Octopus

• 1. Introduction

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  The octopus is a soft-bodied mollusc with eight limbs. In 2010, a specimen named Paul became briefly famous for correctly picking every winner of that year's FIFA World Cup. Paul had no bones and no vocal cords, and by most measures no business predicting anything. Yet people everywhere tuned in to watch him choose. That contradiction runs through the whole animal, a boneless creature that often behaves like something with a plan. Octopuses form the order Octopoda, roughly 300 species, sitting inside the class Cephalopoda alongside squid, cuttlefish and nautiloids. Scientists consider them among the most behaviorally complex animals without a backbone. How does a body with no internal skeleton support senses and reflexes this refined? Why does nearly every octopus die within a few years of birth, many within months of mating? And how did a soft-bodied sea creature end up in ancient myths, erotic art, literary fiction, dinner tables, and even the designs of modern robots? The chapters ahead trace the octopus outward, from its own strange biology, into the ocean it has spread across, and finally into the human stories built around it.

• 2. What's in a Name

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  The scientific name descends from the Ancient Greek oktōpous, a compound of oktō, meaning eight, and pous, meaning foot. That root word was already in use by the physician Alexander of Tralles, who lived from around 525 to 605. The standard English plural today is octopuses, though the Ancient Greek plural, octopodes, has also seen historical use. A third option, octopi, is usually judged incorrect. In both Greek and Latin, the word is a third-declension noun, not the second-declension type that octopi assumes. Octopi was actually the first plural to catch on in English, appearing in the early 19th century. Octopuses overtook it in the latter half of that same century. The Hellenic octopodes arose around the same period but has always remained the rarest of the three. Fowler's A Dictionary of Modern English Usage argues that octopuses is the only acceptable form. It calls octopi misconceived and dismisses octopodes as merely pedantic. Even so, octopodes appears often enough that both the Merriam-Webster 11th Collegiate Dictionary and Webster's New World College Dictionary acknowledge it. The Oxford English Dictionary lists all three forms in order of frequency: octopuses, octopi, and octopodes. It calls octopodes rare and notes that octopi rests on a misunderstanding. The New Oxford American Dictionary's third edition, published in 2010, goes further still. It names octopuses the only acceptable plural and calls octopi incorrect outright. Whichever plural a speaker settles on, the eight foot-derived limbs behind the word do something far stranger than walking.

• 3. A Body With No Bones

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  The giant Pacific octopus, Enteroctopus dofleini, is often cited as the largest member of the group. Adults typically weigh 10 to 50 kg, with an arm span reaching 4.8 m. The largest individual ever scientifically documented reached a live mass of 71 kg. Larger sizes have been claimed, including one specimen reportedly weighing 600 lb with a 32 ft arm span. A carcass of the seven-armed Haliphron atlanticus weighed 61 kg and was estimated to have had a live mass of 75 kg. The smallest known species, Octopus wolfi, measures only around 2.5 cm and weighs less than 1 g.

  A sharp, chitinous beak sits at the center of the mouth, ringed by what was once a simple foot. That foot evolved into eight flexible, prehensile arms, joined near their base by a web. Zoologists label the arms by side and position, such as L1, R1, L2 and R2, grouping them into four pairs. The rear pair mostly walks along the seafloor while the other six forage for food. Behind the head sits the bulbous mantle, holding most of the internal organs and connecting outward through a muscular funnel called the siphon. Soft tissue makes up most of the body, letting even large species squeeze through a gap barely 2.5 cm across.

  Each arm's underside is lined with circular, adhesive suckers built from two parts. An outer disc, the infundibulum, and an inner cup, the acetabulum, are both wrapped in thick muscle under a chitinous cuticle. Muscle contractions seal and release the sucker's grip, letting the octopus anchor itself or manipulate objects. Every one of the eight arms can sense and react to light on its own. That means the octopus can still control its limbs even when its head cannot see them.

  Two cartilage capsules in the skull each hold one large, fish-like eye. A slit-shaped pupil sits just behind a translucent cornea, and it can expand or contract as a retinal pigment screens out bright light. The skin itself is a thin epidermis over a collagen-rich dermis, and that dermis holds the cells responsible for changing colour. A more primitive group, the Cirrina, breaks from this template entirely, with gelatinous bodies, two fins above the eyes, an internal shell, and webbed arms lined with fleshy cirri.

