Let’s begin with a 19th-century mystery. In 1873, merely a year after he had taken the position as naturalist at the Brighton Aquarium in the eponymous town on the southern shore of the United Kingdom, Henry Lee faced a problem. In May 1873, young lumpsuckers (fish shaped as delightfully as they are named, and the quiet force in Ned Bauman’s novel Venomous Lumpsucker) were disappearing from their tank.

On an almost daily basis, Lee’s account tells us, “there was a fresh and inexplicable vacancy in the gradually diminishing family circle.” The fish could not have died, because there were no corpses to be found. They could not have burrowed into the shingle at the bottom of their tank because it was not deep enough. And they could not have leaped out of their abode, because, true to their name, they’re pretty plump.

One morning, though, the culprit was caught red-handed (or red-limbed). Along the lumpsuckers, an octopus lurked. This “marauding rascal” had evidently waited until dark to crawl out of its tank and go find a snack in a nearby one. This is far from the only octopus escape in Brighton Aquarium, as noted by Lee. One had ventured into the crab tank, sadly bereft of snacks, for these crabs were too large. Another had crawled into the lobster tank to pick a fight with the largest lobster in there. And yet another had slithered out of a display vase in Lee’s own office, almost making it out of the door.

Body Artists

The lack of a skeleton certainly helps octopuses with their Houdini-like escape acts. Together with squid, cuttlefish, and nautiluses, our eight-armed (technically, octopuses have arms/limbs and not tentacles) snackers are invertebrates. Instead of bones and vertebrae, they control their movements and body shape with a hydrostatic skeleton, which is a set of muscles that controls the fluid distribution inside the body. This offers a lot of flexibility. An octopus the size of a basketball has no problem fitting itself into an empty peanut butter jar. As long as their only hard part, their beak, fits, then so does the rest of them.

Their body control is remarkably precise as well. Thanks to muscular skin cells called papillae, their skin can go from spiky sandpaper to baby bottom smooth in the blink of an eye. And that’s not all their skin can do.

Several layers of specialized cells contribute to the body art of octopuses and other cephalopods. The top layer of their skin contains thousands upon thousands of chromatophores–cells with an elastic “bag” full of yellow, orange, red, brown, or black pigment. An interplay of muscles and nerves tunes these pigment bags to be spread out so the pigment is visible. When the muscles relax, the elastic pigment bag contracts into a tiny, nearly invisible dot. Under these cells, we find another layer of coloring cells: iridophores, or cells that contain crystals that reflect incoming light in bright greens, blues, silvers, and golds. One more layer down, we find the leucophores, also crystal-carrying cells, but with the crystals so arranged that they form a mirror reflecting the environment (and, for the science fiction writers reading this, these crystals have been successfully introduced into human cells). And then there are a few rare octopuses that live in deeper waters, which add photophores that produce light via bioluminescence to the mix.

Even in their sleep, octopus skin does its marvelous thing. Not unlike our own slumber, octopuses go through a sleep cycle with different states. Quiet sleep–pale skin, unmoving eyes–alternates with active sleep—a firework display of color and texture changes in the skin, accompanied by rapid eye movements. Are the chromatophores simply performing a checkup or are the small Krakens subjectively experiencing an imaginary voyage? Can we read their dreams on their skin?

Among such colorful lives hides a cephalopod secret: they are color-blind. Kind of. Maybe.

Cephalopods have monochromatic vision. They have only one type of light receptor in their eyes and so they (possibly, as we’ll see) perceive the world in grayscale. Perhaps this means that their coloring is just to warn off predators. Perhaps their colors are a by-product of a camouflage system so sophisticated it doesn’t need to “think” about color at all. By matching contrast, brightness, and texture with extraordinary precision, they may accidentally fool color-sighted predators. Or perhaps they’re communicating on another channel. Their skin reflects polarized light, invisible to most predators, a secret visual language readable only by other cephalopods.

And perhaps, we shouldn’t underestimate the octopus’s ability to find their own path. Some researchers suggest that their strangely shaped, off-axis pupils might let them extract color information through chromatic aberration–different wavelengths of light focusing at slightly different depths, which can turn a single receptor into more than monochrome. Maybe they see color, but in an (to us) entirely alien way.

Donut Brains and Clever Arms

Via their large optic lobes, which comprise up to two-thirds of their central brain, octopus vision leads us deeper into their brains. Brains that are very unlike ours. Those two big optical lobes connect to a central brain that is shaped like a donut because the esophagus passes through it. Yes, you read that right. Octopuses have a donut-shaped brain.

From the donut, eight thick nerve cords extend into the octopus’s limbs. Combined, the animal’s limbs house more neurons than the brain. Instead of being analogous to the branching nerves that run through our vertebrate limbs, researchers argue that the arm nerve cords of octopuses belong to the central nervous system (like our spinal cord) rather than the peripheral nervous system (like the nerves in our arms and legs).

Because you can never have enough donuts, the nerve cords in octopian arms meet in a nerve ring under the central brain. Through this ring, each arm is connected to the third arm over, which gives the eight limbs remarkably fine-grained control and coordination, independent of the central brain. That central brain can still take control when the situation requires it to, and it learns from the arms’ input, but the arms often do their own thing. It’s hard to imagine what that would be like, but in Children of Ruin, Adrian Tchaikovsky skillfully introduces a species of uplifted octopuses whose lives are indeed a back-and-forth between arm brains and the central brain.

Not only can the arms perceive light, but they are lined with suckers that allow octopuses to touch and taste their environment via chemotactile receptors. Each sucker is a muscular, cup-shaped organ with two main parts: the infundibulum, or the outer rim that makes contact with a surface, which is lined with a tough, ridged cuticle that helps grip even rough or irregular textures. And then there’s the acetabulum, or the inner chamber that creates a sealed cavity. When the sucker presses against a surface, muscles in the inner chamber expand the inner cavity, which creates a partial vacuum, just like a rubber plunger.

