Shark

• 1. Introduction

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  Sharks range from the dwarf lanternshark, barely 17 centimeters long, to the whale shark, the largest fish alive, which grows 12 to 18.8 meters. More than 500 species span thirteen orders in every sea. Shark-like scales date to the Late Ordovician, 458-444 million years ago, long before dinosaurs. These fish breathe through gill slits that never close and sense the world through channels no human body has. Since 1970, shark populations have dropped 71%, mostly from overfishing and finning. What lets an animal survive hundreds of millions of years? Why has the last half century been so much harder to survive than the rest?

• 2. Deep Time Origins

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  Cladoselache and Doliodus, two shark-like chondrichthyans, first appear in the Devonian Period, 419-359 million years ago. Acanthodians, or spiny sharks, date to the Early Silurian, around 439 million years ago. Fossils from the Early Permian of Russia, called synechodontiforms, may be the earliest true sharks. Modern shark orders diversified in the Early Jurassic, pushing aside the hybodonts that had dominated since the Triassic. The oldest known member of the modern lineage is a genus called Agaleus.

  Sharks sit in the division Selachii, within Elasmobranchii, within the class Chondrichthyes, which also holds the chimaeras. Once thought polyphyletic, sharks are now confirmed monophyletic by molecular studies, alongside their ray relatives, the Batomorphi. The division splits into two superorders, Galeomorphi and Squalomorphi, covering thirteen living orders.

  The Carcharhiniformes, or ground sharks, include the blue, tiger, bull, hammerhead and reef sharks. A nictitating membrane guards their eyes during an attack. The Lamniformes, or mackerel sharks, include the goblin, basking and megamouth sharks, the thresher, mako and great white. This order once held the giant Otodus megalodon too. The Orectolobiformes, or carpet sharks, include the zebra shark, nurse shark, wobbegongs and the whale shark. Sawsharks, in the Pristiophoriformes, carry a long, tooth-studded snout, while the flattened Squatiniformes, or angel sharks, resemble stingrays and skates.

  Classification below the superorder level stays unsettled. Many scientists doubt whether the proposed Hypnosqualea grouping is even valid.

• 3. Built To Hunt

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  A shark can shed 30,000 or more teeth over its lifetime, embedded in the gums rather than fused to the jaw. Most species replace one tooth at a time; the cookiecutter shark instead drops an entire row at once. Crustacean eaters carry flattened crushing teeth, fish eaters carry needle-like teeth, and predators of large mammals carry serrated, triangular cutting teeth. Plankton feeders like the basking shark keep teeth too small to function.

  The rib-less skeleton is built from cartilage, about half the density of bone. This saves energy but leaves sharks vulnerable to their own weight on land. The jaw is reinforced with hexagonal plates called tesserae, crystal blocks of calcium salts arranged like a mosaic. A bull shark, tiger shark or great white can carry two to three layers, and the biggest great whites carry as many as five.

  Most sharks swim with eight fins, built around elastic filaments called ceratotrichia rather than bony rays. Because of this, a shark can only drift toward something directly ahead of it, never tail first. The skin is armored in placoid scales, or dermal denticles, arranged in a helical pattern that smooths water flow. Zebra and tiger sharks carry pigmented denticles that form spots and stripes for camouflage, regrowing even after an injury heals.

  A shark's spine extends into the tail's upper lobe, giving most species a heterocercal tail with an oversized top half. Tiger sharks carry an especially large upper lobe for sudden speed bursts. The porbeagle relies on a large lower lobe to chase mackerel, while the thresher shark whips its upper lobe to stun fish and squid. In the snout, cartilage turns spongy enough to absorb the shock of a direct hit.

• 4. A World Of Senses

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  A shark can detect as little as one part per million of blood in seawater, through an olfactory duct that stays unfused between its two nostrils. Species hunting in murky water carry larger olfactory bulbs than reef sharks in the family Carcharhinidae. Sharks judge scent direction by comparing timing between nostrils, much as mammals localize sound. They often linger near sewage outfalls, drawn by intestinal waste. Nurse sharks carry external barbels to feel out buried prey.

  A tissue called the tapetum lucidum sits behind the retina, reflecting light to sharpen vision in dark water. Sharks can contract and dilate their pupils, a trick no bony fish can manage. Many species close a nictitating membrane while attacking, but the great white lacks one and instead rolls its eyes backward. A study using micro-spectrophotometry on 17 shark species found ten had only rod cells, giving strong night vision but no color perception. The other seven saw only in shades of grey.

