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.
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.
Every whale shark wears a unique constellation of pale spots, so scientists adapted a 1986 star-mapping algorithm built for the Hubble Space Telescope era to match shark flanks the way astronomers match star fields. The method anchors a global photo-ID library with tens of thousands of encounters. Fittingly, the whale shark's name in Madagascar — marokintana — means "many stars."
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.
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.
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.
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.
Dried, untanned shark skin — shagreen, usually from small sharks called dogfish — is a natural abrasive: its denticles cut like 80- to 220-grit paper in one direction and burnish in the other, and cabinetmakers and instrument makers smoothed wood with it for centuries; a microscope study of a 1711 Stradivari cello found what looks like a dogfish denticle lodged in the varnish. Tanned and dyed, shagreen also covered cases, boxes and sword grips. The famous exception is the samurai sword: its hilt covering is called samegawa, “shark skin,” but the museum pieces are wrapped in ray skin. On the katana the shark got the name and the ray did the work.
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.
Voluntary arm shedding, or autotomy, has been documented in at least Abdopus aculeatus and Ameloctopus litoralis. The arm separates at a defined break point and continues writhing while the octopus escapes.
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.
The silky shark's denticles are so small and densely packed that its skin feels sleek rather than sandpapery — the trait that named it. The finish trims drag for a life spent chasing tuna in open water. Even among sharks, there is a best-dressed.
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 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.
When basking shark carcasses decay, the gill apparatus and lower jaw fall away first, leaving a small-headed, long-necked hulk — the source of several famous "sea serpent" and "plesiosaur" strandings, including the 1977 Zuiyo-maru carcass hauled up off New Zealand. Many monsters, examined closely, turn out to be sharks twice over.
Biomechanical modeling published in 2008 estimated the bite force of megalodon, the largest shark that ever lived, at roughly 108,000 to 182,000 newtons — up to about 41,000 pounds of force, the strongest bite calculated for any animal at the time. That is several times the estimated bite of Tyrannosaurus rex. The figure often circulates mislabeled as “psi”; it is total force, and it needed no exaggerating.
The California two-spot octopus has a genome about 2.7 billion DNA base pairs long, slightly smaller than ours. Yet researchers identified about 33,600 protein-coding genes, compared with roughly 20,000 in humans. Surprisingly, this extra genetic complexity did not come from duplicating the entire genome; instead, certain gene families expanded and the genome was extensively rearranged.
The International Shark Attack File puts a U.S. beachgoer's lifetime-scale risk of a fatal shark bite near one in 4.3 million — far below the risks of lightning, bees or the drive to the beach. In a typical year sharks bite about 70 people worldwide, unprovoked, and about six of those bites are fatal.
Heat swells the tower's 7,300 metric tons of iron, lifting it as much as 15 centimeters (6 inches) taller in high summer than in deep winter, by engineers' estimates. And because the sun warms one face faster than the rest, over a clear day the summit traces a little circle about 15 centimeters wide — always leaning gently away from the sun.
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.
While cousins cross oceans, tracked blacktip reef sharks often spend years inside a home range of well under a square kilometer of reef. The fin cutting the lagoon today is likely the same individual as last season's. Not every shark is a wanderer; some are neighbors.
Female common blanket octopuses reach about 2 meters (6.5 feet) long, while males top out near 2.4 centimeters (1 inch). The weight ratio is at least 10,000-to-1 and may reach 40,000-to-1, among the most extreme size gaps known in any large animal. Males are so rarely encountered that the first live one was not photographed until 2002, off the Great Barrier Reef.
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.
The myth that sharks are cancer-proof — spread by a 1990s book and a cartilage-pill industry — is false: scientists have documented tumors in dozens of shark and relative species, including a large tumor on a great white shark's jaw reported in 2013. The legend sold supplements; the biology never cooperated.
The megamouth shark — a blubbery, 16-foot (5 m) filter feeder with a cavernous, glow-lipped mouth — was entirely unknown until one tangled in a U.S. Navy vessel's gear off Hawaii in 1976. Only a few hundred sightings have ever been logged since. The 20th century's biggest zoological surprise swam in from the deep.
The English had sea dog, dogfish and hound-fish — names for the small sharks of home waters — but no name at all for the big tropical animal Captain John Hawkins's men brought to London in 1569. A handbill advertising the specimen said exactly that: there was no proper name for it, “but that certayne men of Captayne Haukinses doth call it a sharke.” Within thirty years “shark” also meant a swindler, and etymologists still argue which sense came first.
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.
A blue-ringed octopus is small and usually drab, but vivid iridescent blue rings appear when it feels threatened. The display serves as a warning; the animal generally tries to escape, and nearly every recorded bite has come after someone picked it up.
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.
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.
A giant Pacific octopus has roughly 200 to 250 suckers on each arm. Monterey Bay Aquarium reports up to 2,240 suckers in a female, about 100 more than in a male. Each sucker can grip independently, and those near the arm tips are the smallest and most sensitive.
