You’re sitting on the seabed, just off the coast of the Indonesian island of Lembeh. You’re not deep—20 feet or so—and there’s plenty of light. As you’d expect in such a tropical place, the water is warm. All around, you see ripples of a fine gray-black sand, covered, in places, with a kind of greenish scum. As you explore, you notice a conch shell. Stoutly made, it has six heavy spikes coming off it. Perhaps the maker is within. Or perhaps the maker is long dead, and the shell now belongs to a hermit crab. Curious, you flip it over. A row of suckers. A pair of eyes.
An octopus. In particular, Amphioctopus marginatus, also known as the coconut octopus. Its common name comes from its habit of hiding in discarded coconut shells (sometimes it even picks them up and carries them about, for use as an emergency shelter). But in fact, any big shell will do—such as a conch.
With a few of its suckers, this octopus is holding two halves of a clamshell. As you watch, it drops them and hoists itself up a little. It gives the impression of evaluating the situation. You make like a statue. After a moment, the octopus climbs out of the shell. Its body is the size of your thumb, its arms perhaps three times that. As it moves onto the sand, it turns a matching shade of dark gray. Is it leaving? No. It snakes several of its arms over the sand, and the rest over the shell. With a single heave, it flips the shell back over and flows inside.
Not wanting to disturb it further, you’re about to swim off, when you notice a small movement. The animal has squirted a jet of water, clearing sand from beneath the lip of the shell. There’s now a small gap between shell and seafloor. In the gap, the eyes reappear. You bring your mask close, and for a moment, you look at each other. Of all the invertebrates—animals that lack a backbone—octopuses are the ones that seem the most like us. In part, it’s the way they return your gaze, as if they’re scrutinizing you. (This sets them apart from plenty of vertebrates too: Most fish don’t appear to stare at you.) In part, it’s their dexterity. Their eight arms are lined with hundreds of suckers; this allows them to manipulate objects, whether it’s to open clamshells, dismantle the filtration system of an aquarium tank, or unscrew lids from jars. This distinguishes them from mammals like dolphins, which, for all their smarts, are limited by their anatomy and can’t easily unscrew anything.
At the same time, octopuses are as alien as any extraterrestrial you might dream up. For starters, they have three hearts and blue blood. When feeling under threat, they squirt a cloud of ink and jet off in another direction. They have no bones. The only hard parts of their bodies are a parrot-like beak and a nub of cartilage around their brain. This makes it easy for them to vanish through tiny cracks—an ability that allows them to escape, Houdini-like, from all but the most octopus-proofed aquarium. Not only can all of their suckers be moved independently; each one is covered with taste receptors—imagine your body covered with hundreds of tongues. Their skin is embedded with cells that sense light. Most otherworldly of all—but let’s wait on that. First, let’s meet another octopus.
You're standing in a small, windowless office in the Natural History Museum in London. In front of you, on a desk crowded with files, lies a slab of pale, fine-grained stone. Beside you, Jakob Vinther, a burly Dane with blond hair and a ginger beard, is pointing at it.
“That thing there is the ink sac,” says Vinther, an expert on fossil invertebrates at the University of Bristol, in Great Britain. “That’s actually pigment—chemically preserved melanin.”
You lean forward to look. The stone is clearly marked with the impression of an octopus. It’s not large: In life the animal would have been perhaps 10 inches long. You can trace the mantle—the baglike structure that housed its gills, hearts, and other vital organs. Ah yes. That dark stain in the middle there—that’s the ink sac. The arms hang down, loosely grouped together, each marked with rows of circles. “And those little round structures,” says Vinther, “those are the suckers.”
Octopus fossils are rare; animals with soft bodies generally leave no trace. This fossil is about 90 million years old, which makes it one of the oldest known octopuses. When this animal lived, the extinction of the dinosaurs was still 25 million years in the future. “It comes from a locality in Lebanon where you find all kinds of wonderfully preserved soft-bodied creatures,” Vinther says. Lampreys. Fire worms. All entombed, long ago, in fine, chalky mud on the floor of a long-vanished sea.
Just as humans are mammals, octopuses are cephalopods. The word is Greek for “head-foot” and refers to their weird anatomy, by which their arms are attached directly to one side of their head while their “torso”—the baglike mantle—is on the other. Cephalopods, in turn, are a type of mollusk—a group that includes snails and slugs as well as clams and oysters.
