Cuttlefish or cuttles[2] 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 15 to 25 cm (6 to 10 in), with the largest species, the giant cuttlefish (Sepia apama), reaching 50 cm (20 in) in mantle length and over 10.5 kg (23 lb) in mass.[3]
Cuttlefish eat small molluscs, crabs, shrimp, fish, octopus, worms, and other cuttlefish. Their predators include dolphins, sharks, fish, seals, seabirds, 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.[4] Cuttlefish also have one of the largest brain-to-body size ratios of all invertebrates.[4]
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 600 m (2,000 ft).[10] 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. By the time the family evolved, ostensibly in the Old World, the North Atlantic possibly had become too cold and deep for these warm-water species to cross.[11] The common cuttlefish (Sepia officinalis), is found in the Mediterranean, North and Baltic seas, although populations may occur as far south as South Africa. They are found in sublittoral depths, between the low tide line and the edge of the continental shelf, to about 180 m (600 ft).[12] The cuttlefish is listed under the Red List category of "least concern" by the IUCN Red List of Threatened Species. This means that while some over-exploitation of the marine animal has occurred in some regions due to large-scale commercial fishing, their wide geographic range prevents them from being too threatened. Ocean acidification, however, caused largely by higher levels of carbon dioxide emitted into the atmosphere, is cited as a potential threat.[13]
Cuttlefish, like other cephalopods, have sophisticated eyes. The organogenesis and the final structure of the cephalopod eye fundamentally differ from those of vertebrates such as humans.[14] Superficial similarities between cephalopod and vertebrate eyes are thought to be examples of convergent evolution. The cuttlefish pupil is a smoothly curving W-shape.[15][16] Although cuttlefish cannot see color,[17] they can perceive the polarization of light, which enhances their perception of contrast. They have two spots of concentrated sensor cells on their retinas (known as foveae), one to look more forward, and one to look more backward. The eye changes focus by shifting the position of the entire lens with respect to the retina, instead of reshaping the lens as in mammals. Unlike the vertebrate eye, no blind spot exists, because the optic nerve is positioned behind the retina. They are capable of using stereopsis, enabling them to discern depth/distance because their brain calculates the input from both eyes.[18][19]
Cuttlefish possess an internal structure called the cuttlebone, which is porous and is made of aragonite. The pores provide it with buoyancy, which the cuttlefish regulates by changing the gas-to-liquid ratio in the chambered cuttlebone via the ventral siphuncle.[22] Each species' cuttlebone has a distinct shape, size, and pattern of ridges or texture. The cuttlebone is unique to cuttlefish, and is one of the features that distinguish them from their squid relatives.[23]