Flat as a doormat, the angel shark buries itself in sand with only eyes showing and explodes upward in a tenth-of-a-second strike on passing fish. Once common from Norway to the Canaries and across the Mediterranean, it has disappeared from about 90% of that range in a century, was listed Critically Endangered by the IUCN in 2006, and hangs on mainly around the Canary Islands. The floor of the sea sometimes has teeth.
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.
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.
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.
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.
The deep-sea goblin shark's jaws shoot forward at prey in what researchers measured as the fastest jaw protrusion known in fishes, snapping out from under its blade-like snout before retracting. Video analysis published in 2016 called the mechanism "slingshot feeding." Its face is a loaded weapon.
In a 2021 experiment, 20 wild-caught bonnethead sharks held in a tank were exposed to magnetic fields mimicking spots hundreds of kilometers (hundreds of miles) from where they were caught — and turned to swim “home,” orienting on magnetic cues alone, the first solid evidence that sharks read Earth's magnetic field as a true map. Some of the longest migrations in the sea may run on planetary GPS.
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.
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.
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.
Great white sharks refuse captivity: they ram tank walls and stop eating, and open-ocean hunters that travel hundreds of miles do not settle in a box. The exception was a juvenile female at the Monterey Bay Aquarium, held 198 days in a 4.5-million-liter (1.2-million-gallon) tank from September 2004 to March 2005 and seen by more than a million visitors before she killed two tankmates and was released. Five more juveniles followed, for 11 days to five months, until the program ended in 2011 when the last died days after release; an adult netted by a Japanese aquarium in 2016 refused food and died in three days. Some apex predators cannot be exhibited — only visited.
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.
The pale deep-sea octopod nicknamed Casper was filmed near Hawaii in 2016 at more than 4,000 meters (13,100 feet) — the deepest record was 4,290 meters (14,100 feet) — a record depth for its group. Related octopods were then found attaching their eggs to the stalks of dead sponges, and those sponges grow only on manganese nodules — potato-shaped metal lumps that take millions of years to form and are prime targets for seabed mining. Remove the nodules and the sponges go, and with them the nursery.
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.
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.
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 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.
The 20-inch (50 cm) cookiecutter shark twists out neat round plugs of flesh from tuna, whales and great whites — and its bites have damaged the rubber sonar domes of U.S. Navy submarines. Its Latin name, Isistius brasiliensis, honors an Egyptian goddess of light, for the glow along its belly. Few animals so small have scarred so many giants.
Bamboo shark embryos, still sealed in their egg cases, freeze — stopping gill movement — when they detect the electric field of a predator outside, resuming only when the threat passes. The survival course begins before hatching.
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.
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.
When an octopus cannot pull a shell open, a muscular salivary papilla studded with minute teeth — separate from the radula, its primary toothed ribbon — can drill through it. Secretions from the posterior salivary glands then help relax the muscle holding the shell closed.
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.
The star-sucker pygmy octopus, Octopus wolfi, lives in the shallow coastal waters of the western Pacific Ocean, measures under 2.5 centimeters (1 inch) in total length and weighs less than a gram (0.04 ounce) — smaller and lighter than a small coin, and small enough to sit on a fingernail. It is named for the ring of tiny papillae fringing the suckers near its arm tips, which let it taste whatever it touches.
The giant Pacific octopus (Enteroctopus dofleini) can span about 4.3 meters (14 feet) from arm to arm. The heaviest individual reliably weighed alive was 71 kilograms (156 pounds), while most adults are far smaller.
A shark's skin is armored with tooth-like denticles whose ridges shed drag, and engineers have been copying them. In 2014 a Harvard team 3D-printed an artificial shark skin and showed it improved swimming performance on a flapping foil; in 2018 the same lab, with the University of South Carolina, molded shortfin mako denticles onto an airfoil and found arrangements that cut drag and added lift at once, improving the lift-to-drag ratio by up to 323% at low angles of attack. Denticle-patterned surfaces are studied for aircraft, drones and wind turbines. Four hundred million years of prototyping ended up in engineering journals.
Unlike whale sharks, which gulp and suction-feed, the basking shark is a pure ram filter feeder: its colossal gape works only when the animal swims forward, forcing water through its gill rakers. Stop the shark and the filter stops with it. For a basking shark, motion is the meal.
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.
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.
Whale shark reproduction was a mystery until 1995, when a female landed in Taiwan — nicknamed “megamamma” — held about 300 embryos at different stages, proving the species bears live young from egg cases hatched internally. At roughly 300, the litter was more than double the largest recorded for any other shark, the blue shark's, at around 135. The biggest fish is also the biggest maternity ward.
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.
Juvenile zebra sharks wear bold dark-and-pale bands; as they mature the bars dissolve into a field of leopard-like spots, and divers routinely misname the adults. One species manages to impersonate two big cats in one lifetime. The zebra grows up to be a leopard.
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.
Sharks first appeared roughly 450 million years ago. Trees — with their distinctive woody trunks and deep root systems — only evolved about 350 million years ago. The predators had a 100-million-year head start.
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.
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.
