Sharks: The Biology Behind the Ocean's Most Misunderstood Predators

Sharks have been refining the same basic body plan for more than 400 million years, surviving multiple mass extinctions that wiped out the vast majority of life on Earth. That longevity comes down to a set of adaptations found nowhere else in quite the same combination: a skeleton made of cartilage instead of bone, an electric sense capable of detecting a prey animal's heartbeat through sand, and a reproductive strategy so slow that it has become the single biggest obstacle to their survival now that humans, not ancient predators, are the threat.

There are more than 500 recognized shark species, ranging from a lanternshark small enough to fit in a hand to a filter-feeding whale shark the length of a school bus, and understanding how a handful of shared traits work explains both why sharks have persisted so long and why so many species are now in serious trouble.

Quick Facts About Sharks

Fact	Information
Common name	Shark
Scientific classification	Class Chondrichthyes, subclass Elasmobranchii
Number of species	536 shark species formally assessed in the IUCN's 2021 global review, part of a broader group of roughly 1,200 sharks, rays, and chimaeras; new species are still being described
Size	From under 20 cm (dwarf lanternshark) to about 12 m (whale shark)
Diet	Highly variable by species — fish, marine mammals, invertebrates, and, in filter feeders, plankton
Habitat	Every ocean, from shallow reefs and coastlines to open ocean and the deep sea
Geographic range	Worldwide in marine waters; a small number of species tolerate freshwater
Lifespan	Commonly decades; the Greenland shark is estimated to live for centuries
Activity pattern	Varies by species — many are most active at dawn, dusk, or night
Conservation status	Varies sharply by species — about 36% of assessed shark species are classified as threatened (IUCN, 2021)
Population trend	Declining overall, driven primarily by overfishing

What Actually Makes a Shark a Shark?

Sharks belong to a group called Chondrichthyes, the cartilaginous fishes, which also includes rays, skates, and chimaeras. The defining feature is right in the name: a skeleton built entirely from cartilage rather than bone, which is lighter, more flexible, and heals differently than bone tissue. Shark skin is covered not in flat scales but in tooth-like structures called dermal denticles, tiny, backward-pointing ridges that reduce drag as water flows over them, which is part of why shark skin feels rough when rubbed one direction and smooth the other. Sharks also lack a swim bladder, the gas-filled organ most bony fish use to control buoyancy, forcing them to solve the same problem with entirely different anatomy, described below. As a lineage, sharks or their close relatives appear in the fossil record more than 400 million years ago, predating trees, and have persisted through multiple mass extinction events that eliminated the vast majority of other vertebrate lineages.

How Sharks Float Without a Swim Bladder

Without a gas-filled swim bladder, sharks rely on two separate systems to avoid sinking. The primary one is an oversized, oil-filled liver, which in some species can make up a quarter or more of total body mass; the liver oil, largely a compound called squalene, is less dense than seawater and provides steady, passive lift, functioning as a kind of biological flotation device rather than an active pump. 

The second system is behavioral rather than anatomical: most sharks generate additional lift the way an airplane wing does, using their pectoral fins and body shape to convert forward swimming motion into upward force, which is why a resting shark with no water flowing over its fins will often sink rather than hover in place. This buoyancy strategy connects directly to how sharks breathe. Many species, including great whites and mako sharks, are obligate ram ventilators, meaning they must keep swimming forward to force oxygenated water over their gills, and stopping entirely can be fatal. Other species, such as nurse sharks and many reef sharks, can pump water over their gills using cheek and mouth muscles even while stationary on the seafloor, a technique called buccal pumping that lets them rest without swimming.

The Electric Sense That Lets Sharks Find Prey They Can't See

Scattered across a shark's snout are hundreds to over a thousand small pores called the ampullae of Lorenzini, gel-filled canals connected to electroreceptor cells capable of detecting extraordinarily faint electric fields in the surrounding water. Every living animal generates a weak electric field through ordinary muscle activity and cellular processes, and sharks are thought to be among the most electrically sensitive animals known, with some research suggesting they can detect voltage gradients on the order of a few billionths of a volt per centimeter. In practice, this lets a shark locate prey buried under sand or hidden in murky water purely from its electrical signature, without needing to see, smell, or hear it, which is especially useful for bottom-feeding species and for hammerheads, whose unusually wide, flattened heads spread the ampullae over a larger area, functioning something like a metal detector sweeping over the seafloor. The same system may also help sharks orient over long migrations, since ocean currents moving through Earth's magnetic field generate faint electric fields large enough for a shark's electroreceptors to detect and use as a navigational cue.

