a hard carapace
(top shell) and another hard shell on the belly called the plastron. The carapace, as with all
other turtles, incorporates their backbone,
sternum and ribs. This is unlike most other
animals whose backbone and ribs are free of a
shell or skin.
Sea turtles are closely related to ancient species
dating back 130 million years to the Cretaceous
Period. Some may be more closely related to
dinosaurs than our present day reptiles. Like all
other reptiles, sea turtles’ body temperature
depends upon their surroundings which explain
their migration southward as northern waters cool
off in the fall. The leatherback turtle, though, has
recently been found to be able to create some body heat, thus making them able to withstand
the chilly waters off Canada and Iceland, where they roam in the summer. Sea turtles are air
breathers, but are capable of holding their breath for quite some time. It is thought that they
are also able to absorb oxygen from their skin, mouth and their cloaca.
SEA TURTLE SPECIES OF THE PACIFIC COAST OF COSTA RICA
OLIVE RIDLEY SEA TURTLE
(Lepidochelys olivacea)
The olive ridley (Photo 4) is the
smallest of the sea turtles. Adults
measure 55 to 70 cm in carapace
length and weigh between 35 and
45 Kg.
They reach sexual
maturity when 10 to 15 years old
and probably live to be 50 or 60.
Nevertheless they are sexually
active for no more than 21 years.
The females usually nest every
two years.
However, they
sometimes nest every three years
Photo 4. Olive ridley sea turtle.
and sometimes every year. A single turtle can nest three times in a season with intervals of
17 to 28 days. The nesting season is from June to December. Olive ridleys also nest on
beaches in Mexico, Nicaragua, Honduras, Panama, Colombia and are found in the Pacific and
Indian oceans.
The average depth of their nests is approximately 45 cm and the average number of eggs per
nest is approximately 105. Eggs incubate in the nests from 40 to 65 days. Hatchlings
measure from 2.5 to 4 cm in carapace length.
In Costa Rica the name for olive ridley is “tortuga lora” which translates directly as parrot
turtle. They are called this because their beaks resemble a parrot’s beak. They are called
“olive ridleys” in English because the color of their shells is olive green. These turtles are
omnivores and eat crustaceans, mollusks, fish and some marine vegetation. Olive ridleys
especially like to float at or near the surface of the water to warm themselves. In fact the
shape of their carapace facilitates absorption of the sun rays. They can dive for up to 30
minutes to depths of 200 m.
Only two species of sea turtles, the kemps ridley and the olive ridley, perform the
phenomenon known as “arribada”. This occurs when thousands of turtles arrive to nest at
one beach at the same time during three or four days, especially when the moon is waning. In
Costa Rica there are two beaches, Ostional and Nancite, where olive ridley arribadas occur.
Both are in the Guanacaste province and the largest arribadas usually occur between the
months of June and December.
However, olive ridleys also nest in solitary fashion on the Pacific coast of Costa Rica.
Results of research projects indicate very low hatching success rates at arribada nesting sites,
apparently due to the high concentration of nesting turtles that mechanically destroy
previously laid nests. The massive destruction of eggs fosters the proliferation of fungi and
bacteria, affecting egg development and causing very low hatch rates (from 1% to 8%). In
contrast, at PRETOMA’s solitary nesting beaches the hatching success is considerably
higher, typically over 80%, a fact which draws attention to the contribution of solitary nesters
to the maintenance of olive ridley populations.
Olive ridleys, like other sea turtles, are in danger of extinction. One of the reasons is the
consumption of turtle eggs by humans and animals. Another reason is accidental capture by
commercial fishing fleets. These turtles feed on shrimp and are caught in shrimp nets and as
they are only able to hold their breath for 15-30 minutes, they drown. They are also
accidentally captured on hooks used by commercial longliners.
Prefrontal
scales
Postorbital
scales
Costal
scutes
Figure 3. The olive ridley sea turtle (Lepidochelys olivacea) have four prefrontal scales, three
pairs of postorbital scales and five to nine costal scutes.
GREEN SEA TURTLE
(Chelonia mydas)
The green turtle (Photo 5) has a
carapace of 80 to 100 cm in length
and can weigh up to 100 Kg.
Green turtles reach sexual maturity
when they are 16-25 years old. The
females nest every two or three
years. A single female can nest up to
three times per season with an
interval of 12 to 14 days. The
nesting season for green turtles on
the Pacific coast of Central America
is from September to March.
Nests are approximately 50 cm deep.
The number of eggs per nest ranges
from 65 to 87. Eggs incubate in the
nests on average 42 to 62 days.
Photo 5. Green sea turtle.
