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 into the plankton.
Nothing it does requires a brain, or even a head – and indeed it has neither. An octopus, on the other hand, is an agile, inquisitive, dextrous, colour-changing, shape-shifting genius.
Most molluscs belong to one of three classes. Bivalves (cockles, clams, mussels, oysters and the like) are characterised by a two-piece hinged shell, and are mostly sedentary filter-feeders. Gastropods (snails, slugs, conches etc) usually possess a single, coiled shell and are mobile grazers or predators.
What are cephalopods?
Then there are the cephalopods, an exclusively marine group comprising octopuses, cuttlefish, 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.
Let’s 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 ‘pen’, 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.
A mollusc’s shell is secreted by an organ called the mantle, visible as fleshy lips around the shell’s opening. In cephalopods, this has been re-engineered as a muscular chamber that provides jet propulsion by pumping water out through a siphon.
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 (‘cephalopod’ derives from the Greek for ‘head foot’) equipped with a pair of sophisticated eyes and sucker-bearing arms or tentacles used to catch and manipulate prey.
Unique to cephalopods is their ability to change skin colour, pattern and texture for camouflage 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.
Octopuses and cuttlefish are widely hailed as the most intelligent invertebrates, capable of solving complex puzzles to access food, using tools, deceiving rivals and even planning for the future by rejecting one treat today on the promise of two tomorrow.
The group also includes the largest living invertebrate, the colossal squid, which can weigh half a tonne. Cephalopods, it seems, can be big and clever.