Sepioidea

(invertebrate zoology) An order of the molluscan subclass Coleoidea having a well-developed eye, an internal shell, fins separated posteriorly, and chromatophores in the dermis.

An order of the class Cephalopoda (subclass Coleoidea) including the cuttlefishes (Sepia), the bobtail squids (Sepiola), and the ram's-horn squid (Spirula). The group is characterized by an internal shell that is calcareous and broad with closely packed laminate chambers (the cuttlebones of cuttlefishes). The mouth is surrounded by ten appendages (eight arms and two longer tentacles) that bear suckers with chitinous rings. The tentacles are contractile and retractile into pockets at their bases (see illustration).

cuttlefish and of (b) a cut-tlebone shown in ventral view and (c) in cross section.">
Diagram of some features of (a) a cuttlefish and of (b) a cut-tlebone shown in ventral view and (c) in cross section.

Cuttlefishes are common benthic or epibenthic (living on or just above the bottom, respectively) animals that occur in the warm and temperate waters of the nearshore and continental shelf zones of the Old World, but they are excluded from the Western Hemisphere (North and South America). They prey on shrimps, crabs, small fishes, and other cuttlefishes. The sexes are separate, and during mating, which follows a colorful ritualistic courting behavior, sperm is transferred to the female in cylindrical packets (spermatophores) by a modified arm (the hectocotylus) of the male.

The cuttlebones are used to control the buoyancy of the cuttlefishes as fluid is pumped into and out of the laminar chambers. Dried cuttlebones are a source of calcium for cage birds and are used for fine jewelry molds, dentifrices, and cosmetics. The cuttlefishes eject an attention-getting blob of brownish-black ink when threatened by predators, then change color, become transparent, and jet-swim away, leaving the predator to attack the false body (pseudomorph) of ink. Artists have used the ink, called sepia, for centuries. Cuttlefishes are important in world fisheries; about 200,000 metric tons are caught each year for human food. See also Cephalopoda; Coleoidea.