They swim by jet propulsion, forcibly expelling water through a siphon. 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 European cuttlefish, Sepia officinalis, possesses a degenerate internal shell composed of lime, which is popularly called cuttlebone. 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.