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.
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.
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.
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.
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.
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 have no gas-filled swim bladder; buoyancy comes from an enormous oil-rich liver that can approach a quarter of a shark's weight in some species. The oil, squalene, is lighter than water, letting a heavy cartilage body hang almost weightless. A shark floats on its own chemistry.
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.
The Port Jackson shark lays a spiral, corkscrew-shaped egg case that the mother wedges into rock crevices, where the threads lock it against the surge. Beachcombers who find washed-up cases call them mermaid's purses. Evolution invented the screw thread long before people did.
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.
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.
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.
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.
Blue-ringed octopus saliva contains tetrodotoxin, a potent neurotoxin with no known antivenom. Symbiotic bacteria in the posterior salivary glands produce the toxin, and females also coat their eggs with it.
When food is thick, a whale shark can hang nearly vertical in the water and gulp — using suction rather than forward motion to draw in prey-rich water. Snorkelers at feeding aggregations often meet the giants bobbing upright like feeding balloons. The largest fish can dine without swimming an inch.
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.
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.
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.
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 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.
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.
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.
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.
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 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.
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.
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.
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.
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.
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