Boneless arms and hundreds of suckers provide our eight-limbed friends with an uncanny dexterity, which they sometimes apply to carry coconut shells as a makeshift shelter. One species even co-opts the tentacles of the fearsome Portuguese man o’ war as a weapon.

So far, no cephalopod has unambiguously passed the traditional mirror test for self-awareness. Yet, the skin of the octopus contains a “self-recognition” chemical that prevents the arms from getting stuck on each other. Like the octopus, self-awareness can come in different shapes. So can intelligence.

Alien Intelligence

Sometime between 550 and 700 million years ago, predating the famous Cambrian explosion, lived a wormy thing with simple eyespots that was the ancestor of both humans and cephalopods. Since then, our eight-armed fellow earthlings and we have gone our diverging paths.

And yet, one of evolution’s many wonders is that it can arrive at similar traits through different routes. This is known as convergent evolution and, as we’ve seen, eyes are a great example. The octopus and the human eye have both evolved independently from simple eye spots on our great-great-great-etc.-grandworm. Beyond some differences (pupil shape and the lack of retinal blind spot in cephalopods), there are also several structural similarities–cornea, pupil, lens, fluid, retina . . . Different details, same blueprint.

Can we say the same for intelligence? Of course, the term “intelligent” can mean many things. Let’s refine it here to “possessing a relatively large brain-to-body mass ratio and displaying complex and adaptable behavioral complexity that includes responsiveness to environmental changes and the ability to acquire and use new information.” I’m sure not everyone agrees with that proposal and there are valid reasons to question the brain-to-body mass part. Still, if we stick to it for now, cephalopods certainly seem, as Shelby Van Pelt’s novel Remarkably Bright Creatures (now also a Netflix movie) suggests, pretty clever. Their apparent intelligence, coupled with the large evolutionary distance between them and other animals we tend to consider intelligent, has made cephalopod minds one of the truly other other minds, as argued in Peter Godfrey-Smith’s aptly titled Other Minds.

If we accept octopuses into the group of remarkably clever creatures, however, we must face some explanatory challenges.

Mischievous as ever, these tricky invertebrates challenge some of our cherished hypotheses on the evolution of intelligence.

Intelligence likely evolves for a combination of reasons, but octopus life history appears to lack characteristics we usually notice in other animals that we place highly on the intelligence scale. For example, some proposed explanations for why certain animals have evolved big, clever brains rest on any combination of relatively long lives, complex social networks, and an extended period of parental support.

As far as we know, octopuses and their cephalopod cousins don’t tick those boxes. Most octopuses are lucky if they get a handful of years. The nautiluses probably take the Methuselah award with a lifespan of around twenty years. Parental support in octopuses focuses on oxygenating the eggs. Females stop eating and protect their eggs until they hatch. But, soon after hatching, the mother passes away from the starvation she puts herself through. Great sacrifice notwithstanding, does this count as extended parental support? And finally, octopuses are quite solitary creatures.

In other words, they grow smart alone and die young.

As with their eyes, octopuses may have found another evolutionary path to intelligence, and that may have a lot to do with getting squishy. A recent hypothesis suggests that it is the loss of their external shells that may have driven their smarts. When your protective shell drops away, you suddenly have to worry a lot more about predators and you, as an invertebrate, become a lot more flexible. To deal with the predators, octopuses may have evolved a fast life. A single reproduction event with lots of progeny is a bet-hedging strategy when faced with a lot of predators. Meanwhile, their bodily flexibility opened up several new ways to physically explore complex environments. Octopuses also tend to engage in extractive feeding in which they have to pry open shells, for example. That requires dexterity and spatial reasoning. Add to that a body that has evolved to contain a multi-sensory communication mechanism, and perhaps you end up with an alternative recipe for intelligence.

How To Care For Your Octopus

Octopuses, I wrote earlier, are loners, living lives of dodging being eaten (including by other octopuses), finding food and shelter, and mating. Mostly, this is true. A little over a decade ago, though, scientists stumbled upon Octopolis in the waters of Australia’s Jervis Bay, where about a dozen octopuses occupied the same site. A few years later, and a few hundred meters from Octopolis, the researchers found Octlantis, a larger congregation that includes twenty-three dens and fifteen octopuses.

At these sites, where the cephalopods occur in higher densities than we would expect, they also display new behaviors. They form dominance hierarchies, evict one another from dens, throw debris at each other, engage in boxing matches, and puff themselves up in intimidating dominance displays (including the ominous Nosferatu pose). I leave the parallels with human big city life for the reader.

While not quite the highly intelligent octopus species in Ray Nayler’s The Mountain in the Sea (as far as we know anyway), this previously hidden world of octopus behavior and, in a sense, culture, raises questions about their welfare. Here we have clever animals that continue to surprise us, that have evolved intelligence and (perhaps) color vision in ways we hadn’t imagined before, and that experience pain.

Yet, in considerations of animal welfare, invertebrates, including our clever cephalopods, have long tended to be little more than an afterthought. Fortunately, that has been changing, and, in some countries, cephalopods in research have been granted (a smidge of) legal protection. More generally, there is progress as well. For example, the Animal Welfare (Sentience) bill in the UK was amended in 2021 to expand its scope, based on scientific evidence that cephalopods and decapods (crabs, lobsters) are sentient creatures that deserve greater welfare protection than they previously received.

Despite the title of this essay, octopuses are not aliens. They may be alien to us, but they are earthlings. Just like us, they are the descendants of a wormy thing that lived over half a billion years ago. In a way, we’ve lost sight of them since then. And not only because they are masters of disguise.