Stories
Secrets of the Shell
A turtle doesn't wear its shell — it is the shell. An anatomy of the most famous body part in the reptile world.
The shell is bone
A turtle's shell is not armor it wears. It is the ribcage and spine themselves, flattened and fused with bones formed in the skin — the body itself. The backbone runs along the inside of the carapace, which is why a turtle can no more leave its shell than you can leave your ribs.
Covering the bony shell are keratin plates called scutes — the same material as your fingernails. The seams of the bones and the seams of the scutes are deliberately offset, overlapping like staggered brickwork: one of the shell's structural secrets.
Every scute has a name
On the carapace, the plates running down the center are the vertebrals; the large plates flanking them are the costals; the small plates rimming the edge are the marginals; the single plate above the neck is the nuchal. When researchers identify a species, they are often counting exactly these.
The belly shell is the plastron, joined to the carapace at the sides by a bony bridge — all of it, together, is the shell.
Growth rings
Look closely at a scute and you may see concentric lines like tree rings. These growth annuli mark seasons of fast and slow growth, and give a rough idea of age — though extra rings can form within a year, and old ones wear away, so ring-counting is an estimate, not a birthday.
Some aquatic turtles shed the outer layer of their scutes as they grow. A translucent plate drifting in the tank may simply be the shell molting.
Frequently asked
Q. Can a turtle feel its shell? — Yes. The shell carries nerves; turtles feel touch through it, and some visibly enjoy a gentle scratch. It is not a box.
Q. What if a shell cracks? — The shell is bone, so a crack is a fracture. Skip the DIY glue and consult a reptile veterinarian.
Q. Where does the hexagonal kikkō pattern come from? — From the shapes of the scutes themselves. See the Dictionary of Turtle Words for the full story.
