Cuttlebone, also known as cuttlefish bone, is a hard, brittle internal structure (an internal shell) found in all members of the family Sepiidae, commonly known as cuttlefish, a family within the cephalopods.
Cuttlebone is composed primarily of aragonite. It is a chambered, gas-filled shell used for buoyancy control; its siphuncle is highly modified and is on the ventral side of the shell. The microscopic structure of cuttlebone consists of narrow layers connected by numerous upright pillars.
Depending on the species, cuttlebones implode at a depth of 200 to 600 meters (660 to 1,970 ft). Because of this limitation, most species of cuttlefish live on the seafloor in shallow water, usually on the continental shelf.
The largest cuttlebone belongs to Sepia apama, the giant Australian cuttlefish, which lives between the surface and a maximum depth of 100 meters