Radiocarbon dating of proteins in their eye lenses shows Greenland sharks live at least 272 years — longer than any other animal with a backbone. The oldest of the 28 sharks measured in the landmark 2016 study, a female just over 5 m (16 ft) long, was estimated at roughly 392 years old. Females don't even reach maturity until around 150, and a big one cruising the Arctic today has been swimming for some three centuries.
On Great Barrier Reef platforms cut off at night by spring low tides, the epaulette shark tolerates hours of severe hypoxia — oxygen near 5% of normal — and at least an hour of total anoxia, with no drop in its brain's energy stores and no neurons lost afterward. It manages this by throttling demand rather than boosting supply: an adenosine-driven slowdown leaves the shark in a coma-like torpor until oxygen returns, when it snaps back to alertness within moments. Researchers at Griffith University and the University of Auckland study its brain mitochondria for clues to limiting stroke and heart-attack damage, where the harm comes as oxygen floods back. A reef shark's hour without oxygen is a lesson medicine wants.
Behind a shark's retina lies the tapetum lucidum, a layer of mirror-like plates that bounces unabsorbed light back through the photoreceptors for a second pass. The same structure makes a cat's eyes shine in headlights. In dim water, a shark's eye reads every photon twice.
Megalodon, the extinct giant that grew several times the size of any shark alive, left teeth so like a great white shark's that the family tree looks obvious — but fossil evidence places the two on separate lineages, the white shark's ancestry tracking closer to ancient mako-like sharks. Megalodon's line ended; it left no descendants. The great white is the giant's rival's grandchild, not the giant's.
Pairs of larger Pacific striped octopuses mate face to face with their suckers aligned, share food and may occupy the same den for days. Females produce multiple clutches over months instead of a single terminal brood.
Cirrate octopuses, the deep-sea group nicknamed dumbo for the ear-like fins above their eyes, propel themselves by slowly beating those fins rather than by jetting water. A web of skin joins the arms into an umbrella. The deepest octopus ever filmed was one of them, foraging almost 7 kilometers down.
Sand tiger shark embryos hatch inside their mother and the largest in each uterus devours its smaller siblings, a behavior called intrauterine cannibalism. Each of the two uteri ultimately delivers a single battle-tested pup. The nursery is the first hunting ground.
An octopus has one brain, but a highly distributed nervous system. The central brain in its head does the learning, the remembering and the deciding, while each of the eight arms carries its own large cluster of neurons that can sense touch and taste and control much of the arm's movement without waiting for orders. Those clusters are nicknamed mini-brains, but nothing in an arm learns or remembers.
Given eight seemingly interchangeable arms, an octopus still plays favorites: in a 2006 experiment, every animal tested reached into a maze with the same preferred arm again and again, and individuals favor one eye as well. Unlike human handedness, though, the species has no side — roughly half the animals lean left and half lean right, so the preference is personal rather than built into the octopus.
The bold dark bars that name the tiger shark belong mostly to youth: they fade as the shark matures, leaving big adults far plainer than their portrait. The name records the juvenile. By the time a tiger shark is truly formidable, it has nearly erased its tiger.
The parrot-like beak is the only hard part of an octopus; everything else is muscle and skin that can be poured through a gap. The Alaska Department of Fish and Game estimates that a 23-kilogram (50-pound) giant Pacific octopus can pass through a hole about 5 centimeters (2 inches) across. One researcher watched an octopus of that size cross a research vessel's deck, find the scupper drain and be gone inside a minute.
Fossil sites in Panama, Spain and the southeastern U.S. preserve concentrations of small juvenile teeth of megalodon, the extinct giant shark that hunted the oceans until about 3.6 million years ago — evidence the giants birthed and raised young in warm, food-rich shallows; 2025 growth analyses indicate newborns already stretched about 3.6–3.9 m (12–13 ft). The deadliest daycare in history had ocean views.
Cornered, the swell shark gulps water to double its girth and wedge itself into rock crevices; under blue light its skin also fluoresces brilliant green, visible to other swell sharks' eyes. It is both a pufferfish impersonator and a secret neon sign.
