The thresher's scythe-like upper tail can equal the length of its body, and high-speed video published in 2013 shows the shark whipping it overhead to stun whole groups of sardines before circling back to swallow the dazed. It is the only shark that hunts with a bullwhip.
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.
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.
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.
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.
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.
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.
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.
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.
By 2020, pocket sharks were known from exactly two specimens: a 40 cm (16 in) female trawled near the Nazca Ridge off Chile in 1979, and a 14 cm (5.5 in) male netted in the Gulf of Mexico in 2010 and named the American pocket shark in 2019. Each carries a slit-like pocket above the base of each pectoral fin, and a 2020 study of the Gulf specimen's pocket glands found the cellular machinery for secreting a luminous fluid. A shark barely longer than a smartphone is built with glowing pockets.
Juvenile scalloped hammerheads cruising sun-drenched shallows darken dramatically as melanin builds in their skin — one of the first demonstrations of true sunlight-driven tanning in a fish, published in Nature in 1996. Sharks kept shaded stayed pale while sunlit sharks browned. The beach tan predates the beach towel by millions of years.
Portuguese dogfish have been recorded around 3,700 m (12,100 ft) deep — among the deepest confirmed for any shark — hunting in freezing blackness at pressures hundreds of times the surface's. Even the abyss has its patrols.
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.
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.
The bluntnose sixgill belongs to the Hexanchiformes, the oldest-known group of modern sharks, already swimming by the Early Jurassic about 195 million years ago — before most dinosaurs, though not before the first. It keeps six gill slits where nearly every other living shark has five, and patrols deep water worldwide, rising toward the surface at night. Meeting one is the closest thing to diving with the Jurassic.
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 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.
During more than 500 diver hours off Indonesia, researchers watched veined octopuses dig up coconut-shell halves, stack them and carry them as far as 20 meters (66 feet) before assembling them into shelters. Because the shells provide no protection while being transported and are saved for later use, the behavior was classified as tool use.
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.
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 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.
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.
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.
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.
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.
Greenland shark stomachs have yielded remains of seals, reindeer and even polar bear, though the sluggish sharks are believed to scavenge much of it and ambush sleeping seals for the rest. In the Arctic food web, the slowest fish still collects the largest tolls.
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.
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.
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.
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.
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.
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.
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.
Jaws (1975) invented the modern summer blockbuster — wide release, saturation advertising, the first film past $100 million at the U.S. box office — and fixed the shark in the public mind as a man-hunting monster. Author Peter Benchley spent his later life publicly regretting the panic and campaigning for shark conservation. One fictional shark changed how a planet saw real ones.
Octopuses retune their nervous systems with the seasons: they can extensively edit their RNA, altering the proteins produced from the same genes. When California two-spot octopuses were acclimated to 13 °C (55 °F) instead of 22 °C (72 °F), editing changed at more than 20,000 sites, including sites in proteins involved in nerve signaling and intracellular transport.
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.
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.
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.
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.
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.
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.
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.
At the base of Davidson Seamount off California, about 3,200 meters (10,500 feet) down, roughly 6,000 nesting pearl octopuses were counted in a 2.5-hectare patch, and researchers estimate the full site holds more than 20,000 nests. Named the Octopus Garden, it is the largest octopus aggregation known anywhere. The animals migrate there to mate, nest and die.
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.
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.
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.
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.
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.
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.
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 trials with eight giant Pacific octopuses, two repeatedly jetted water at a floating pill bottle so a tank current carried it back, then jetted it away again. Researchers classified the repeated, non-feeding behavior as exploratory play. Later studies found similar play with other objects and in adults as well as juveniles.
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.
Harvard researchers identified a family of chemotactile receptors found only in cephalopods and expressed in the suction cups themselves. They detect molecules that dissolve poorly in water — exactly the compounds that would cling to a crab's surface rather than drift away. So an octopus probing a dark crevice is not groping blindly. Each sucker is reporting flavor.
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.
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."
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.
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.
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.
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.
To purge something indigestible, a shark can perform gastric eversion — pushing its whole stomach out through its mouth, rinsing it inside-out in seawater, then swallowing it back into place. Scientists have filmed the maneuver taking only seconds. It is the ocean's most extreme reset button.
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.
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.
A white shark tagged in South Africa — nicknamed Nicole — swam to Australia and back, about 20,000 km (12,400 miles) round trip in under nine months, the fastest transoceanic return migration then recorded for a marine animal. She navigated with no coastline for weeks at a stretch.
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.
Lacking the swim bladder most fish use to make noise, sharks were long declared voiceless. Then in March 2025 researchers published the first documented shark sounds: rig sharks in New Zealand snapping out 48-millisecond clicks, likely with their flattened teeth, when handled. Five hundred species just became an open question.
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.
Octopus photoreceptors are arranged in two channels that detect the orientation of polarized light. Behavioral tests show they can distinguish differences in polarization angle of only a few degrees. This can make transparent prey visible even when brightness and color provide little contrast.
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.
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.
Shark young are pups, and in nearly all species they are born fully armed and immediately abandoned; nursery lagoons, not mothers, keep them alive through their first years. A newborn lemon shark's first lesson is that no one is coming.
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.
Sharks retain urea and other compounds in their blood until their body fluids match or exceed seawater's concentration, so they never dehydrate in the ocean the way bony fish do. It is also why poorly handled shark meat reeks of ammonia. Sharks solved seawater chemistry from the inside.
Hundreds of suckers per arm will stick to almost anything they meet, which raises an obvious problem for an animal with eight boneless arms. Israeli researchers found that amputated octopus arms grabbed petri dishes readily but would not grab octopus skin, and gripped dishes smeared with skin extract about ten times more weakly. The signal works in the arm itself, with no help from the brain. The octopus solved knots with chemistry.
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.
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.
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.
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.
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.
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.
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.
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.
Stefano Lorenzini described the jelly-filled snout pores of sharks in 1678, but their electric-sensing purpose was only demonstrated experimentally in the 20th century — three centuries between discovery and explanation. Sometimes science finds the organ long before the sense.
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 central brain is not a lump above the mouth. It is a donut of fused lobes encircling the esophagus, split into a supraesophageal mass handling learning and memory and a subesophageal mass handling movement. Food passes through the middle of the brain on its way to the gut. Whatever an octopus swallows, it swallows through its own head.
Repeated behavioral tests have found consistent individual differences in octopus activity, reactivity and avoidance. Later work in another species found similar temperament differences early in life, showing that individuals of the same species can behave predictably differently.
An Australian aquarium's zebra shark named Leonie reproduced with a mate for years, then — separated from males — produced viable young alone; DNA testing in 2017 ruled out stored sperm and confirmed parthenogenesis. It was the first shark seen switching reproductive modes within one lifetime. She didn't lose the option; she changed the setting.
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.
The Greek roots behind Cephalopoda translate as head and foot, describing a body plan in which the two are fused. The eight arms are modifications of the same muscular foot that a snail crawls on, rearranged into a ring around the head. The mouth and beak sit underneath, at the center of that ring. An octopus is a mollusk that walks on a reinvented snail foot.
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.
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.
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 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.
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.
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.
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.
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.
The salmon shark's heart is loaded with SERCA2, a calcium-pump protein expressed at levels closer to a mammal's heart than a fish's, keeping cardiac muscle contracting in water that would stop most fish hearts. The finding, published in Science in 2005, helped explain how a warm-bodied shark patrols sub-Arctic seas. Its cold tolerance is written in molecules.
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.
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.
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.
That is every fact on the shelf — all 199 of them.
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