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The Coleoidea contains two subdivisions. One, the Belemnoidea, had members that were common inhabitants of neritic waters during the Cretaceous but became extinct and the end of this Period.
Figure. Left - Side view of a fossil belemnoid "Acanthoteuthis" from Solenhofen, Germany and in the collection of the U. S. National Museum of Natural History. The equal arms are lined with hooks as is characteristic of this group. Right - Fossil hooks of Belemnoteuthis (?) from Lyme Regis, Dorsetshire, England and in the collection of the U. S. National Museum of Natural History. The shape of the hooks is seen along with the way they may have lined up on the arm. Photographs by R. Young.
The other subdivision, the Neocoleoidea, contains two extant groups: the Octopodiformes (octopods and vampire squid) and the Decapodiformes (squids and cuttlefishes). In addition, a variety of fossil groups exist that may fall into either of these two groups of neocoleoids. The extant coleoids occupy all major habitats in the ocean from intertidal to great depths (deepest record is 7279m, Aldred et al., 1983; Voss, 1988,p. 266) and from south to north polar regions. Although some loliginid squids and some sepiolids can tolerate reduced salinity, no cephalopod occurs in fresh water. In many regions of the oceans, coleoids are dominant members of the ecosystem and are often targets of major fisheries.
Coleoids generally use mantle-generated jet propulsion to obtain high swimming speeds. This is an effective but energetically expensive means of locomotion (Webber and O'Dor, 1985) and, as a result, some groups utilize the fins as the primary locomotor organ. This is especially common in deep-sea species (e.g., cirrate octopods, octopoteuthid squid, magnapinnid squid) where, in dark waters, the value of speed is reduced.
When cephalopods first evolved, the primary feature that separated them from other molluscs was a shell with buoyancy chambers. This allowed cephalopods to invade the pelagic realm where predators and competitors were few. When the shell became internal in coleoids the bouyancy chambers were retained in belemnoids. In most present day neocoleoids, the buoyant shell has been abandoned (it is retained in Spirula and the Sepiidae) and many squids and octopods are negatively buoyant. While this is not a problem for those octopods that are benthic, negatively buoyant squids, however, must swim constantly to stay afloat. Some squids and pelagic octopods, however, have gained near-neutral buoyancy via the retention of light ions (especially ammonium in place of sodium) and the reduction of heavy tissues (Clarke, et al., 1979). This type of neutral buoyancy is found in many deep-sea species since reduction of muscular and cartilagenous tissues can be tolerated in this habitat. In a few cases oil provides buoyancy (Bathyteuthidae) and in one case (Ocythoidae) a true swimbladder has evolved (Packard and Wurtz, 1994).
Coleoid arms are unique grasping structures that possess a variety of different types of suckers and hooks. The arms and suckers are also provided with a variety of sensory organs that can be a primary means of investigating the immediate environment. This is especially true of the octopods where the arms can hunt for unseen prey under rocks. Prey are seized and passed to the mouth then cut into pieces by sharp beaks and swallowed. Because of the circumesophageal organization of the molluscan central nervous system and its elaboration into a complex brain in cephalopods, the cephalopod esophagus passes through the center of the brain; this arrangement limits the maximum size of particles that can be swallowed.
Survival in the sea depends not only on obtaining food but also on avoiding predators. Although preyed upon by many vertebrates (especially marine mammals) and other animals, coleoids excel at camouflage and deception (see Hanlon and Messenger, 1996). They easily exceed all other animals in their ability to change color quickly through the use of chromatophoric organs (chromatophores) and various types of reflectors. The chromatophores contain muscles under nervous control; quick muscle contraction produces rapid color changes that immediately changes the appearance of the animal. This camouflage, combined with their ability to secrete clouds or globs of ink, and the ability of many to alter their skin texture and arm postures, provides them exceptional concealment abilities.
Another, perhaps surprising feature of coleoids, since many species reach a large size, is their short life span. Most species grow rapidly and live for one year or less. Reasons for this are not well understood. However, this is part of a life-history strategy that seems designed for rapid increase in population size. One result of this is that cephalopod fisheries typically fluctuate wildly in annual catch rates.