Room 12 pupils of Hoon Hay School, asks :-

If crabs have teeth where are they?

Harry Taylor, a Zoologist at the University of Canterbury, responded.

Crabs do have teeth but not in their mouths. There are teeth on the inside of their claws and also inside their stomachs.

Crabs have differently-shaped teeth for different jobs. Paddle crabs eat shellfish, like cockles and pipis, which they dig out of the sand. The larger right claw has rounded teeth near the hinge. These are used like nut-crackers to open small shellfish, or to chip bits off the edge. The left claw has sharper teeth, like the cutting teeth of cats and is inserted between the shell halves to cut the muscle holding them together. Claws of paddle crabs can be snapped shut very quickly and are also used to catch small fish. They are also used defensively and can inflict a painful wound to an attacker, or to bathers' feet!

Shore crabs, like the hairy-handed crab or the smooth shore crab, eat plant and animal material washed in by the tide. Their claws bear small, even teeth to grip pieces of food while they are torn up by the other claw and fed into the mouth. Mud crabs do not have teeth on their claws. The claws are used simply to scoop mud, which contains fine plant material, into the mouth.

Teeth inside the stomach form the "gastric mill" which grinds the food finely before digestion. In meat-eating crabs the teeth are large and knobbly, like the molars (back teeth) of pigs and people, to crush and chew the food. In eaters of seaweed and mud, the teeth are more like those of sheep and horses, having many ridges to grind up tough plant fibres.