In 2020 researchers reported that whale shark eyes are covered with more than 3,000 minute dermal denticles — the tooth-like scales that armor shark skin — apparently protecting eyes that have no lids. As a second defense, the shark can retract each eyeball deep into its socket. Even its stare wears chain mail.
The shortfin mako carries its Māori name into science and sport worldwide; in New Zealand tradition mako teeth were treasured ear pendants of high status. One of the ocean's fastest animals is one of the few whose everyday English name is borrowed from a Polynesian language.
The hammerhead's wide-set eyes give it a panoramic view and, when it sweeps its head, effectively 360-degree vision — a 2009 study also confirmed excellent stereo depth perception from the exaggerated eye separation. The strangest head in the sea turns out to be a surveying instrument.
Shark jaws hang in slings of muscle and ligament, letting the whole apparatus thrust forward and gape wide independent of the head — the mechanism behind slow-motion footage of white sharks seeming to launch their mouths at prey. The skull aims; the jaw travels.
From 2007 to 2011, researchers from the Monterey Bay Aquarium Research Institute returned 18 times to the same site in Monterey Canyon and found the same scarred Graneledone boreopacifica female guarding about 160 eggs. She was never observed feeding and grew progressively thinner. Her 53-month brood remains the longest recorded for any animal.
Tagged white sharks from California converge each year on a remote mid-Pacific region with little obvious food — nicknamed the White Shark Café — where they dive hundreds of times a day; a 2018 expedition found hidden layers of prey and possible mating behavior. Half the mystery is solved; the reservation system is not.
Octopuses is the standard English plural. Octopi is also widely used, but it applies a Latin-style ending to a word derived from Greek. Octopodes follows the Greek plural pattern. Major dictionaries record all three forms.
Cameras trained on gloomy octopuses in Jervis Bay, Australia, caught 102 throws in 24 hours of footage from a group of about ten animals. Each throw involves gathering silt, shells or algae in the arms and repositioning the siphon under the arm web to blast the material several body lengths. Throws in social contexts were more forceful, more often silt than shells, and more often made by dark-colored individuals — and they frequently connected. Targeted throwing had previously been seen only in some social mammals.
The white shark's two-tone paint job is engineering: slate-gray above blends into deep water for eyes looking down, while the pale belly dissolves into surface glare for eyes looking up. The scheme, countershading, hides a two-ton predator in open water. The most famous silhouette in the sea is built to have none.
Of 500-plus shark species, exactly three became filter-feeding giants — the whale shark, the basking shark and the megamouth — each evolving the strategy separately. Gigantism in sharks arrived not with bigger prey but with smaller. The three largest mouths in the family eat things you can barely see.
The most rigorous global estimate, published in Science in 2024, found fishing kills roughly 80 million sharks annually — about 25 million of them from threatened species — and the toll rose even as anti-finning laws increased tenfold. Sharks, by comparison, kill about six people worldwide in a typical year. For every human lost to a shark, humans take more than 10 million sharks.
Researchers at UC Santa Barbara shone light on isolated patches of skin from the California two-spot octopus and watched the color-changing chromatophores expand, then relax in darkness. The tissue responded fastest to blue light near 480 nanometers, the same wavelength the eye's pigment favors, and the skin turned out to express rhodopsin. They named the effect light-activated chromatophore expansion. The skin does not form images — but it knows when the lights change.
Greenland shark meat is loaded with trimethylamine oxide and urea, enough to sicken anyone who eats it fresh. Icelanders ferment and air-dry it for months into hákarl, a pungent national delicacy served in small cubes. The world's longest-lived vertebrate is also one of its most acquired tastes.
A shark's skin is armored with millions of tiny tooth-like scales called dermal denticles — complete with enamel-like coating, dentine and pulp. The denticles channel water to cut drag, a design so effective that competitive swimsuits mimicking it preceded the 2010 ban on high-tech racing suits. Stroke a shark tail-to-head and it feels like sandpaper.
In round-the-clock observations at 22 °C (72 °F), nocturnal octopuses averaged about 12 quiet-sleep bouts of 50 minutes each plus roughly 10 brief active-sleep bouts. Together, these amounted to about 10 hours of sleep per day. Warmer water shortened the interval between active bouts.