  Blood moves through a closed system driven by three separate hearts. One main systemic heart handles general circulation, while two smaller branchial hearts push blood through the gills. The systemic heart shuts down whenever the octopus swims, which drains its energy quickly. That is largely why octopuses spend most of their time crawling instead of swimming. Instead of haemoglobin, octopus blood carries the copper-based protein haemocyanin, dissolved directly in the plasma, which tints it blue. Pumping that thicker blood can require pressure exceeding 75 mmHg, though in cold, low-oxygen water haemocyanin actually outperforms haemoglobin at carrying oxygen.

  Breathing draws water into the mantle cavity, across the gills, and back out through the siphon. That system pulls up to 65% of the oxygen from water at 20 degrees Celsius. The skin absorbs oxygen too, supplying around 41% of the total while resting. That figure drops to 33% while swimming and as low as 3% after a large meal.

  Digestion begins at a beaked buccal mass and a chitin radula that shreds food before it reaches the crop, the stomach, and a caecum that filters particles from liquid and absorbs fat. The digestive gland breaks nutrients down into structures nicknamed brown bodies. Waste gets compacted into what the scientific literature calls faecal ropes and is expelled through the funnel. Paired organs called nephridia, equivalent to kidneys, filter mostly ammonia out of the blood and route the resulting urine into the mantle cavity.

  Tucked beneath the digestive gland, the ink sac stores a fluid made largely of melanin. When threatened, the octopus mixes it with mucus and blasts it out through the siphon as a thick, dark cloud, though the cirrate group generally lacks this organ. The order's scientific name, Octopoda, was assigned in 1818 by the English biologist William Elford Leach, who had classified the group under a different name, Octopoida, the year before. Because octopuses are built almost entirely from soft tissue, they leave few fossils behind, which is part of why so much about their inner workings still surprises researchers.

• 4. A Mind Spread Wide

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  Octopuses carry more than 500 million neurons, a nervous system roughly on par with a dog's. Only about a third of those neurons sit inside the brain itself. Two-thirds run instead through the nerve cords of the arms, letting each arm make its own decisions when it receives the right information. A severed arm can keep moving and responding to touch on its own, since it was never simply taking orders from the head. Unlike most other animals, octopuses do not appear to organize movement in the brain using an internal map of their own bodies.

  In A. aegina, colour vision is present. In O. vulgaris, it is absent, a split that shows how differently species can perceive the world through the same camera-like eye design. Light-sensitive cells called opsins sit in the skin itself, helping the animal pick colours that match its surroundings, working independently of the eyes. One theory holds that oddly shaped pupils, described as U-shaped, W-shaped or dumbbell-shaped, let a single-photoreceptor eye split light by wavelength. That would effectively fake colour vision, at some cost to image sharpness. Two organs called statocysts sense the body's orientation relative to gravity and time. They keep the pupils oriented horizontally at all times and may double as hearing organs; the common octopus can detect sound between 400 Hz and 1000 Hz, hearing best around 600 Hz. Chemoreceptors built into the suckers let an octopus taste anything it touches. Separate sensors recognize the octopus's own skin, stopping its arms from accidentally gripping each other. Despite all this, octopuses seem to have poor proprioception and must watch their arms just to know where they are.

  In laboratory mazes and problem-solving tasks, octopuses show evidence of both short- and long-term memory. They can be trained to tell shapes and patterns apart. The veined octopus collects discarded coconut shell halves and carries them off to assemble into a shelter later, a behaviour researchers count as genuine tool use. Some octopuses have been seen jetting water at a floating bottle again and again, in what researchers describe as play. They are also notorious for breaking out of aquarium tanks, sometimes climbing into a neighbouring tank in search of food. Reports of observational learning exist too, though researchers still dispute whether the evidence holds up. That same skin, wired with its own light sensors, does more than taste and feel. It is also the octopus's primary tool for disappearing entirely.