These nest sporadically on the
Pacific coast of Costa Rica on beaches such as Naranjo, Nombre de Jesus, Playa Caletas and
Rio Oro among others.
Green turtles are for the most part herbivores and feed on algae. They are found in Mexico,
El Salvador, Nicaragua, Costa Rica and Colombia.
Green turtles are endangered principally due to consumption of their meat and eggs.
Prefrontal
scales
Postorbital
scales
Costal
scutes
However, they are also captured accidentally by commercial fishing fleets. Over the past few
years, dozens of dead green turtles have washed up Pacific beaches in Costa Rica. They have
had injuries on their abdomens or evidence of capture in nets and on hooks.
HAWKSBILL SEA TURTLE
(Eretmochelys imbricata)
The hawksbill turtle (Photo 6) has a
carapace length of 70 to 95 cm and
weighs between 42 and 77 Kg.
Females nest every two or three years
and in a single season can nest up to five
Figure 4. The green sea turtle (Chelonia mydas ) has two prefrontal scales, four pairs of
postorbital scales and four costal scutes.
times in intervals of 14 to 16 days. The
nesting season on the Pacific coast is
from May to January.
The average number of eggs in each nest
is 160. Eggs incubate for 47 to 75 days.
The average nest depth is 40 cm.
Hatchlings have 3.8 to 4.5 cm carapace
length. Hawksbills feed mostly on
marine sponges but also consume other invertebrates and algae.
Hawksbills are near extinction because
Photo 6. Hawksbill sea turtle.
their beautiful shells are used for
different products and to obtain the shell
the turtle must be killed. Their eggs are also consumed by humans.
Prefrontal
scales
Postorbital
scales
Costal
scutes
Figure 5. The hawksbill sea turtle (Eretmochelys imbricata) has four prefrontal scales, three pairs of
postorbital scales and four costal scutes.
LEATHERBACK SEA TURTLE
(Dermochelys coriacea)
The leatherback turtle (Photo 7) is
the largest sea turtle and the heaviest
reptile on the planet. These turtles
are found in the Atlantic and the
Pacific.
In general, the Pacific
leatherbacks are slightly smaller.
They weigh between 300–400 Kg
and have an average carapace length
between 1.4 and 1.8 m.
Leatherbacks reach sexual maturity
after 10 years. Females nest every
two or three years and are able to
nest up to six times during a nesting
season with intervals of nine days.
The nesting season on Pacific
beaches lasts from September to
March.
Photo 7. Leatherback sea turtle.
The average number of eggs per leatherback nest is between 80 and 90 with approximately
30 infertile eggs. Average depth of nests is 75 cm. Eggs incubate from 50 to 70 days.
Hatchlings have an average carapace length from 5-6.5 cm.
Leatherbacks migrate great distances. From their nesting beaches in the tropics they migrate
to temperate zones and even sub-polar regions where they have feeding grounds. Satellite
tags on these turtles have shown that after nesting in Costa Rica, they migrate south and west
to Coco Island and the Galapagos and continue on along the coast of South America all the
way to the cold waters of Chile.
Of all the sea turtles, leatherbacks are able to dive to the deepest depths – down to 800 m. It
is also the fastest swimmer and is able to travel 70 Km per day.
Unlike other sea turtles, the leatherback does not have a carapace of bone and scales. Rather
their carapace is cartilage, rich in oil with bony plates and covered with skin that is very dark
in color with white spots. Their skin is smooth and lacks scales. Leatherbacks are the only
turtles capable of regulating their body temperature and maintain it above that of surrounding
waters. This allows them to survive in cold waters where they have feeding grounds. Their
diet is very specialized as they feed strictly on jelly fish.
Leatherbacks’ most important nesting beaches are found in Mexico, Nicaragua and Costa
Rica. Unfortunately leatherbacks are disappearing at alarming rates. Playa Grande, in the
Leatherback National Park in Costa Rica, is the most important leatherback nesting beach on
the Pacific, yet numbers of nesting turtles have plummeted over the past decade. Optimistic
estimates put the number of remaining leatherbacks world wide at 30 000. Eastern Pacific
Leatherbacks are showing the greatest decline and it is estimated that less than 1000 nesting
females are left in existence. This species could be extinct within 10-15 years if something is
not done to stop the decline.
Longitudinal
ridges
Figure 6. Head of leatherback sea turtle (Dermochelys coriacea) has no scales, and there are
seven prominent ridges along the length of the torso.
Life cycle
The seven different species of marine turtles inhabit both temperate and tropical regions,
utilizing both marine and terrestrial habitats during different phases of their life cycle.
