Cephalopods are an exclusively marine group comprising octopuses, cuttlefish, squids and nautiluses says Stuart Blackman <\/p>
Does any major group of animals include members as strikingly different as molluscs? Think of a cockle and an octopus. The former spends its life within a shell buried in sand, where it filters microscopic food, surfacing only to cast its eggs or sperm<\/a> into the plankton.<\/p> Nothing it does requires a brain, or even a head \u2013 and indeed it has neither. An octopus, on the other hand, is an agile, inquisitive, dextrous, colour-changing, shape-shifting genius.<\/p> Most molluscs belong to one of three classes. Bivalves (cockles, clams, mussels, oysters <\/a>and the like) are characterised by a two-piece hinged shell, and are mostly sedentary filter-feeders. Gastropods (snails, slugs<\/a>, conches etc) usually possess a single, coiled shell and are mobile grazers or predators.<\/p> Then there are the cephalopods, an exclusively marine group comprising octopuses<\/a>, cuttlefish<\/a>, squids and nautiluses, as well as the extinct ammonites and belemnites. Like other molluscs, cephalopods are rubbery, non-segmented animals usually equipped with some sort of shell. But they have taken the basic molluscan body plan and transformed it almost beyond recognition.\u00a0<\/p> Let\u2019s start with the shell. Among living species, only the ammonite-like nautiluses have a conventional external shell. In squid, it has been reduced to a long, flattened internal structure called a \u2018pen\u2019, which stiffens the body. In cuttlefish, it is a larger, lightweight, chalky internal structure involved in maintaining buoyancy. Most octopuses, meanwhile, have no shell at all.\u00a0<\/p> A mollusc\u2019s shell is secreted by an organ called the mantle, visible as fleshy lips around the shell\u2019s opening. In cephalopods, this has been re-engineered as a muscular chamber that provides jet propulsion by pumping water out through a siphon.\u00a0<\/p> Another organ typical of molluscs is the foot. Used by gastropods for gliding and by bivalves for burrowing, in cephalopods it has transformed into a head (\u2018cephalopod\u2019 derives from the Greek for \u2018head foot\u2019) equipped with a pair of sophisticated eyes and sucker-bearing arms or tentacles used to catch and manipulate prey.<\/p> Unique to cephalopods is their ability to change skin colour, pattern and texture for camouflage<\/a> and communication. Colour-change is achieved with pigmented cells that can quickly expand or contract. Mysteriously, cephalopods are able to match a background colour despite being colour-blind.<\/p>What are cephalopods?<\/h2>