• 5. Camouflage, Ink, and Escape

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  Octopus skin holds specialized cells called chromatophores, sacs of yellow, orange, red, brown or black pigment. Most species deploy three of these colours at once, though some use only two and others four. Additional cells, reflective iridophores and white leucophores, add shine and pale tones to the mix. Running the whole chromatophore system at full activation costs nearly as much energy as the animal spends at rest. Muscles in the skin also change its texture, letting the mantle mimic the bumpy look of algae-covered rock, a trick most developed in shallow-water, day-active species and largely absent in nocturnal or deep-sea ones. One display, called the passing cloud, sends moving waves of dark colouration across the body to confuse onlookers. A second, the moving rock trick, has the animal creep across open ground at the same speed as the surrounding current, disguised as a drifting stone.

  In one study of Enteroctopus dofleini, 66% of animals examined bore scars, and half were missing at least one arm. Predators include fish, seabirds, sea otters, seals and their relatives, whales and dolphins, and other cephalopods. An octopus may hide in its den for as much as 40% of the day. Some species back up camouflage with the opposite strategy: warning colouration or a deimatic bluff meant to look as large and dangerous as possible. The Atlantic white-spotted octopus, Callistoctopus macropus, turns redder and grows bright white spots when performing this display. The blue-ringed octopus keeps its warning colours hidden inside muscular skin folds until threatened, when the folds contract and iridescent blue rings flash into view. Every octopus carries venom, but only the blue-ringed octopuses are considered lethal to humans. If a predator grabs hold, some octopuses can shed the trapped arm entirely and regrow it later. The mimic octopus goes further still, combining its flexible body with colour change to imitate more dangerous animals outright, including lionfish, sea snakes and eels.

  Roughly 150 recognised species of parasites, including cestodes, nematodes and copepods, are known to trouble octopuses. Tiny worms called Dicyemidae live in the renal appendages of many species, though scientists still debate whether the relationship is parasitic or simply symbiotic. A coccidian parasite in the genus Aggregata can cause serious gut illness. A bacterium, Vibrio lentus, is known to cause skin lesions, exposed muscle and occasionally death. In response, an octopus's blood cells, haemocytes, hunt down and destroy invaders and also help heal wounds. These defenses travel with the octopus into every environment it occupies, from shallow tide pools down to deep hydrothermal vents on the seafloor.

• 6. Life in the Open Ocean

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  The Hawaiian day octopus, Octopus cyanea, makes its home on coral reefs, one of many habitats octopuses occupy across every ocean on Earth except fresh water. As juveniles, common octopuses shelter in shallow tide pools, while the argonaut drifts through open, pelagic water. Abdopus aculeatus keeps close to shore in seagrass beds. Some species push into far harsher terrain: the spoon-armed octopus, Bathypolypus arcticus, survives 1000 m down, and Vulcanoctopus hydrothermalis lives around hydrothermal vents 2000 m deep. Megaleledone setebos and Pareledone charcoti manage the frigid waters of the Antarctic. Only a single confirmed record places any octopus in the hadal zone, a species of Grimpoteuthis, the dumbo octopus, photographed at a depth of 6957 m.

  The Larger Pacific striped octopus breaks the group's general pattern of solitude, living instead in colonies of up to 40 individuals. Most other octopuses live alone, meeting mainly to compete over dominance or mates, likely because plentiful food collides with a shortage of good den sites. Octopuses den in crevices among rocks or man-made debris, and smaller species will squat in an abandoned shell or a discarded bottle. They can find their way home without retracing the path they took leaving, and they do not migrate. Leftover shells and bones pile up outside a den as a midden, which draws in scavenging fish, molluscs and echinoderms. On rare occasions, octopuses hunt cooperatively alongside fish, apparently managing their partners' behaviour by punching them.

  The giant Pacific octopus favours bivalves such as the cockle Clinocardium nuttallii, along with clams, scallops and crabs. Its prey more broadly ranges across crustaceans, bivalves, gastropods, fish and other cephalopods, including members of its own species. It passes over moon snails as too large and skips limpets, rock scallops, chitons and abalone because they cling too tightly to the rock. Small cirrate octopuses, in genera such as Grimpoteuthis and Opisthoteuthis, feed instead on polychaete worms, copepods, amphipods and isopods. To get into a shell, an octopus was once thought to drill with its radula. The actual tools are tiny teeth on the tip of the salivary papilla, paired with an enzyme in the toxic saliva that dissolves the shell's calcium carbonate. The process can take hours, though the prey dies almost the instant the shell gives way. Tough-shelled crabs get drilled while soft-shelled ones are simply torn apart. Grimpoteuthis instead swallows prey whole through a small or absent radula, and the deep-sea genus Stauroteuthis has converted its suckers into light-producing photophores that appear to lure prey straight toward its mouth.