Cephalopods were among the first predatory animals to hunt in the ancient seas. They evolved more than 500 million years ago—long before fish got going—from a small animal with a shell like a witch’s hat. Indeed, if you were to travel back in time 450 million years, some of the fiercest predators in the oceans would have been shell-wearing cephalopods. Some of these were apparently enormous: The shell of the long-extinct Endoceras giganteum may have been more than 18 feet long.
Today there are more than 750 known living species of cephalopods. Besides around 300 species of octopus, these include an array of squid and cuttlefish (both of which have 10 limbs) and a few species of nautilus—peculiar animals that have 90 tentacles and live in shells.
Modern octopuses are a diverse bunch. The giant Pacific octopus, Enteroctopus dofleini, is, as its name suggests, gigantic. Each arm of a large individual can be 6.5 feet long, and the whole animal can weigh more than 200 pounds. Others, such as the star-sucker pygmy octopus, Octopus wolfi, are teensy, weighing just a fraction of an ounce. Some octopuses have a tiny mantle but immensely long arms; others are more evenly proportioned. Most clamber around on corals, mud, or sand, swimming only to get from A to B or to escape from a predator, but a few have taken to cruising the ocean currents. You can find octopuses from the tropics to the poles, from coral reefs to sand flats, from tide pools to the abyss. At least you can if you can spot them.
Back in Lembeh, it’s a sunny morning. You’re swimming over a shallow reef. Your guide—a man named Amba—makes a hand sign that shows he’s seen an octopus. A big octopus. Where? You look around. No octopus. Just rocks, covered with corals and sponges of different colors. Amba gestures insistently: big octopus! You look where he’s pointing. Nope: nothing. Wait. Look again. That patch of dark, velvety coral, that one over there. That’s no coral. That’s a day octopus, Octopus cyanea. Astonishing to have missed it: It’s as big as a dinner plate.
Octopuses and cuttlefish that live in shallow water and hunt during the day are the world champions of disguise. Of course, disguise is not unusual: Many creatures have evolved to look like something they’re not. That orange sponge over there, for example, is not a sponge but a frogfish, lying in wait for an unwary fish. That leaf you see drifting above the sand is not a leaf; it’s a fish that’s evolved to look like a leaf. That leaf—that one there, scuttling over the sand—really is a leaf, but it is also really scuttling. A crab has taken it and stuck it to its shell. That small anemone is a sea slug that has evolved to impersonate an anemone. And everywhere you look, patches of sand get up and walk about (tiny crabs with sand-colored shells) or swim off (sand-colored flatfish).
What makes octopuses and cuttlefish (and to a lesser extent, squid) different is that they can disguise themselves on the fly, now looking like coral, now like a clump of algae, now like a patch of sand. It’s as if they use their skin to make three-dimensional images of objects in their surroundings. How do they do it?
Octopus disguise has three main elements. One is color. Octopuses generate color through a system of pigments and reflectors. The pigments—usually tones such as yellows, browns, and reds—are kept in thousands of tiny sacs in the top layer of the skin. When the sacs are closed, they look like minute speckles. To show the pigment, an octopus contracts the muscles around the sac, thus pulling it open and revealing the color. Depending on which sets of sacs an octopus opens or closes, it can instantly produce patterns such as bands, stripes, or spots. The reflecting cells come in two types. The first type reflects back the light that arrives—thus causing the skin to appear white in white light, red in red light, and so on. The second type is like a living soap bubble that presents different colors when seen from different angles. Together, the reflectors plus the pigment organs allow an octopus to create a huge variety of colors and patterns.
The second element of disguise is skin texture. By contracting special muscles, octopuses can change their skin from smooth to spiky. The effect can be extreme. The algae octopus, Abdopus aculeatus, generates temporary wispy structures that give the impression that the animal is just a piece of seaweed. The hairy octopus, a creature yet to be scientifically described, has evolved a permanently wispy look and is hard to tell apart from a scrap of red algae.
The third part of disguise is posture. The way an octopus holds itself can make it more or less conspicuous. Some octopuses, for example, will ball themselves up like a lump of coral and, using just two of their arms, creep slowly across the seafloor. (No, no, don’t look at me—I’m just a rock …)
How did octopuses get so good at this? The short answer is: evolution. Through tens of millions of years, individuals that were better at disguise were more likely to evade predators and leave offspring. And plenty of animals—including eels, dolphins, mantis shrimps, cormorants, many fish, and even other octopuses—are enthusiastic eaters of octopuses. Because octopuses have no bones, predators can eat the whole animal. As Mark Norman, a world expert on living cephalopods at Museum Victoria, in Melbourne, Australia, puts it, “These animals are pure meat walking around—they’re filet mignons.”