Endless Rows of Teeth: How Sharks Replace What They Lose

A shark's mouth doesn't contain a fixed set of teeth the way a mammal's does. Instead, teeth are arranged in multiple rows behind the functional front row, embedded in the gum tissue rather than fused to the jawbone, and as front teeth are lost to wear, breakage, or feeding, replacements rotate forward from behind on an ongoing basis throughout the shark's life. 

Depending on species and diet, a shark can go through many thousands of teeth over a lifetime, and because tooth shape is closely tied to feeding strategy, it varies enormously across species: triangular, serrated teeth suited to slicing flesh in species like the great white, needle-like teeth for gripping slippery fish in species like the mako, and flattened, plate-like teeth for crushing shellfish in species like the Port Jackson shark. This continuous replacement system means a shark's dental arsenal is essentially self-maintaining, which matters given how much physical stress feeding puts on individual teeth.

Why Sharks Reproduce So Slowly, and Why That Matters

Shark reproduction spans a wider range of strategies than almost any other vertebrate group. Some species are oviparous, laying tough, leathery egg cases, sometimes called mermaid's purses, that develop unattended in the environment. Most species are ovoviviparous, retaining fertilized eggs inside the body until birth without a placental connection to the mother. A smaller number of species are fully viviparous, developing a placenta-like structure that transfers nutrients directly from mother to embryo, similar in function to mammalian pregnancy.

What unites nearly all shark species, regardless of reproductive mode, is that the process is slow: gestation can run well over a year in some species, litter sizes are typically small compared to bony fish, and many sharks don't reach sexual maturity until they're a decade or more old, in some species not until their twenties or thirties. This combination of traits, long gestation, small litters, and late maturity, means shark populations simply cannot rebound from heavy fishing pressure the way faster-reproducing fish can, and it's the central biological reason overfishing has hit sharks disproportionately hard compared to most other marine animals.

Conservation: A Slow-Reproducing Group Under Fast Pressure

Shark conservation status has to be read against the reproductive constraints described above. A widely cited 2013 study led by Dalhousie University researcher Boris Worm, published in the journal Marine Policy, estimated that roughly 100 million sharks were killed by commercial fishing in 2000, with a plausible range between 63 million and 273 million annually given the uncertainty in unreported and discarded catch, and found that sharks were being fished at a rate exceeding their average population rebound rate. A more recent and more comprehensive assessment, published in Current Biology in 2021 by the IUCN's Global Shark Trends project, formally evaluated 536 shark species and found that about 36% are now classified as threatened with extinction, more than double the proportion found in a comparable 2014 assessment, making sharks and rays collectively the second-most threatened vertebrate lineage after amphibians.

The primary driver is overfishing, both targeted catch for fins, meat, and liver oil, and incidental bycatch in fisheries aimed at other species; coastal and tropical species face disproportionately high risk because they overlap most heavily with human fishing activity. Conservation responses include CITES trade restrictions on the most heavily targeted species, national and regional finning bans that require sharks to be landed with fins still attached, and a small number of countries that have established dedicated shark sanctuaries in their waters. Status still varies enormously by species: some coastal and pelagic sharks remain data deficient or of lower concern, while many hammerhead, mako, and thresher shark species are now classified as endangered or critically endangered.

Ecological Role

As apex or near-apex predators in most marine food webs, sharks help regulate the population and behavior of the species below them, and removing sharks from an ecosystem has measurable downstream effects rather than simply leaving a gap. In one well-documented case along the U.S. East Coast, the decline of large shark populations was linked by researchers to a subsequent boom in one of their prey species, cownose rays, which in turn increased predation pressure on scallop beds, illustrating how a shark decline can ripple through several additional links in a food web before the effect becomes visible to people. Beyond direct predation, sharks influence prey behavior simply through the risk of being hunted, often keeping herbivorous fish and other grazers moving across a wider area rather than concentrating in one spot, which can help prevent overgrazing of coral reefs and seagrass beds. Scavenging species also play a role in removing dead or dying animals from the water column and seafloor, contributing to nutrient cycling in ways that are still being actively studied.