A giant Pacific octopus can grow to tens of kilograms, but a hatchling weighs only about 22 milligrams and is roughly the size of a rice grain. The young drift near the surface as plankton for weeks or months before settling to the seafloor, and only about one in a hundred survives that stage.
Vision-model research simulating a white shark's eye found that surfers paddling overhead closely match the silhouette and motion of seals — supporting the long-held mistaken-identity explanation for many bites, which are frequently single strikes without feeding. To a young white shark looking up, a surfboard is a seal-shaped question.
Jelly-filled pores on a shark's snout — the ampullae of Lorenzini — detect the faint electric fields of living bodies: in Kalmijn's 1982 experiments, sharks and rays responded to voltage gradients of just 5 nanovolts per centimeter, five billionths of a volt. A flounder hiding under sand still broadcasts its heartbeat. To a shark, every animal in the sea is a battery that cannot be switched off.
By day, nurse sharks heap together under ledges — sometimes dozens deep, flank on flank — and individuals return to the same resting sites again and again. For animals stereotyped as loners, it is conspicuously social sleeping. The reef's most relaxed predator naps in a pile of friends.
Male and juvenile blanket octopuses tear stinging tentacles from Portuguese men-of-war and hold them in their arms, apparently as weapons or deterrents. Skin chemistry is thought to keep the stinging cells from firing against them.
Vertebrate and cephalopod camera eyes evolved independently, but their retinas are wired differently. In vertebrates, nerve fibers lie in front of the photoreceptors and the optic nerve creates a blind spot. In octopuses, the photoreceptors face incoming light and the nerve fibers exit behind them, so there is no comparable blind spot.
In 2021 researchers confirmed that the kitefin shark, a deep-sea species reaching about 5 ft 11 in (1.8 m), produces its own blue-green light — making it the largest known luminous vertebrate on Earth. Its glowing belly likely erases its silhouette against the faint light above, a cloaking trick called counterillumination.
Nurse sharks generate suction among the strongest measured in any fish, inhaling prey from crevices with a sound like a slurp — one possible origin of their name. They can pump water over their gills while lying still, so they spend daylight dozing in piles on the seafloor.
In Hawaiian tradition, certain sharks were ʻaumākua — ancestral guardian spirits of specific families — fed, respected and never hunted by those they protected. The man-eater of one culture's nightmares was another culture's grandmother.
Most octopus species live for roughly a year or less. The giant Pacific octopus is unusually long-lived at about three to five years, while the deep-sea Graneledone boreopacifica has been recorded brooding its eggs for 53 months.
Named for its schedule, the night shark spends days hundreds of meters down and rises toward the surface after dark, guided by oversized eyes with a vivid green shine. Fishermen named it before scientists explained it. Its workday begins when the ocean's lights go out.
Sharks were long assumed to follow the stronger scent, but a 2010 experiment on smooth dogfish — fitted with headgear that piped squid odor to each nostril separately — showed they turn toward whichever nostril catches the smell first, even when the other nostril gets the stronger dose, on time gaps as short as a tenth of a second. Odor in the sea arrives in ragged patches, so timing beats concentration for tracking a plume back to its source. The researchers suspect the hammerhead's wide-set nostrils sharpen this stereo sense further. The famous nose is less a detector than a compass.
The siphon is a muscular funnel that forcefully expels water from the body. An octopus uses it for jet propulsion, to eject ink, and to aim water beneath the arm web strongly enough to throw silt, shells and other debris.
Female sharks of several species can store viable sperm in specialized glands for months to years after mating — one brownbanded bamboo shark produced a pup after 45 months of isolation, a record documented in 2015. Shark family planning operates on geological patience.
The octopus genome contains 168 protocadherin genes, which help regulate how neurons develop and connect. That is about ten times the number found in other invertebrates and more than twice the number in mammals. Large protocadherin families had previously been considered mainly a vertebrate feature.
Before petroleum, oil rendered from shark livers lit lamps, tanned leather and greased machinery; entire fisheries — from Achill Island's basking sharks to the World War II hunt for vitamin A-rich soupfin sharks — rose and crashed on shark oil. Sharks powered human nights before electricity did.
