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 hidden at 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, or common European, cuttlefish, Sepia officinalis, possesses an 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, which they display when they are signaling or are disturbed; they also can change color and texture to camouflage themselves. 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. Cuttlefish are classified in the phylum Mollusca, class Cephalopoda, order Sepiida.
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