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.
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 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.
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.
Sawsharks wield flattened, tooth-edged snouts hung with a pair of trailing barbels, slashing through fish schools and raking sand for buried prey — unlike the larger sawfish (a ray) they resemble. It is a weapon, a metal detector and a shovel in one nose.
When the Moon formed about 4.5 billion years ago, the young Earth spun so fast that a full day lasted only around six hours. The Moon's tides have been braking the planet ever since — for about a billion years in the middle, the day even stalled near 19 hours while sun-driven tides in the atmosphere pushed back. Today the day is still stretching, by about two milliseconds every century.
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.
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.
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.
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.
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.
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.
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 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 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.
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.
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 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.
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.
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.
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 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.
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.
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.
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.
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.
The whale shark is the largest fish alive, and the biggest reliably measured individual reached 18.8 m (61 ft 8 in). It feeds almost entirely on plankton, straining tiny drifting life through a mouth up to 1.5 m (5 ft) wide. The ocean's biggest shark eats some of its smallest creatures.
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.
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.
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.
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.
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.
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.
Researcher Jennifer Mather observed a wild octopus leave its den, collect three rocks, set them aside, carry them back and stack them across the entrance before resting. The sequence has been discussed as possible planning, although that interpretation remains debated.
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.
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.
The great white shark gives more of its brain to smell than any other shark: its olfactory bulbs — the paired lobes that process scent — run to about 18% of its brain mass.
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.
The animal long called the common octopus, Octopus vulgaris, is now understood as a species complex rather than a single worldwide species. Genetic studies have identified at least six regional species that can be extremely difficult to distinguish by appearance alone.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 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.
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.
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.
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.
Field measurements put the Greenland shark's heart rate near one beat every ten seconds, among the slowest recorded for any fish — machinery throttled down for water near freezing. The same economy drives its glacial growth and centuries-long life. It is an animal living in slow motion.
Sharks of all species combined kill people in the single digits in a typical year — the International Shark Attack File's recent five-year average is eight unprovoked deaths worldwide — while mosquito-borne diseases kill hundreds of thousands, malaria alone about 600,000. In the coastal United States, lightning kills roughly 38 people a year to sharks' one every other year, and dogs about 35. Fear tracks teeth, not numbers.
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.
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.
At the Seattle Aquarium, eight giant Pacific octopuses met two unfamiliar people dressed the same way for two weeks: one always fed them, the other always touched them with a bristly stick. By the end, the animals showed measurably different body patterns, arm movements and funnel orientation toward each person, and the larger ones changed their breathing rate. The uniform trick was the point. They were not reading the clothes.
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.
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.
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.
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 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.
That is every fact on the shelf — all 199 of them.
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