Global assessments led by IUCN specialists conclude that roughly one in three species of sharks, rays and chimaeras is threatened with extinction, chiefly from overfishing. The animals that outlasted five mass extinctions are struggling with a sixth made of nets and hooks.
Because cartilage rarely fossilizes, the shark fossil record is overwhelmingly teeth — shed continuously for 400 million years — plus occasional vertebrae and rare body impressions. Some fossil beds hold thousands of teeth per square meter. Sharks wrote their autobiography almost entirely in bite.
The dwarf lanternshark of the deep Caribbean matures at around 6 to 8 inches (15–20 cm) — smaller than many goldfish — and carries light-producing organs along its belly. The ocean's most feared family includes a member you could hide in a coat pocket.
Radiocarbon from Cold War bomb tests, lodged in the growth bands of two dead whale sharks' vertebrae, proved in 2020 that each band marks one year — and put one shark, a 10 m (33 ft) female stranded in Pakistan, at 50 years old, the oldest ever verified. Earlier modelling built on those yearly bands suggests the largest may live a century. The biggest fish in the sea is also one of its slow-burning elders.
Behind its small gripping front teeth, the Port Jackson shark carries broad, flattened molars — crushing plates that pulverize sea urchins, snails and crabs. Its scientific name, Heterodontus, means "different teeth." It is a shark that grinds its dinner like a mill.
The underside of a hammerhead's wide head is studded with electroreceptors, and the shark swings it across the seafloor to scan for the faint fields of stingrays buried in sand — the width of the hammer widens the search area with every pass, an advantage Kajiura and Holland measured in 2002. Great hammerheads then use the hammer itself to pin a ray to the bottom while they bite. The strange head is both the search coil and the weapon.
Scientists fitted tiger sharks in the Bahamas with cameras and trackers, and the sharks' commutes across their feeding grounds helped confirm the largest known seagrass ecosystem on Earth — at least 66,000 km² (25,000 sq mi) and up to 92,000 km² (35,000 sq mi) across the Bahama Banks, enough to raise the world's known seagrass cover by more than 40% in a 2022 study. The apex predator moonlighted as a survey vessel.
Octopuses are carnivores, and for nearly every species studied the staple is crustaceans: crabs above all, with shrimp and lobsters behind them. Mollusks come second — clams, snails and scallops, pulled apart, drilled through or relaxed with paralyzing saliva. Larger species take fish as well. And the giant Pacific octopus is recorded eating smaller octopuses, its own species included.
Two close cousins carry opposite toolkits: the great white's broad triangular teeth are serrated like steak knives for shearing blubber, while the shortfin mako's are smooth, narrow spikes for pinning slippery fish. Read the tooth and you read the menu. In sharks, cutlery is destiny.
Sharks produce unusually small, rugged antibody fragments called VNARs — roughly a tenth the size of human antibodies — able to reach folds on viral proteins that larger molecules cannot. Lab studies have shown shark VNARs neutralizing SARS-CoV-2 and related coronaviruses, and researchers are developing them against cancers and other targets. The ocean's oldest immune system is auditioning for ours.
A shark's entire skeleton is cartilage — the same flexible tissue as a human ear — reinforced with mineralized plates where stress is highest. Cartilage is lighter and bends without breaking, part of why sharks are such economical swimmers. It is also why complete shark fossils are so rare while their teeth litter the world.
In History of Animals, Aristotle called the octopus a stupid creature because it would approach a person's lowered hand. Yet he also accurately described its specialized mating arm, a feature European naturalists later had to rediscover.
Small deep-sea lantern sharks grow light-producing organs called photophores along their undersides, matching the dim glow filtering from above so predators looking up see nothing. Some also flash light along spines and fins. In the deep, the best hiding place is brightness.
An octopus can rebuild a lost arm, including its muscles, suckers and nerve cord. The wound is first covered by a layer of epithelium, then new tissue is organized into the structures of a functional replacement arm rather than a simple scar.
