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Cephalopods, the class of mollusks which scientists classify octopuses, squid, cuttlefish and nautiluses, can change color faster than a chameleon. They can also change texture and body shape, and, and if those camouflage techniques don't work, they can still "disappear" in a cloud of ink, which they use as a smoke-screen or decoy. Cephalopods are also fascinating because they have three hearts that pump blue blood, they're jet powered, and they're found in all oceans of the world, from the tropics to the poles, the intertidal to the abyss. Cephalopods have inspired legends and stories throughout history and are thought to be the most intelligent of invertebrates. Some can squeeze through the tiniest of cracks. They have eyes and other senses that rival those of humans.
The class Cephalopoda, which means "head foot", are mollusks and therefore related to bivalves (scallops, oysters, clams), gastropods (snails and slugs), scaphopoda (tusk shells), and polyplacophorans (chitons). Some mollusks, such as 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. Most mollusks are protected by a hard external shell and many of them are not very mobile. Although nautilus has an external shell, the trend in cephalopods is to internalize and reduce the shell. The shell in cuttlefish, is internal and is called the cuttlebone, which 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 altogether.
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 in the late Cambrian period 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!
Cephalopods were once one of the dominant life forms in the world's ocean. Today there are only about 800 living species of cephalopods (compare that with 30,000 living species of bony fish, see FishBase). However, in terms of productivity, some scientists believe that cephalopods are still giving fish a run for their money.
Many species of cephalopods to grow very fast, reproduce over a short period of time, and then die. Scientists classify this weed-like life history as "r-selection" - the r refers to exponential growth. If you were to clear cut an oak forest, the first plants to grow would not be more oak trees - it would be weeds. In life history terms, cephalopods are the weeds of the seas. With over-fishing and climate change, there may be more biomass of cephalopods now than anytime in recent history.
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The Cephalopod Page (TCP), © Copyright 1995-2018, was created and is maintained by Dr. James B. Wood, Associate Director of the Waikiki Aquarium which is part of the University of Hawaii. Please see the FAQs page for cephalopod questions, Marine Invertebrates of Bermuda for information on other invertebrates, and MarineBio.org and the Census of Marine Life for general information on marine biology.