  In 2005, researchers found that Abdopus aculeatus and the veined octopus, Amphioctopus marginatus, can walk on just two arms while disguising the rest of their body as drifting plant matter. That trick lets them flee a predator unrecognized. Crawling is otherwise an octopus's slowest gait, and jet propulsion its fastest, powered by water forced out through the siphon. The veined octopus also performs a stilt walking gait, stacking coconut shells and carrying them underneath its body on two arms while its remaining, stiffened arms support an awkward, upright shuffle. Some species crawl briefly out of the water altogether, moving between tide pools. Some swim by flattening their bodies and splaying their arms, a posture that may generate lift and outpace ordinary swimming. Jet propulsion, while fast, is costly. The mantle pressure needed to fire the jet is high enough to momentarily stop the heart, leaving the animal with an oxygen deficit. Cirrate octopuses cannot jet at all. They swim using their fins, contract their arms and web for sudden take-off bursts, or inch forward slowly with a rippling motion called pumping. Every one of these strategies, denning, hunting, walking on two arms, serves an animal that, in almost every species, has only a few years left to use them.

• 7. Short, Furious Lives

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  A specialised arm called the hectocotylus, usually the R3 arm in the genus Octopus, carries no suckers at its spoon-shaped tip. Its one purpose is loading spermatophores, or sperm packets, into a female's mantle cavity. Each octopus carries a single gonad, a testis in males or an ovary in females, connected to the mantle cavity through a duct. An optic gland releases hormones that trigger sexual maturity, ageing and gamete production, with environmental cues like temperature, light and nutrition setting its timing.

  In the giant Pacific octopus, courtship involves the male changing his skin's texture and colour before clinging to or positioning himself beside the female. He may first use the hectocotylus to clear out any spermatophore already left by a rival. He then draws a fresh one from his own spermatophoric sac and guides it into the opening of the female's oviduct. Two spermatophores pass over this way, each roughly a metre long, with the empty ends sometimes left protruding from the female, and a hydraulic mechanism inside eventually releases the sperm. More than a month after mating, a female giant Pacific octopus lays her eggs. The species can produce up to 180,000 in a single clutch, compared with as many as 45,000 for O. rubescens and 500,000 for O. vulgaris. She guards and tends the eggs for around five months, 160 days, before they hatch, a process that can stretch to ten months in colder water such as off Alaska. The argonaut takes a different path: the much larger female secretes a thin, cornucopia-shaped shell, lays her eggs inside it, and broods them there while continuing to swim.

  Most hatchlings emerge as tiny, planktonic paralarvae, drifting for weeks or months while feeding on shrimp, isopods and amphipods before settling to the seafloor to mature. Species that lay larger eggs skip that drifting phase entirely, hatching straight into a bottom-dwelling form resembling a miniature adult. Among them are the southern blue-ringed, Caribbean reef, California two-spot and Eledone moschata octopuses.

  Even the longest-lived octopuses survive only up to about four years. Some species complete their entire life cycle in under six months. Once maturity hits, males begin to senesce almost immediately, while females hold off until after they lay a clutch of eggs. Senescence itself can drag on for weeks or stretch to a few months. During this time the octopus stops eating, weakens, grows sluggish, and develops lesions as its cells break down faster than they can be replaced. Many die of starvation or fall prey to a predator before the process finishes. The trigger sits in the optic glands. When researchers experimentally removed them from octopuses after spawning, the animals lived and stayed active for noticeably longer. That same optic gland, quietly running out a biological clock in a matter of months, sits behind an animal that people have been carving into pottery and coinage for well over three thousand years.

