Cuttlefish or cuttles are marine molluscs of the order Sepiida. They belong to the class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique internal shell, the cuttlebone, which is used for control of buoyancy.
Cuttlefish have large, W-shaped pupils, eight arms, and two tentacles furnished with denticulated suckers, with which they secure their prey. They generally range in size from , with the largest species, the giant cuttlefish (Sepia apama), reaching in mantle length and over in mass.
Cuttlefish eat small molluscs, crabs, shrimp, fish, octopus, worms, and other cuttlefish. Their predators include dolphins, sharks, fish, seals, seabirds, humans, and other cuttlefish. The typical life expectancy of a cuttlefish is about 1–2 years. Studies are said to indicate cuttlefish to be among the most intelligent invertebrates. Cuttlefish also have one of the largest brain-to-body size ratios of all invertebrates.
The "cuttle" in cuttlefish comes from the Old English name for the species, cudele, which may be cognate with the Old Norse koddi (cushion) and the Middle Low German Kudel (rag). The Greco-Roman world valued the cuttlefish as a source of the unique brown pigment the creature releases from its siphon when it is alarmed. The word for it in both Greek and Latin, sepia, now refers to the reddish-brown color sepia in English.
The earliest fossils of cuttlefish are from the end of the Cretaceous period, represented by Ceratisepia from the Late Maastrichtian Maastricht Formation of the Netherlands. Although the Jurassic Trachyteuthis was historically considered possibly related to cuttlefish, later studies considered to be more closely related to octopus and vampire squid.
The family Sepiidae, which contains all cuttlefish, inhabits tropical and temperate ocean waters. They are mostly shallow-water animals, although they are known to go to depths of about . They have an unusual biogeographic pattern; they are present along the coasts of East and South Asia, Western Europe, and the Mediterranean, as well as all coasts of Africa and Australia, but are totally absent from the Americas.