  Most sharks hear best between 20 and 1000 Hz, through a small opening on each side of the head leading into the inner ear. That opening also connects to the lateral line.

  Hundreds to thousands of jelly-filled pores called the ampullae of Lorenzini give sharks the sharpest electrical sense of any known animal. They detect a living creature's electromagnetic field and find prey buried in sand. Hammerhead sharks lean on this sense especially heavily.

  Hair-like cells called neuromasts, tuned to 25-50 Hz, let a shark feel pressure changes from obstacles and struggling prey it cannot yet see. Its field of view can swap between monocular and stereoscopic at any moment.

• 5. Life Underwater

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  A shark's liver can make up to 30% of its body mass, packed with squalene oil that offsets its lack of a swim bladder. Sand tiger sharks instead gulp air into the stomach as a makeshift bladder. Nurse sharks stay negatively buoyant to rest on the seafloor. Stroked on the nose or flipped upside down, many sharks enter tonic immobility, a stillness researchers use to handle them safely.

  While swimming, sharks push water over the gills by moving forward, called ram ventilation. At rest, most instead pump water across the gills. A few species have lost that ability and must swim constantly or suffocate. The spiracle, a modified opening behind the eye, helps bottom-dwelling sharks draw in water.

  Most sharks are poikilothermic, matching the sea's temperature. The Lamnidae family, including the shortfin mako and great white, run warmer via red muscle and a heat-trapping network called the rete mirabile, or "miraculous net". The whale shark instead relies on sheer size to hold body heat when diving cold. The scalloped hammerhead closes its mouth and gills while diving to around 800 meters to stay warm.

  Shark blood runs isotonic with seawater through a urea concentration up to 2.5%, plus a compound called trimethylamine N-oxide. A dead shark's body smells of ammonia as bacteria break that urea down. The bull shark alone evolved kidneys that excrete large amounts of urea. In 1930, researcher Homer W. Smith found shark urine too low in sodium to explain their salt balance. In 1960, scientists at the Mount Desert Island Biological Laboratory in Salsbury Cove, Maine, traced the answer to a salt-secreting rectal gland.

  Food in a shark's J-shaped stomach can take a long time to digest. If indigestible, the shark vomits it or turns its stomach inside out through its mouth. A structure called the spiral valve packs a long absorptive surface into a short intestine. Under blue light, the swell shark and chain catshark glow, fluorescing from a metabolite of kynurenic acid.

  Sharks stay common to about 2000 meters and grow rare below 3000. The deepest confirmed sighting, a Portuguese dogfish, was recorded at 3700 meters. The bull shark and river sharks move between salt and fresh water, while the Ganges shark lives in freshwater exclusively.

  A cruising shark covers about 8 km/h, jumping past 19 km/h when feeding. The shortfin mako, the fastest shark alive, can burst to 50 km/h, and the great white manages similar bursts. Sharks can cover 70 to 80 km in a day.

  Sharks that breathe via spiracles can rest motionless without truly sleeping. Species that must swim to breathe undergo sleep swimming instead, driven by the spinal cord rather than the brain in the spiny dogfish. In 2016, a great white shark was filmed in this state for the first time.

• 6. Feeding Strategies

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  Basking, whale and megamouth sharks each filter plankton differently. Basking sharks use ram feeding with mouths open, whale sharks use suction, and megamouth sharks lure prey with luminescent mouth tissue in the deep ocean. All three rely on gill rakers, sieve-like filaments similar to the baleen of great whales.

  Cookiecutter sharks slice circular plugs of flesh from far larger animals, using teeth that look enormous against their small bodies. Researchers believe they latch on with thick lips and twist to tear flesh free.

  Angel sharks and wobbegongs camouflage on the seabed and suck in passing prey. Many bottom dwellers crush crustaceans with flat, molar-like teeth.

  The viper dogfish points its teeth outward to strike and swallow prey intact. Sawsharks stir prey from the seabed or slash at swimming fish with their tooth-studded rostra.

  The bonnethead shark is the only known omnivorous species, eating mostly crustaceans and mollusks but also enough seagrass to extract nutrients from roughly half of what it consumes.

  Some social species, including the whitetip reef shark, hunt cooperatively in packs to herd prey no lone shark could corner.

• 7. Birth And Behavior

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  Most sharks live 20-30 years, though the spiny dogfish can live more than 100 and whale sharks may too. A study of a 5.02 meter Greenland shark aged it at 392 years, give or take 120, making it the longest-lived vertebrate known.

