Turtles are an ancient group of reptiles who have unique shells that protect their bodies. The collective name turtle includes sea turtles, freshwater turtles and the land-based tortoises. Being reptiles, all turtles lay eggs on land and breathe air. They evolved over 200 million years ago and are one of the most specialized groups of all vertebrate animals. All up there is around 220 living species of turtles and they are found mostly in tropical and sub-tropical climates.

Turtles provide no parental care to their children and it is estimated that only 1 in 1000 turtles survive to adulthood. Adult turtles however are one of the longest living animals on Earth and individuals from many species can live for over 100 years. Unfortunately, a large number of turtle species are currently endangered due to human activities.

Turtle shell

A turtle is easily identifiable because of the giant shells it carries on its back. Shell provide protection from predators and are key to their long-lasting survival but they have also limited the diversity of turtle species. The shell is fused with both the ribs and the spine of a turtle. The top half of the shell is known as the ‘carapace’ and the bottom shell is called the ‘plastron’. The carapace and plastron are connected together via bony structures called ‘bridges’. The carapace is usually covered in a layer of scales, called scutes, which are helpful for identifying different turtle species.

The shape and structure of a turtle’s shell is often a reflection of the environment the turtle lives in. Most land-based tortoises have high domed shells, whereas sea turtles and freshwater turtles are more likely to have lower shells to reduce drag as they swim through water. Soft shelled turtles are especially fast swimmers. Partly because they have lighter shells but also because they have large webbed feet. The pancake tortoise is an exception as it has a flat shell and lives on land. Its flat shell enables it to squeeze into small crevasses between rocks.

Turtle Physiology

Green sea turtleTurtles have an interesting physiology that is influenced a lot by the presence of their shell and the fact that their body temperature is regulated by the surrounding environment. Turtles, along with other reptiles, will often spend a significant amount of time basking in the sun in order to warm their body up and increase their metabolism.

A turtle’s shell constricts the expansion of its ribs for breathing. Turtles overcome this problem with muscles that increase the space surrounding their lungs as they breathe in. This provides room for the lungs to expand. The muscles then squeeze the same space and cause air to be forced back out of the lungs as they breathe out.

Turtle’s hearts have the ability to control whether blood is sent to the lungs or the rest of the body. Depending on the importance of oxygen uptake or temperature regulation at any given time, the heart will send more blood to the lungs or to other parts of the body.

Sea turtles

Of the approximately 220 species of turtle, only eight live in the sea. This includes the black sea turtle, flatback sea turtle, green sea turtle, hawksbill sea turtle, Kemp’s Ridley sea turtle, leatherback sea turtle, loggerhead sea turtle and olive ridley sea turtle. The sea turtles are highly specialized for life in water with legs that have been modified into flippers, no teeth and flatter shells to reduce drag. They are found mostly in warm tropical and sub-tropical waters and make huge migrations across thousands of miles of ocean.

The only time sea turtles ever leave the water is when females come ashore to lay eggs above the high tide line. Females return to the same stretch of beach that they were born on, they bury their eggs in sand and then return immediately back to the water. Once the juvenile turtles hatch, they scramble to the water. Many newly hatched turtles are eaten by birds, crabs, sharks and humans and it is estimated that only 1 in a 1000 survive to adulthood. Once male juvenile sea turtles reach the sea, they never again leave the water.


Tortoises are a group of land-based turtles from the family Testudinidae and generally only enter water to drink or bath. Tortoises are extremely long-lived animals and some individuals are known to have lived for well over 150 years. Most species are herbivorous whilst a couple feed on insects and other invertebrates. The majority of tortoises have high domed shells and elephant-like feet. Some species have spade shaped front feet that they use to dig burrows.

Interesting facts

  • The gender of a turtle is determined by the temperature that its egg is incubated at. Global warming is creating problems for turtle sex ratios because warmer temperatures are increasing the amount of females born relative to males.
  • A musk turtle is able to climb trees.
  • The largest turtle is the leatherback which can weigh more than 600 kg (1300 lbs.)

Last edited: 16 December 2015