thumb|Cuttlebone of [[Sepia officinalis (left to right: ventral, dorsal, and lateral views). The cuttlebone is about 15cm in length.]]
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thumb|[[Tortoise with cuttlebone]]
thumb|Fossil cuttlebone of the [[Pliocene species Sepia rugulosa]]
thumb|Fossilised cuttlebone-like [[gladius (cephalopod)|gladius of Trachyteuthis]]
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, within the cephalopods. In other cephalopod families it is called a gladius.
Cuttlebone is composed primarily of aragonite. It is a chambered structure that the animal can fill with gas or liquid for buoyancy control. On the ventral (bottom) side of the cuttlebone is the highly modified siphuncle; this is the organ with which the cuttlebone is filled with gas or liquid. The microscopic structure of cuttlebone consists of narrow layers connected by numerous upright pillars.
Depending on the species, cuttlebones implode at a depth of . Because of this limitation, most species of cuttlefish live on the seafloor in shallow water, usually on a continental shelf.
Upon the death of a cuttlefish, its body decomposes, leaving only the cuttlebone, which often washes up on beaches.
Human uses
In the past, cuttlebones were ground up to make polishing powder, which was used by goldsmiths. The powder was also added to toothpaste, and was used as an antacid for medicinal purposes and 20th centuries.
Bird calcium supplement
Today, cuttlebones are commonly used as calcium-rich dietary supplements for caged birds, chinchillas, hermit crabs, reptiles, shrimp, snails, and laying hens. These are not intended for human consumption. They are commonly available at pet stores.
Lime production
As a carbonate-rich biogenic raw material, cuttlebone has potential to be used in the production of calcitic lime.
Jewelry making
Because cuttlebone is able to withstand high temperatures and is easily carved, it serves as mold-making material for small metal castings for the creation of jewelry and small sculptural objects.
It can also be used in the process of pewter casting, as a mould.
Internal structure
The microstructure of the cuttlebone consists of two components, horizontal septa and vertical pillars. Both components are composed predominantly of aragonite. The horizontal septa divide the cuttlebone into separate chambers. These chambers are supported by the vertical pillars which have a corrugated (or "wavy") structure. The horizontal septa are typically thicker than the vertical pillars and consist of a double-layered structure. The upper layer of the septa and walls consist of vertically aligned crystals, whereas the bottom sublayer consists of nanorods rotated with respect to each other to form a "plywood" structure. In addition, due to its mechanical properties, cuttlebone has been used as scaffolding in superconductors and tissue engineering applications. The light weight of the cuttlebone derives from its high porosity (over 90% by volume).