hard, brittle internal structure found in all members of the family Sepiidae

Cuttlebone is a hard brittle structure found inside cuttlefish. Like seashells, the cuttlebone is made of calcium carbonate. The cuttlebone is special as it is covered with tiny holes that the cuttlefish can fill with water or air. This allows the cuttlefish to control its ability to float or sink in water. In the past, cuttlebones were used to make polishing powder. This power was then added to toothpaste to make it look white. Today, cuttlebone is mainly used for caged birds and turtles in their habitats, so they get their calcium easily and fast.

A cuttlebone from a sepia
Common cuttlefish Sepia officinalis
Tortoise with cuttlebone