Sea Turtles are a group of shelled reptiles and belong to the order Testudines or Chelonia. Also included within this order are the terrapins and tortoises.
Within the Chelonia, there are 13 families. The eight species of modern Sea Turtles are found within two of these, the Dermnochelyidae and the Cheloniidae.
The first family has only one Sea Turtle, the Leatherback. The other seven species of Sea Turtle are found in the other family - the Cheloniidae.
Sea Turtles are large animals, the Leatherback attaining a length of 2m and a weight of over 500 kg. (one caught off Wales in 1991 weighed 995 kg!)

The Sea Turtles were common in the Cretaceous period of 130 million years ago.
Their fossil record may well extend back at least 200 million years. Early Sea Turtles were alive with the dinosaurs although the present day genera and eight species originated within the last 60 to 10 million years.


Triassochelys - Skull of an early turtle ( 160 million years ago?), with a shell like that of modern forms, but still possessing teeth on the palate.
Like modern Sea Turtles, the head, tail, and limbs could not be withdrawn into the shell. Protection was gained by the addition of spines.
The aquatic chelonians showed various modifications and there were several lines of evolution. There was a reduction in the bony shell, presumably to save on weight and possible because of the absence of enemies.

Archelon of the cretaceous period ( 3.6 metre shell) was very similar to the modern Cheloniidae.
The fossil history of the chelonia before the triassic period is not all that clear: there is little real evidence to tell us how, when and why one early reptilian group shortened their bodies and covered them with armour for protection.

Recently some bones of a giant sea turtle was found in South Dakota, USA. The fossilized remains of this turtle represent the largest sea turtle ever found - probably 20 feet (six metres) wide! It has been dated at around 70 million years old, when much of the area was covered by water.
Sea Turtles are one of the most attractive, yet vulnerable, of all marine vertebrates. They are widely exploited by man for their eggs, meat and shell. Their survival is seriously threatened by many factors such as pollution, loss of nesting sites, fishing and tourism. All eight species are endangered.

Sea Turtles are adapted for life in the oceans. They are large long-lived animals ( 60+ years?) with relatively streamlined shells. The swimming limbs are held out to the sides more than downwards, making movement on land very difficult. The forelimbs or front flippers are generally elongated and provide the main propulsive force. The hind flippers are large and spadelike and the females use these when digging their nests in the sand.

Loggerhead turtle - internal bones of forearm
Sea Turtles absorb a great deal of salt from their diet and when they drink sea water. They have salt glands in their eye sockets which enables them to excrete excess salt. The salt concentration can be twice as much as in sea water. When female turtles nest they are said to cry: in reality, they are excreting salt via their eye glands.

Sea Turtles feed in a variety of ways although all hatchlings are carnivores, eating slow moving animals near the surface. The adults are essentially carnivores though stomach contents often include vegetable matter. Each species has its own food preference but all tend to be opportunistic. Food ranges from bottom dwellers such as crabs to floating jellyfish. Green turtles are herbivores, feeding on sea grasses, algae and mangrove shoots.

There is some evidence that young Sea Turtles can confuse oil droplets for food and that some adults eat plastic bags mistaking them for jellyfish.

Jelly fish, Laganas Bay, Greece.
It may take 15-30 years for Marine Turtles to reach sexual maturity.

All Sea Turtles have very similar life-histories, mating taking place in coastal waters and the females return to their natal beaches (the place they were born) to lay their eggs.

Some species are less specific ( philopatric) and return to the same area or stretch of coastline. Most species lay about 100 eggs in a flask shaped hole in the sand. Depending on the species, they may lay 2 - 6 times in a season although they may not lay again for 2 - 4 years.

From June to August the Loggerhead and Green turtle visit certain beaches of the Mediterranean to nest. ( The Leatherback does not nest in the Mediterranean)

The female turtle comes ashore at night, digs her nest and lays about 100 soft shelled eggs, which look similar to ping pong balls.

The eggs are then covered with sand and left to hatch.
The nests are about 45 cm deep.

The eggs must remain undisturbed in the warm sand for about 55 or so days before hatching.

If the temperature goes above 28 degrees C the eggs will hatch as females,

below this temperature males will hatch.

Although females lay thousands of eggs each summer, very few hatchlings survive to adulthood. ( 1 in 1000 ? )
Together, the hatchlings dig their way out of the nest.

Usually emerging at night, the group makes its way down the beach and enters the sea.

This race to the sea is important for the hatchlings' biological cycle.

Loggerhead turtles emerging from their nest.
Lights near the beach disorientate them, causing the hatchlings to

wander in the wrong direction.

If this happens they will die of dehydration or be eaten by predators.

This migration to the sea is made more difficult by vehicle tracks and sand

pits made during the day by tourists.

They are too small to climb out and soon die of dehydration in the hot sun of the morning.
For roughly 72 hours they swim directly out to sea to escape shore based predators,

then they begin to feed on passing morsels of food.

It is believed that they drift with the currents, but little is known

of hatchling and pre-adult turtle behaviour.

This period in their life history is known as the lost years.