Cephalopods

Octopuses, squids, cuttlefish and the chambered nautilus belong to class Cephalopoda, which means 'head foot'. Cephalopods are mollusks and therefore are related to bivalves (scallops, oysters, clams), gastropods (snails and slugs), scaphopoda (tusk shells), and polyplacophorans (chitons). Some of these mollusks, like the bivalves, don't even have a head, much less something large enough to be called a brain! Yet cephalopods have well developed senses and large brains and are thought to be the most intelligent of all invertebrates. Most mollusks are protected by a shell and many of them are not very mobile. Although the ancient nautilus has an external shell, the trend in cephalopods is to internalize and reduce the shell. The shell in cuttlefish, when present, is internal. The cuttlebone from cuttlefish is sold in many pet shops to supply calcium to birds. Squid also have a reduced internal shell called a pen. Octopuses lack a shell all together.

Cephalopods are found in all of the world's oceans, from the warm water of the tropics to the near freezing water at the poles. They are found from the wave swept intertidal region to the dark, cold abyss. All species are marine, and with a few exceptions, they do not tolerate brackish water.

Cephalopods are an ancient group that appeared some time in the late Cambrian several million years before the first primitive fish began swimming in the ocean. Scientists believe that the ancestors of modern cephalopods (Subclass Coleoidea: octopus, squid, and cuttlefish) diverged from the primitive externally shelled Nautiloidea (Nautilus) very early - perhaps in the Ordovician, some 438 million years ago. How long ago was this? To put this into perspective, this is before the first mammals appeared, before vertebrates invaded land and even before there were fish in the ocean and upright plants on land! Thus, nautilus is very different from modern cephalopods in terms of morphology and life history.

Cephalopods were once one of the dominant life forms in the world's oceans. Today there are only 650 or so living species of cephalopods (compare that with 30,000 living species of bony fish). However, in terms of productivity, some scientists believe that cephalopods are still giving fish a run for their money.