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Cuttlefish are squid-like cephalopod mollusks of the family Sepiidae, in the order Sepioidea. Cephalopod literally means "head-footed animal" and is the name given to advanced mollusks (such as cuttlefish, squid and octopus) whose heads are circled with tentacles. Cuttlefish have a relatively well-developed brain, sensitive organs of smell and hearing, highly developed eyes, and a relatively advanced reproductive system.

There are more than 100 species of cuttlefish common in the warmer waters of the Mediterranean, the European Atlanticcoast, and abundant in the Indian Ocean, and western Pacific. Cuttlefish are found in shallow, sandy, coastal waters, where they feed upon their usual diet of Sepia latimanus, a cuttlefish, off Mota Island, Vanuatu. © Fred McConnaughey, National Audubon Society Collection/Photo Researchers, Inc. Reproduced with permission. shrimp. Cuttlefish are not found in the oceans around the United States. The smallest species of cuttlefish (Hemisepies typicus), grows to about 3 in (7.5 cm) long, while the largest species (Sepia latimanus) can reach up to 5.5 ft (1.6 m) in length. The best known species of cuttlefish Sepia officinalis, or common cuttlefish, grows up to about 3 ft (91 cm) long, including its tentacles.

Cuttlefish have ten tentacles (decapod), eight of which are short and have rows of suckers at their ends. The other two tentacles are longer and are retractable tentacles that can be used to catch prey. These tentacles have club-shaped ends with suckers, which can catch prey faster than the tongue of a lizard or frog, and can retract into sockets beside each eye. The cuttlefish mouth bears a strong beak-like structure that can bite and tear the prey, and cuttlefish salivary glands can secrete an immobilizing poison with the saliva.

The skin of a cuttlefish has pigment cells (chromatophores) that are under nervous and hormonal control, which enables the animal to become red, orange, yellow, brown, or black. Cuttlefish are often colored brownish green with white, irregular stripes that provide a perfect camouflage among seaweed. Cuttlefish also have a purple ribbon-like fin running the length of the body, and they are iridescent in sunlight. Cuttlefish can change their color and pattern at will, in a fraction of a second, a behavior which is thought to be a form of communication. They can become invisible by taking on the colors and designs of their surrounding environment, including other cuttlefish.

The body of a cuttlefish is a flattened oval and supported by a shield shaped, internal, calcareous (calcium) shell that contains a number of tiny, gas filled chambers. The cuttlebone has a hydrostatic function—it can change the proportion of gas and liquid it contains, thus controlling its specific gravity. Cuttlebones are used in bird cages as a good source of calcium, and to help keep the bird's beak trimmed. Cuttlefish bone is also pulverized and used in polish.

These mollusks swim by undulating their side fins and by using a funnel in the mantle cavity to maintain a stationary position in the water and to propel itself backward with a great deal of speed if necessary. The cuttlefish can control the direction of the funnel, and so control the force with which the water is expelled. Another defense capability of the funnel is a brownish black ink (called sepia) that is ejected when danger is sensed. The pigment in the ink is made of copper and iron, which are extracted from the cephalopod's blood. The sepia ink is the original India ink and is used by artists and industries as a pigment for paints, inks, and dyes.

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