In cephalopod anatomy, arms carry suckers along their whole length while tentacles are longer, retractable and suckered mainly at the tip. Squid and cuttlefish have both — eight arms plus two tentacles. Octopuses have eight arms and nothing else. The word "tentacle" is almost universally applied to them and is almost universally wrong.
A female giant Pacific octopus hangs her eggs from the roof of her den in hundreds of strands of roughly 250 eggs each, commonly totaling 18,000 to 74,000 and occasionally reaching 100,000. For six or seven months she fans them, cleans them and blows water over them, and does not eat. As the eggs hatch, she blows the young free of the den with her last strength, then dies. Each hatchling emerges the size of a rice grain, weighing about 22 milligrams, and of the tens of thousands, a handful will reach adulthood.
The mimic octopus, first seen off Sulawesi in 1998 and named Thaumoctopus mimicus, flattens, contorts and recolors itself into imitations of flatfish, lionfish and banded sea snakes. Reports credit it with impersonating a dozen or more marine animals, nearly all of them venomous or unpalatable.
The male seven-arm octopus (Haliphron atlanticus) hides its eighth arm, a specialized mating arm, in a pouch beneath the right eye, so only seven are visible. This deep-sea species feeds on jellyfish, eating the bell and sometimes carrying the trailing tentacles.
The pigeye shark — a stocky, snub-snouted, small-eyed cousin of the bull shark — patrols murky Indo-Pacific shallows and is so often mistaken for the bull that scientists struggle to count it. Its name is not a nickname; it is the official common name. Somewhere a shark answers, formally, to "pigeye."
With pectoral fins shaped like wings and a tail delivering forward thrust, many sharks fly through water more than they float in it — negatively buoyant bodies held aloft by hydrodynamics, sinking whenever they glide too slowly. A great white shark is, structurally, a heavier-than-water aircraft.
Across 500-plus species, sharks cover nearly every reproductive playbook: some lay armored egg cases, some nourish live young with placentas rivaling mammals', and some hatch eggs internally before live birth. No other vertebrate group mixes its maternity wards so freely.
Tiger sharks are the ocean's least fussy eaters: documented stomach contents include sea turtles, seabirds, other sharks — and human garbage from license plates to tires, which earned them the nickname "wastebaskets of the sea." Their serrated, can-opener teeth shear through turtle shell. Curiosity, in a tiger shark, is a digestive strategy.
The shortfin mako is the fastest shark in the world — a torpedo-shaped, crescent-tailed hunter that runs down tuna and swordfish in the open ocean.
Jacques Cousteau called the oceanic whitetip "the most dangerous of all sharks" — not for beaches, but for the open sea, where it reaches shipwrecks and air crashes first. It is widely implicated in deaths among the roughly 900 sailors set adrift by the USS Indianapolis sinking in 1945, though exposure and thirst killed many. The shark you have never heard of owns the ocean's worst disasters.
Feeding trials published in 2018 showed bonnethead sharks digesting and absorbing nutrients from seagrass, not merely swallowing it while hunting crabs — the first shark shown to be a true omnivore. More than half its diet can be plants. Somewhere in the hammerhead family, a shark went partly vegetarian.
In the late 1950s, Eugenie Clark trained lemon sharks at Florida's Cape Haze laboratory to push a target that rang a bell for food, then to pick the correct target over a wrong one — and after a 10-week winter layoff, the sharks took the task straight back up. In 2018, Port Jackson sharks in a Macquarie University lab learned to swim to a feeding station whenever a jazz track played, though asked to tell jazz from classical, they struggled. The reputation for mindless instinct does not survive contact with a curious shark.
Detached arms continue responding to stimuli and activating their suckers long after they lose contact with the brain — in laboratory observations, for more than an hour. In one experiment an octopus handled its own amputated arm freely but held it only at the skinless cut end and would not eat it. The self-recognition signal apparently survives the separation.
Helicoprion, a 270-million-year-old shark relative, grew a spiral whorl of teeth like a circular saw blade set in its lower jaw — a structure so bizarre that scientists argued for a century about where on the body it belonged until CT scans settled it in 2013. Evolution once shipped a power tool with fins.