Now let's turn to the matter of the octopus’s nervous system. A typical pond snail has just 10,000 neurons; lobsters have around 100,000; jumping spiders, perhaps 600,000. Honeybees and cockroaches, which after cephalopods have a claim to be the planet’s most neuronally rich invertebrates, have around a million. So the 500 million neurons of the common octopus, Octopus vulgaris, put the animals into a completely different league. In terms of their neuron count, they are better endowed than a mouse (80 million) or rat (200 million) and almost on a par with a cat (around 700 million). Yet while vertebrates keep most of their neurons in their heads, two-thirds of an octopus’s are in its arms. What’s more, nervous systems take a lot of energy to run and can evolve to be large only when the benefits outweigh the costs. So what’s going on?
Peter Godfrey-Smith, a philosopher turned octopus biologist at City University of New York and the University of Sydney, in Australia, suggests that several forces may have helped the octopus develop a complex nervous system. The first is its body. Nervous systems, after all, evolve in tandem with bodies, and the octopus body has evolved to be unusually complex. Being boneless, an octopus can extend any arm in any direction at any point; unlike you or me, it’s not limited to moving at shoulder, elbow, or wrist. This gives the octopus an enormous range of possible movements; also, each arm can be doing something different. An octopus on the hunt can thus be an impressive sight. It can have every arm stretched out over the sand, each one exploring, rummaging, probing into holes. If one arm startles a shrimp, two more can reach out to catch it. Octopuses also have all those suckers that can be moved independently, not to mention the structures and mechanisms for controlling skin color and texture. At the same time, the animal has evolved the capacity to receive and process a huge amount of incoming sensory information: taste and touch from the suckers, gravity sensed by structures called statocysts, as well as all the information that its sophisticated eyes collect.
On top of this, many octopuses live in spatially complicated environments—they must navigate on, around, and through reefs. Having no body armor, they need to keep a sharp lookout for predators, and in case camouflage is not enough, they need to know where to hide. Finally, octopuses are fast, agile hunters that catch and eat a wide variety of animals, from oysters to crabs to fish. Boneless bodies, complex environments, diverse diets, avoiding predators—all factors, Godfrey-Smith suggests, that can drive the evolution of intelligence.
Yet while octopuses clearly have complex nervous systems, are they, in fact, smart? Evaluating the intelligence of other animals is tricky at the best of times, and sometimes tells us more about ourselves than about the animal in question. Markers of intelligence in birds and mammals, such as the ability to use tools, often don’t make much sense for an octopus: Its whole body is a tool. It doesn’t need a tool to reach into crevices—it can just reach in—or to pull oysters apart.
That being said, experiments starting in the 1950s and 1960s showed that common octopuses are good at tasks involving learning and memory—two attributes that we associate with intelligence. Indeed, a particular part of the octopus brain, the vertical lobe, is dedicated to such tasks. I’m stressing the common octopus here because it has been studied the most, by far. Octopus species do differ somewhat in the organization of their brains, and as only a few have been studied, no one knows whether all of them are equally gifted. Roy Caldwell, an octopus researcher at the University of California, Berkeley, says, “Some that I’ve had in the lab seem to be as dull as toast.” Name names? “Octopus bocki, a tiny little octopus.” What makes it dull? “It just doesn’t seem to do much.”
But perhaps whether they are smart or dull—whether they are pondering philosophy or lunch, or not thinking anything at all—is less important than the fact that they are just all around astonishing. Enchanting.
Let’s go on one final dive. It’s dusk in Lembeh. You’re kneeling by a rocky slope. In front of you, swimming cheek to cheek, a pair of small fish are spawning. An eel is curled in a hole. A large hermit crab, in its borrowed shell, comes clunking past. And there, sitting on a rock, is a small algae octopus.
As you watch, it starts to move. One moment it seems to float, levitating like an eight-armed yogi. Another moment it appears to glide. Now it starts to crawl over the rocks—but whether it pulls itself with the arms in front or pushes with the arms behind, you cannot say. As it moves down the slope, one arm finds a tiny hole, and, one arm after another, the animal streams into it. Gone. No—not quite. The tip of an arm reaches out of the hole, feels around, grabs some small stones, and pulls them over the entrance. There. All secure for the night.