Molluscs belonging to the class Cephalopoda and the order Sepiida, they are most closely related to squid, octopuses and nautiluses. With eight arms, two tentacles with denticulated suckers used for securing prey and large W-shaped pupils and being named for their unique internal shell, the cuttlebone. They generally range in size from 15-25cm, while the largest species, Sepia apama, can reach 50cm in mantle length and over 10.5kg in mantle weight.
Cuttlefish have the one of the largest brain-to-body size ratios in the world and recent studies show them to be amongst the most intelligent invertebrates. Eating other cuttlefish, small molluscs, crabs, shrimp, fish, octopuses and worms. Their predators include dolphins, shark, fish, seals, sea birds and other cuttlefish. Their life expectancy is generally one to two years.
The cuttlebone that cuttlefish are so famous for is a porous structure formed of aragonite, that provides the cuttlefish with buoyancy, which it regulates by altering the gas-to-liquid ratio in the chambered cuttlebone with the ventral siphuncle.
Often referred to as the chameleons of the sea, cuttlefish have the remarkable ability to change the colour and texture of their skin to communicate with other cuttlefish, camouflage themselves and as a deimatic display to warn off potential predators. This colour changing ability is produced by groups of blue, red, yellow, black and brown pigmented chromatophores above a layer of reflective iridophores and leucophores. With up to 200 of these specialized pigment cells per square millimeter, this would be equivalent to about 359DPI if they were individually controlled.
Even though they lack the ability to perceive colour, cuttlefish are able to rapidly change the colour of their skin to match their surroundings, even in complete darkness, through some mechanism which is not yet understood.
The Greco-Roman world valued cuttlefish for the unique brown pigment which they release when attacked, which they called sepia, a word still used in modern english to refer to a brown pigment. The cuttlebone on the other hand was traditionally used by jewellers and silversmiths as moulds for small objects, and more recently as a source of dietary calcium for parakeets and other caged birds.