  Sharks are K-selected reproducers, investing in a small number of well-developed young. A single cycle can produce 2 to over 100 young. Maturity arrives late; lemon sharks mature only around ages 13-15.

  Males carry modified pelvic fins called claspers, used to deliver sperm internally, roughly analogous to a mammalian penis. Mating is rarely observed: catsharks mate with the male curling around the female, while less flexible species swim parallel as he inserts a clasper. Females in larger species often carry bite marks from males gripping them to hold position.

  Documented cases exist of female sharks producing pups through parthenogenesis without contacting a male, confirmed by genetic fingerprinting that ruled out stored sperm. Scientists call it a rare, last-resort strategy that may have contributed to the blue shark's decline off the Irish coast. Mammals are the only major vertebrate group in which it has never been observed.

  Other species, including the horn shark, catshark, Port Jackson shark and swellshark, lay eggs in a leathery case called a mermaid's purse. Hammerheads and the requiem sharks, including the bull and blue sharks, instead give birth to fully formed young with no egg stage.

  The image of the shark as a lone hunter fits only a few species. Most live social, sedentary lives with apparent distinct personalities. More than 100 scalloped hammerheads have been recorded congregating around seamounts in the Gulf of California. Oceanic whitetip sharks dominate similarly sized silky sharks at feeding sites. Even solitary sharks travel thousands of miles a year to shared breeding or hunting grounds, in migrations that may rival birds in complexity.

  Sharks carry brain-to-body ratios similar to mammals and birds, showing curiosity and play-like behavior. Juvenile lemon sharks show observational learning from watching other sharks investigate objects. Approached too closely, a shark may respond with a threat display, an exaggerated swimming burst whose intensity tracks how threatened it feels.

• 8. Sharks And Humans

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  In 2006, the International Shark Attack File investigated 96 alleged attacks, confirming 62 as unprovoked and 16 as provoked. Between 2001 and 2006, unprovoked attacks averaged 4.3 deaths worldwide a year. Of more than 470 known species, only four, the great white, oceanic whitetip, tiger and bull shark, have driven a significant number of fatal unprovoked attacks.

  Researchers believe most attacks happen when a shark mistakes a swimmer for usual prey like a seal. A California State University Shark Lab drone study showed juvenile sharks swimming up to people without biting, leading researchers to conclude humans and sharks can share the water.

  Public fear was shaped by isolated incidents like the Jersey Shore attacks of 1916, and by fiction, especially the Jaws film series, based on Peter Benchley's novel. Both Benchley and director Steven Spielberg later spoke out against the man-eating image their work helped create.

  The Monterey Bay Aquarium became the first facility to hold a great white shark long-term, keeping a young female for 198 days in September 2004 before release. Before that, only benthic species like hornsharks, leopard sharks and catsharks had reliably survived a year in captivity. Home-aquarium species need tanks built for adult lengths near 3 feet and lifespans to 25 years.

  In Hawaiian mythology, families claimed a shark aumakua, a deified guardian believed to drive fish into their nets and ward off danger. Kamohoali'i, the most revered shark god and older brother of the goddess Pele, could take human or fish form and held a sacred site on the Kilauea crater's summit cliff. On Tutuila in American Samoa, the site called Turtle and Shark preserves a legend of two humans transformed into those animals.

  Sharks are not immune to cancer or disease, despite persistent belief resting on only anecdotal support. Claims that fins prevent cancer or cartilage treats osteoarthritis have no scientific backing.

  In 2008, researchers estimated nearly 100 million sharks were killed yearly by commercial and recreational fishing. Fin yields alone implied about 100 million killed in 2000 and 97 million in 2010, with fins selling for around $300 a pound by 2009. Shark fin soup remains a status symbol in parts of Asia, despite high concentrations of the neurotoxin BMAA found in fins.

  Queensland's government drum lines have killed roughly 50,000 sharks since 1962, including 10,480 between 2001 and 2018. New South Wales nets killed 352 tiger sharks and 577 great white sharks between 1950 and 2008, among 15,135 marine animals total. George H. Burgess, former director of the International Shark Attack File, called culling 'a form of revenge, satisfying a public demand for blood and little else.'

  South Africa became the first country to legally protect great white sharks, in 1991. The U.S. closed a major finning loophole with the Shark Conservation Act, passed December 2010 and signed into law that January. By January 2019, twelve U.S. states and three Pacific territories had banned the sale or possession of shark fins. A 2020 survey of 371 reefs across 58 nations found no sharks on nearly one in five reefs studied, a gap tied to how well each nation protected them.