cuttlefish, common name applied to cephalopod mollusks that have 10 tentacles, or arms, 8 of which have muscular suction cups on their inner surface and 2 that are longer and can shoot out for grasping prey, and a reduced internal shell enbedded in the enveloping mantle. The body is short, broad, and flattened. Cuttlefish are carnivorous and excellent at capturing prey with their arms.
Although good swimmers, they are not as fast as the related squids, but like the squids cuttlefish have lateral fins used as stabilizers and for steering and propulsion. They swim by jet propulsion, forcibly expelling water through a siphon. During the day they lie buried in the bottom of the ocean; at night they swim and hunt for food.
Except for the squid genus Loligo, cuttlefish have the best cephalopod eyes, which are highly complex. When disturbed, cuttlefish eject a cloud of dark brown ink from an ink sac for protection. The ink gland and ink sac are specializations of the rectal gland. The ink is composed mostly of melanin and has been used as the artist's pigment, sepia. All cuttlefish are dioecious, i.e., the sexes are separate.
The common, worldwide, deepwater cuttlefish, genus Spirula, is considered a "living fossil" because it possesses a remnant of the external shell of the ancient cephalopods. These cuttlefish have a small, coiled internal shell containing a bubble of gas (nitrogen), which serves as a float in the ocean. The European cuttlefish, Sepia officinalis, possesses a degenerate internal shell composed of lime, which is popularly called cuttlebone. Within the narrow spaces between the thin septa of the shell are fluid and gas (mostly nitrogen), which give the organism buoyancy. These cuttlefish are found in the Mediterranean and E Atlantic. The cuttlebone is used for pet birds as a source of lime salts. Sepia are able to undergo a complex of color changes ranging from pink to brown with varying stripes and spots, usually displayed when they are disturbed. The eggs, deposited singly and attached by a stalk to objects on the ocean bottom, are extremely large, up to .6 in. (15 mm) in diameter. The smallest cuttlefish, Idiosepius, inhabits tide pools and attains a length of .6 in. (15 mm). Cuttlefish are classified in the phylum Mollusca, class Cephalopoda, order Sepioidea.
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