Fascinating Facts About Sharks

Not all sharks need to keep swimming to survive. As covered above, obligate ram ventilators like great whites do need constant forward motion to breathe, but species capable of buccal pumping, including nurse sharks, can rest motionless on the seafloor indefinitely.

Some sharks are warm-bodied in a cold ocean. A small number of species, including the great white and several mako sharks, are regional endotherms, meaning they can keep certain muscles and organs warmer than the surrounding water using specialized blood vessel arrangements, which improves swimming performance and digestion in cold water.

The whale shark, the largest living fish, eats almost nothing but plankton. Despite reaching lengths comparable to a school bus, whale sharks are filter feeders, straining tiny plankton and small fish from the water rather than hunting large prey.

Greenland sharks may be the longest-lived vertebrates known. Radiocarbon dating of eye tissue in a 2016 study estimated some individuals at several centuries old, with sexual maturity possibly not reached until around 150 years of age, an extreme example of the slow-life-history pattern common across the group.

Shark skin inspired real engineering. The drag-reducing structure of dermal denticles has been studied and adapted in swimsuit fabric design and some aircraft surface coatings aimed at reducing friction.

A shark's bite is only part of its sensory toolkit. Beyond electroreception, sharks have a highly developed sense of smell capable of detecting extremely dilute concentrations of chemical cues in water, along with a lateral line system that senses vibration and water movement from nearby objects.

Most shark species are not a meaningful threat to humans. Of the more than 500 recognized species, only a small handful are regularly involved in documented incidents with people, and unprovoked shark bites overall remain a rare event relative to the number of people who enter shark habitat every year.

Frequently Asked Questions

Do sharks have to keep moving or they'll die?
Only some species. Obligate ram ventilators, including great white and mako sharks, need continuous forward swimming to push oxygenated water over their gills. Other species, like nurse sharks, can actively pump water over their gills while resting, so the "must always swim" rule doesn't apply to sharks as a whole.

Why do sharks need to be fished less to recover, compared to other fish?
Most sharks mature late, sometimes not until well into their second or third decade of life, and produce relatively few offspring per reproductive cycle compared to bony fish. That combination means a heavily fished shark population takes far longer to rebuild than a similarly fished population of a faster-reproducing species.

Are all sharks dangerous to humans?
No. The overwhelming majority of shark species pose little to no realistic threat to people, either because of their small size, deep-water habitat, or diet of small prey. Only a limited number of larger species are involved in the rare documented incidents that make headlines.

How do sharks find prey they can't see?
Primarily through electroreception via the ampullae of Lorenzini, which can detect the faint electrical field produced by any living animal's muscle activity, supplemented by an acute sense of smell and a lateral line system that detects water movement and vibration from nearby objects.

Why are shark fins specifically such a big conservation issue?
Shark fins are extremely valuable in certain international markets, primarily for shark fin soup, which has driven a large-scale, historically under-regulated fishery focused specifically on removing fins, sometimes from live sharks that are then discarded. This targeted demand adds pressure on top of general overfishing and bycatch, which is why finning bans specifically are a common conservation policy tool.

Do sharks lay eggs or give live birth?
Both, depending on species. Some sharks are oviparous and lay egg cases externally; most are ovoviviparous, retaining eggs internally until birth without a placental connection; and a smaller number are fully viviparous, with a placenta-like structure connecting mother and embryo during development.

Sources & Further Reading

• IUCN SSC Shark Specialist Group — Frequently Asked Questions
• Current Biology / Global Shark Trends Project via James Cook University — "Sharks and rays under threat"
• Smithsonian Magazine — "More Than One-Third of Shark Species Are Threatened With Extinction"
• Dalhousie University — "Unsustainable fishing: Dal researchers on the global shark decline"
• Florida Museum of Natural History (University of Florida) — Shark Biology
• ReefQuest Centre for Shark Research — Electroreception in the White Shark
• The Pew Charitable Trusts — "New Study Says 100 Million Sharks Killed Annually"