Octopuses drag prey home and discard the shells outside the entrance, building a heap called a midden. Divers use these piles of cracked crab and clam shells to locate dens that would otherwise be invisible. The habit is old enough that Aristotle noted it more than 2,000 years ago.
Many large sharks mature astonishingly late — white sharks at roughly 26–33 years by vertebral studies, Greenland sharks near 150 — and then produce few young after long pregnancies. That slow arithmetic is why overfishing empties shark seas so fast and refills them so slowly.
The great white is stocky, and gets stockier as it grows — which the 2025 megalodon study concluded caps it near 7 m (23 ft), because a bulky body shoves aside too much water to be worth hauling any bigger. Every giant that swims is built long and lean instead: the whale shark, the basking shark, the great whales. The ocean's most feared predator is held back by its own build.
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.
A great white breathes only by moving: it swims with its mouth open so water washes over its gills, and if it stops, it suffocates. Makos are built the same way. Most sharks are not — they can pump water across their gills while resting, which is how a nurse shark sleeps motionless on the bottom. “All sharks die if they stop swimming” is folklore with a grain of truth stretched over 500 species.
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.
Great white sharks, shortfin makos and their mackerel-shark kin run heat exchangers of tightly packed blood vessels that keep swimming muscles, eyes and brain well above sea temperature — regional warm-bloodedness in a fish. A warm retina sees fast prey in cold water. The great white hunts pre-heated.
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.
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.
For centuries Europeans prized glossopetrae — "tongue stones" — as magical charms against poison, believing they fell from the sky or grew from serpents' tongues. In 1666 the Danish anatomist Nicolas Steno dissected a great white shark's head and showed they were shark teeth, a founding moment for paleontology. A superstition dissolved in one dissection.
The epaulette shark of Australian reefs walks across exposed coral flats on its paddle-shaped fins, wriggling between tide pools. It survives the trip by tolerating extreme oxygen loss that would injure most vertebrates, shutting down non-essential brain functions until it reaches water. It is a fish that hikes between puddles.
White sharks lack the protective third eyelid many sharks deploy, so in the last instant of a strike they rotate each eye backward in its socket, exposing tough white tissue. The predator goes momentarily blind precisely when a seal's claws are closest. It trades sight for eyes it can keep.
A resting octopus lies pale and still, then erupts for about a minute into twitching arms, darting eyes, quickened breathing and racing skin patterns before going quiet again. The phase has a raised arousal threshold, is regulated by prior sleep, and produces neural activity resembling wakefulness, making it broadly comparable to mammalian REM sleep.
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.
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.
In April 2019, cameras lowered into the Java Trench during the Five Deeps Expedition recorded a dumbo octopus foraging at 6,957 meters (22,825 feet), in water of about 1.5 °C (35 °F). A second animal turned up at 5,760 meters (18,898 feet). The previous reliable record, a black-and-white photograph, had stood at 5,145 meters for half a century.
The eel-like frilled shark, a deep-water relic with 300 needle teeth arranged in frills, is estimated to carry its young for around 3.5 years — among the longest gestations proposed for any vertebrate. In the cold deep, even birth runs in slow motion.
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.
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.
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.
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.
A whale shark's back is covered in some of the thickest skin in the animal kingdom — up to about 10 cm (4 in) — and it hardens the surface further by flexing when boats or predators come close. The gentlest giant wears the heaviest coat.
A 2009 genetic survey found venom proteins across octopuses, cuttlefish and some squid, tracing them to a single ancient venomous ancestor. Most species use theirs quietly, paralyzing a clam until its shell relaxes open. Only the blue-ringed octopuses of the genus Hapalochlaena carry a toxin dangerous to humans.
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.
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.
Female bull sharks deliver pups in estuaries and river mouths, and the young may spend years in low-salinity nurseries where big marine predators cannot follow. Fresh water is not a stunt for the species; it is a life stage. Some of the ocean's most formidable sharks learned to swim in rivers.
In 2016 staff at the National Aquarium of New Zealand in Napier found Inky's tank empty and a trail of water across the floor. He had exploited a lid left ajar after maintenance, crossed roughly 2.5 meters (8 feet) of floor and squeezed into a drainpipe about 50 meters (160 feet) long that emptied into Hawke's Bay. He was never seen again. His tankmate, Blotchy, stayed put.
The salmon shark of the North Pacific maintains body temperatures around 21–26 °C (70–79 °F) even in near-freezing water — among the warmest bodies of any fish — letting it chase salmon where most predators stiffen. It is a shark wearing its own wetsuit of blood.
Satellite-tagged whale sharks travel thousands of kilometers to arrive precisely when food blooms — coral spawn off Australia, fish eggs off Yucatán — forming seasonal gatherings of hundreds, the largest known assemblies of giant fish. Somehow, the world's biggest fish keeps a calendar.
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.
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.
Behind a whale shark's mouth, up to 1.5 m (5 ft) across, the esophagus narrows to a passage only several centimeters (a few inches) wide. Anything oversized gets spat back out.
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.
That is every fact on the shelf — all 199 of them.
Explore all general-knowledge topics and the A-to-Z lists.
We Value Your Opinion Comments