• 8. Sea Monsters, Suppers, and Robots

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  Octopus imagery shows up on Minoan coins as far back as around 1650 BCE and on Mycenaean Greek pottery between roughly 1200 and 1100 BCE, evidence that seafaring cultures noticed the animal early. A Hawaiian creation myth casts the octopus as the sole survivor of a previous age of the world. Norway's kraken and the Ainu's Akkorokamui, a monster worshipped within Shinto tradition, both draw on octopus-like imagery, and some scholars connect the snake-haired Gorgon Medusa of Greek myth to the same creature. In the Japanese Asuka-era legend Taishokan, a female diver fights an octopus to reclaim a stolen jewel, a story that later inspired woodblock prints. In Victor Hugo's 1866 novel Travailleurs de la mer, known in English as Toilers of the Sea, a battle with an octopus plays a central role. The animal turns up again as a film villain in 1948's Wake of the Red Witch, and in Thomas Pynchon's 1973 novel Gravity's Rainbow, as an octopus named Grigori that attacks a woman on a beach. Political cartoonists have long drawn octopuses to represent sprawling empires and corporations, their arms standing in for far-reaching influence.

  Japanese shunga erotic art frequently features octopuses, most famously in Katsushika Hokusai's 1814 woodblock print Tako to ama, The Dream of the Fisherman's Wife, which shows a woman sexually entwined with a large and a small octopus. That image later inspired Pablo Picasso's 1903 drawing An Erotic Drawing: Woman and Octopus.

  One diver reported a nearly perfectly camouflaged 2.4 m Pacific octopus that lunged at him and wrestled over his camera before finally releasing it, an encounter another diver caught on video. Octopuses generally steer clear of people, though verified conflicts like this one do happen. Every octopus carries venom, but only the blue-ringed octopuses are lethal to humans. Their bites, usually small and painless, are delivered only when the animal is provoked or accidentally touched, and are reported every year across a range stretching from Australia to the eastern Indo-Pacific. Their venom contains tetrodotoxin, which can apparently seep through skin given enough contact time, blocking nerve signals to the muscles. Death, when it happens, comes through respiratory failure and the resulting oxygen loss to the brain. There is no antidote, but a patient kept breathing artificially typically recovers within 24 hours. Bites from other, non-lethal captive octopus species have also been recorded, leaving only temporary swelling behind.

  Fishing fleets around the world caught between 245,320 and 322,999 metric tons of octopus a year from 1988 to 1995. That global catch peaked at 380,000 tons in 2007 before falling by about a tenth by 2012. Fishers take octopus by pots, trapping, trawling, snaring, drift fishing, spearing, hooking and simply catching them by hand, and many more turn up as bycatch, while commercial octopus farming remains controversial. Octopus meat is prepared differently across the Mediterranean and Asian coasts where it is widely eaten. In Korean san-nakji it is served live or still wriggling. If a diner does not prepare the dish with care, the severed arms' suckers can choke a person, and at least one such death was recorded in 2010, prompting animal welfare groups to object to eating octopus alive on the grounds that it can feel pain.

  The Greek philosopher Aristotle, who lived from 384 to 322 BC, wrote in his Historia animalium that the octopus "seeks its prey by so changing its colour as to render it like the colour of the stones adjacent to it; it does so also when alarmed." Aristotle also noticed the hectocotylus arm and guessed it played a role in reproduction, a claim that sat ignored until the 19th century. The French zoologist Georges Cuvier described the arm in 1829 and mistook it for a parasitic worm, naming it Hectocotylus octopodis. The German zoologist Heinrich Müller later assumed it was built to detach during mating, until the Danish zoologist Japetus Steenstrup showed in 1856 that it transfers sperm and only rarely comes loose. More recently, researchers sequenced the full genome of the California two-spot octopus, opening a window onto its molecular adaptations. The philosopher Peter Godfrey-Smith, studying what intelligence even means, has compared octopuses to hypothetical intelligent life from another planet. In the United Kingdom, the common octopus was, from 1993 to 2012, the only invertebrate protected under the Animals (Scientific Procedures) Act 1986, before a European Union directive extended the same protections to all cephalopods in 2012. In 2015, a team in Italy built soft-bodied robots that could crawl and swim using only minimal computation. In 2017, a German company fitted a soft, pneumatically controlled silicone arm with two rows of suckers, capable of grasping a metal tube or a magazine and pouring water from a bottle into a glass. If a simple silicone arm can already pour water and grip a magazine, the question now facing robotics labs is how much more of the octopus's own engineering still remains to copy.