Male argonauts detach a specialized mating arm inside the female, where it can continue moving on its own. In 1829, Georges Cuvier mistook these arms for parasitic worms and named them Hectocotylus octopodis. The term hectocotylus is still used for a cephalopod's specialized mating arm.
The first dinosaurs appeared roughly 230–240 million years ago; the shark lineage was already more than 200 million years old by then and had come through three mass extinctions — the end-Ordovician, the Late Devonian and the end-Permian, the worst of them all. When Tyrannosaurus finally showed up, sharks were the ocean's grizzled veterans. Dinosaurs came and went; the sharks stayed.
One of the world's most heavily fished sharks is also among its longest-lived: Pacific spiny dogfish are estimated to live 80 years or more, maturing only after decades. The 3-foot (0.9 m) shark defends that long life with two mildly venomous dorsal spines. Longevity in the sea does not require size.
Megalodon — Otodus megalodon, an extinct shark that hunted the world's oceans from about 20 million to 3.6 million years ago and is known mainly from its serrated teeth, some the size of a human hand — was far larger than any shark alive. A 2025 study by 28 specialists, comparing fossil vertebrae from Belgium and Denmark with the proportions of 165 living and extinct sharks, put its maximum length at up to 24.3 m (80 ft) and its weight near 94 metric tons.
Around Greenland, the sharks' eyes are frequently parasitized by Ommatokoita elongata, a pale copepod about 3 cm (1.2 in) long that anchors itself to the cornea — for decades, reason enough to assume the hosts were all but blind. A 2026 study of Greenland shark eyes found a pristine, purely rod-based retina tuned to deep-water blue, with no sign of degeneration even in a specimen more than 130 years old, and a cornea that still passes most light despite the parasite. Video shows the sharks turning their eyes toward light. The longest-lived vertebrate is not blind; it just lives where there is little to see.
The common octopus runs on roughly 500 million neurons, by far the largest nervous system of any invertebrate — a honeybee has about a million. Two-thirds of them sit in the arms rather than the central brain, each arm carrying a nerve cord that handles touch, taste and movement locally. The brain sets the mission; the arms work out the details.
Hunting seals at Seal Island, South Africa, great whites rocket upward from deep water and launch fully clear of the surface, striking from below at speeds researchers estimate near 40 km/h (25 mph) at breach. A two-ton fish becomes briefly airborne. The ambush comes from the third dimension.
Australia's wobbegong sharks are flattened ambush hunters fringed with weed-like skin flaps, camouflaged so well against reef floors that their name is thought to come from an Aboriginal word associated with shaggy beards. Prey swims to them. The rug moves last.
Two branchial hearts drive blood through the gills; a third systemic heart sends it out to the arms, organs and brain. When the animal jets, pressure inside the mantle stops blood returning, and the systemic heart — with nothing to pump — stops until the jetting ends. Fast swimming therefore starves an octopus of oxygen at the exact moment it needs it most. This makes sustained jetting physiologically costly compared with crawling.
In 2001 a hammerhead pup appeared in a Nebraska zoo tank holding only females, and DNA testing later proved the first confirmed virgin birth in sharks — parthenogenesis, reproduction with no father at all. Zebra sharks and other species have since repeated the trick in captivity. Some sharks simply do not wait for a mate.
Common octopuses can lay about 100,000 to 500,000 small eggs that hatch into planktonic paralarvae, while giant Pacific octopuses lay tens of thousands. Some deep-sea octopods produce only about 30 large, yolk-rich eggs whose young hatch as miniature bottom-dwelling octopuses.
Many sharks enter tonic immobility — a limp, trance-like state — when rolled onto their backs, a reflex researchers use to handle them safely. Orcas appear to exploit the same off switch, inverting great whites until they drown. Even apex predators carry a reset button.
Every octopus studied carries a single type of visual pigment, which by the usual definition makes the animal color-blind. Yet octopuses produce camouflage that matches background colors closely enough to fool spectrophotometers. The paradox has been open since the pigment was first measured in 1958.