Nesting behaviour
Female turtles crawl ashore during the night to find an ideal nest location in which to lay their
eggs. The female turtle will clear vegetation and other debris away from the chosen areas
with her frontal flippers before digging her nest using her hind flippers. Depth of the nest
depends on the species of turtle ranging from 40cm to 100cm. Number of eggs laid and size
also depends on the species. Once all eggs have been laid the female covers the nest and pats
down the sand using her whole body, once satisfied that her eggs are safe she quickly returns
to the ocean.
Egg development
How fast an embryo becomes a hatchling is determined largely by temperature, so nest
location is important, shaded nests can take up to 70 days to incubate whereas nests in direct
sunlight can take as little as 45 days to hatch. Temperature of the nest is an extremely
important part of egg development as it determines the sex of the hatchlings, at 28oc
hatchlings develop into males and at 31oc or above they will produce females.
Hatchlings
After roughly 2 months hatchlings are ready to emerge. It takes 24 to 48 hours for the
hatchlings to reach the surface. Hatchlings travel up the nests in groups the hatchlings at the
top knock sand from the roof of the nest which filters down to the hatchlings at the bottom
that stamp it into the floor of the nest. As this continues the nest rises slowly until they break
the surface. In general hatchlings emerge only during the night when temperatures have
dropped and the sand is cooler.
Visual cues guide the hatchlings from the nest towards the sea following the brighter seaward
horizon. Once entering the ocean hatchlings initially orient seawards by swimming into the
waves. Experiments have demonstrated that hatchlings can transfer a course initiated on the
basis of waves or visual cues to a course mediated by a magnetic compass. After they have
entered the ocean little is known about where hatchlings go, it is thought that they migrate
drifting pelagically in oceanic gyre systems, this period is known as the “lost years”.
Maturity
When hatchlings have grown into juveniles sea turtles migrate back to tropical and temperate
coastlines where they utilize these habitats to feed and further develop until they have
reached maturity, taking between 15-30 years depending on the species. Upon maturity as the
nesting season approaches adults start migrating back to nesting beaches, as mating grounds
usually lie adjacent to the nesting beach. It is thought that adult sea turtles find their way back
to the same nesting beach where they were born by a number of different methods including
magnetic navigation, celestial cues, chemical concentrations in sea water and memory of land
marks. Once mating has taken place the female crawls ashore to lay her eggs, returning year
after year for decades to repeat the process.
Threats
Harvest for consumption: to date many cultures still consume both sea turtle eggs and meat.
Here in Costa Rica it is very common on all sea turtle nesting beaches to encounter people
stealing turtle eggs illegally. Ostional is the only nesting beach in Costa Rica where it is still
legal to take turtle eggs. In many Central American and Asian countries other parts of the
turtle can be used such as the shell, oil and skin.
Commercial fishing: trawling and longlining severely
effect sea turtle populations annually through incidental
capture resulting in high mortality. 250,000 sea turtles are
captured annually through trawling 60% of which result
in mortality, 15,000 of these are captured in Costa Rican
waters. 300,000 turtles are captured annually by long
liners of which 5% result in mortality.
Marine debris and pollution,
Ocean Plastic, hundreds of thousands of sea turtles, whales, other
marine mammals and more than 1 million seabirds die each year
from ocean pollution and ingestion or entanglement from marine
debris. Plastic bags have been well documented to cause mortality
in leatherback turtles, as a floating plastic bag can be easily
confused as a jellyfish, which is the primary food source of leatherbacks.
Coastal Development, - loss and degradation of numerous nesting beaches is occurring
through factors associated with coastal development. Beachfront armoring have caused the
loss or depleted success of many nesting beaches worldwide. 40% of Florida’s beaches are
now classified as critically eroded due to this type of landscape changes. Increased
beachfront lighting deters females from coming ashore to nest and disorients newly
emerged hatchlings making it more difficult for them to reach the sea. Increased traffic
both on the beach and around coastal waters, driving on nesting beaches can cause sand
compaction resulting in lower nest success. Driving at night can disturb nesting females and
disorientate newly emerged hatchlings. Increased traffic around coastal waters can increase
risks of collisions causing severe injuries or mortality in sea turtles.
Climate change: Increased atmospheric temperatures will raise sand temperatures
affecting the incubation process of sea turtle nests. The sex of hatchlings are determined by
nest temperatures, if this increases substantially the ratio of males to females will be
considerably effected and could also result in no hatchlings emerging. The increased risk of
rising sea levels will considerably effect female turtles as more nesting beaches will become
eroded resulted in fewer areas where they can nest successfully.