Can a Turtle Live Without Its Shell? How the Shell Is Built

The shell is part of the skeleton

Cartoons often show turtles slipping out of their shells like a person stepping out of a coat. Real turtles are built completely differently. During development in the egg, the ribs of a turtle grow sideways rather than curving down around the body, and they fuse with plates of bone that form in the skin. Together they make the carapace, the upper shell.

The backbone runs along the middle of the carapace and is fused to it for most of its length. Only the neck and tail vertebrae stay free to move. This means removing the shell would mean removing the turtle’s spine and ribs, which no animal could survive.

Turtles are the only living animals whose shoulder blades sit inside the rib cage. Their ribs grew outward over the shoulders and hips during evolution, wrapping the limbs inside the shell. The museum skeleton in the photo shows this clearly: the ribs fan out under the carapace like the spokes of an umbrella.

How is a turtle shell built?

A turtle shell has two layers.

The bony layer. In a typical turtle the carapace is made of about 50 bones, including the ribs, parts of the backbone and a ring of edge bones. The plastron, or lower shell, has about nine bones, some of which come from the collarbones and the bone between them. In most turtles the carapace and plastron are joined on each side by a bony bridge. Box turtles have flexible joints instead, which lets them close up, as explained in Can box turtles close their shells?

The scutes. Over the bone lies a layer of thin plates called scutes, made of keratin, the same tough protein as fingernails, hair and horn. A typical turtle has 5 large scutes down the middle of the back, 4 pairs along the sides and a row of smaller scutes around the edge.

The seams between the scutes do not line up with the seams between the bones underneath. Like bricks laid in an offset pattern, this overlap makes the shell stronger, because no weak line runs straight through both layers.

The shell is not dead armor. The bone has a blood supply, and there are nerves under the scutes, which is why turtles can feel touch through their shells.

How does a turtle shell grow?

A turtle hatches with its shell already formed, though it is soft at first and hardens over the first years of life. As the turtle grows, the bones of the shell grow at their edges and the scutes grow too.

In many land turtles, such as the eastern box turtle and tortoises, each scute grows by adding new keratin around its edge. This leaves rings that can be used to estimate the age of young turtles. Many water turtles, such as painted turtles and sliders, shed the thin outer layer of their scutes in pieces as they grow, revealing a fresh layer beneath.

Shell shape also depends on the environment. The Galápagos giant tortoise has a domed shell on wetter islands and a saddle-shaped shell, raised at the front, on dry islands where it must stretch its neck to reach tall plants.

Turtles with unusual shells

Not every turtle has a hard, plated shell:

Even in these turtles the shell is still part of the skeleton and cannot be removed.

How does a turtle breathe inside a rigid shell?

Most animals with lungs breathe by moving their ribs. A turtle’s ribs are locked into its shell, so it cannot do that. Instead, turtles use special muscles in the body cavity, along with movements of the legs and neck, to squeeze and expand the space around the lungs. Some water turtles can also take up a little oxygen through the skin and the lining of the cloaca, the rear opening of the body.

How did turtles get their shells?

Fossils show that the shell evolved in steps over millions of years. Early turtle relatives such as Eunotosaurus, which lived about 260 million years ago, had broad, flattened ribs. Odontochelys, found in rocks about 220 million years old in China, already had a full plastron but only a partial carapace of widened ribs. Fully shelled turtles followed soon after. Many scientists think broad ribs first helped these animals dig, and only later became armor.

Myths about turtle shells

Myth: a turtle can find a new shell. No. That idea comes from hermit crabs, which live in empty snail shells. A turtle grows its own shell and keeps it for life.

Myth: the shell is just dead armor. No. It is living bone with blood vessels and nerves, and it can slowly repair damage. Injured wild turtles should be taken to a licensed wildlife rehabilitator rather than treated at home.