Cephalopods to Wrap Around Your Head

Illustration of large orange octopus; illustration from different angles of a pink speckled octopus

Left: Illustration of Polypus levis (Benthoctopus levis). Right: Illustration of Eledonella pygmaea (Bolitaena pygmaea). Both images from "The Cephalopoda" by Carl Chun, via Biodiversity Heritage Library.

What do you think of when you hear “octopus”? An animal whose bottomless eyes not only meet your gaze but seem to understand you? A distinct form of animal life, uniquely different from all the others but also just so uncannily familiar? Just maybe, they are the second smartest animal life on the planet after humans? 

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Blueish octopus with arms spread on a rocky, green sea floor
An octopus of the Muusoctopus genus.

Octopuses are all of those things, but to a zoologist, they are members of the class Cephalopoda in the phylum Mollusca. To us, this means they don’t have a backbone, not even a skeleton, properly speaking; the closest relatives of cephalopods might be snails or clams! But they separated from those animals a long time ago (the first fossil we recognize as a cephalopod was alive some 520 million years ago, so odds are, even before then). Cephalopods followed their own unique evolutionary path to become smart, mobile predators in today’s oceans.  

What makes cephalopods mollusks? There are three key features: their anatomy, with three hearts that pump their blue, copper-rich blood through their vessels; their DNA, which links with their shelled relatives; and a thin layer of mucus-y slime that covers their epidermis. Some anatomical features seem really odd to us chordates, but they are just part of being a mollusk. For one, the cephalopod esophagus (the muscular tube that carries food from the mouth to the stomach) goes through the brain. Not exactly ideal if you eat anything that might have sharp things in it—there is a scientific report of a dead octopus having had its brain speared by a spine in its esophagus. Shudder.   

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An octopus with a white body and reddish arms among many small branch-like protrusions on the sea floor.
An octopus of the genus Muusoctopus among tubeworms of the genus Ridgeia near a hydrothermal vent. 

Although in school they tell you that all animals have a skeleton, either inside or outside, cephalopods don’t have a fixed skeleton. Rather, they make their own skeleton as needed, just like your tongue works without a skeleton (it would be really hard to talk if you had bones in your tongue, wouldn’t it?). Your tongue and cephalopod bodies work the same way: their muscles make an ever-changing skeleton that moves in more ways than it could if it had a traditional skeleton.  

Octopuses move around by walking on their eight arms, mostly—some octopuses spend their lives swimming, but most of them walk. Their cousins the squids rule the waves as well as the deeper ocean that generally lacks waves. Squids swim their whole lives, and their entire bodies are streamlined, with a fin at one end and their eight arms and tentacles at the other. Those tentacles can shoot out a surprising distance from the animal, hitting unsuspecting prey like a fish with a powerful force. The poor fish is stunned senseless, so the squid grabs it and draws it into its sucker-covered arms from which there is no escape. Then, the squid preferentially bites into the base of the fish’s skull, paralyzing it so it can’t even put up a fight. Most octopuses use poison to do the same thing (render the poor prey senseless, that is). 

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Three illustrations of a white squid with reddish spots and large green eyes
Illustration of the squid Abraliopsis morisii from "The Cephalopoda" by Carl Chun.

That hardly sounds fair, does it? But cephalopods make their living by killing and eating other animals. When they were little, other animals like fishes, shrimps, and even other octopuses, would do their best to kill and eat them, so turnabout is fair play. As every animal in the ocean loves to eat cephalopods, it is a great reason for them to grow big, and quickly, which they do.  

Janet Voight, associate curator of zoology, is a specialist in cephalopod mollusks, especially octopuses.