In the Red Sea, day octopuses hunt alongside goatfish, groupers and other reef fish, and researchers recorded them delivering swift explosive jabs with one arm at particular fish partners. The team called the behavior active displacement. Sometimes it clears a competitor off prey; sometimes there is no prey to grab at all. The scouting fish get punched least, and the loafers get punched most.
Bull sharks regulate the salt in their bodies so well that they thrive in fresh water, and they have been recorded more than 1,600 km (1,000 miles) up the Mississippi — famously near Alton, Illinois, in 1937. Lake Nicaragua's “landlocked” sharks turned out to be ordinary bull sharks: tagging from 1966 to 1972 showed them commuting both ways along the San Juan River from the Caribbean, and they most likely breed at sea. The shark in the river is not a legend.
The argonaut is a small octopus that lives in open water, and the female builds herself a thin white shell using two of her arms. She swims to the surface, traps a gulp of air inside the shell, then dives — and deep down the squeezed air balances her weight exactly, so she floats in midwater without swimming.
The shark lineage has endured for more than 400 million years, swimming through all five of Earth's great mass extinctions — including the one that ended the dinosaurs. They watched the rise and fall of nearly everything else in the ocean.
Sharks are tuned to low-frequency pulses — the irregular vibrations of a struggling fish — which they detect with inner ears and a lateral-line system running the length of the body. Long before a shark sees you, it has heard and felt you.
Octopuses are semelparous: females produce a single clutch, guard it without feeding, and die around the time it hatches. Males typically die not long after mating. The one clear exception found so far, the larger Pacific striped octopus, spawns repeatedly and keeps eating — which is why researchers now study its optic glands looking for what the rest of the order is missing.
Captive octopuses routinely learn to open screw-top containers for food. At the Seattle Aquarium, some also worked out childproof pill-bottle caps. In a baby-bottle test, one octopus bypassed the screw top entirely by tearing off the nipple.
Chromatophores hold only yellow, red, brown and black pigments, so no combination of them can produce the blue-green sheen a living octopus sometimes shows. Two other cell types do that work: iridophores, stacks of protein plates that split light by interference, and leucophores, which scatter whatever light falls on them and so look red in red light and blue in blue. Part of octopus color is not paint at all. It is optics.
Octopus marginatus and Abdopus aculeatus have been filmed walking on two alternating rear arms while wrapping the other six around the body. One resembles a drifting coconut and the other a clump of algae. The behavior was the first documented bipedal locomotion in an animal without a rigid skeleton.
The cookiecutter's lower teeth are fused into a single serrated band that sheds whole, not tooth by tooth — and the shark swallows the discarded band, likely recycling its calcium. It eats pieces of giants, and then it eats its own knives. Nothing in its economy goes to waste.
Shark teeth are not anchored in bone; they sit in the gums on a conveyor of replacements, with new rows rotating forward as old teeth fall away. Some species may cycle through 30,000 or more in a lifetime. Losing a tooth costs a shark about a week, not a dentist.
Octopuses use copper-based hemocyanin instead of iron-based hemoglobin to carry oxygen. Oxygenated hemocyanin is blue and remains effective in cold, oxygen-poor water, although it carries less oxygen per unit of blood than hemoglobin.
Tagged blue sharks ride ocean gyres on migrations spanning thousands of miles — Atlantic individuals recaptured across the ocean from where they were marked — following temperature bands and prey like long-haul truckers of the open sea. Their round trips can circle an entire ocean basin.
Expeditions to Kavachi, an active undersea volcano in the Solomon Islands, filmed hammerheads and silky sharks living inside its acidic, superheated crater plume — a discovery nicknamed "Sharkcano" that NASA later observed erupting from orbit. Some sharks chose the one neighborhood everything else evacuated.
The broadnose sevengill — a stout, spotted cow shark with seven gill slits instead of the usual five, up to about 3 m (10 ft) long — is one of the few sharks that regularly prey on seals, and it does not always do it alone. Off South Africa and California, observers have watched sevengills hunting in loose packs, a strategy David Ebert described in 1991 as social facilitation, alongside stealth, ambush and sudden bursts of speed. Adults have only two known predators of their own: orcas and great whites. A shark shaped like a Jurassic leftover runs the coast like a wolf pack.
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