Whale sharks are the largest fish in the world, and almost nothing about their bodies is typical of a shark. The mouth sits at the very front of the head, rather than underneath it. The eyes are armored with scales and can be pulled inside the skull. The skin is the thickest of any animal. And roughly a third of the body length is taken up by a mouth-and-gill chamber built to strain plankton out of the sea. This tour of the world’s biggest fish draws on Whale Sharks: Biology, Ecology, and Conservation, the first scientific textbook on the species.
What makes a whale shark’s body plan unusual?
Almost every distinctive feature of a whale shark (Rhincodon typus) traces back to one job: filtering tiny animals from open water. The changes start at the front of the head and run all the way to the tail.
Most sharks have a snout — a rostrum — that sticks out in front of a mouth set underneath. Its flat underside works like a wing, generating lift at the front of the animal to balance the lift the tail generates at the back. A whale shark has no rostrum at all. Its mouth is terminal: it opens right at the front of the head, and it is the only living shark built that way. The missing lift is made up for by unusually large, broad pectoral fins and the wide flat expanse under the head — the same planing surface, moved behind the mouth instead of in front of it.
Behind the head, long ridges and valleys run down the body and join the keels either side of the tail base. The first dorsal fin — the classic “shark fin” — sits about halfway down the body, much further back than on a reef shark or a tiger shark. Otherwise the fins are unremarkable in everything except size.
How big is a whale shark’s mouth?
A whale shark’s mouth can be up to 1.5 meters (5 ft) across. It is wide, square, and blunt, and it works less like a bite and more like an intake.
That shape suits the way whale sharks feed. They tilt the body up at roughly 25° — often with the top jaw clear of the water — and actively suck in mouthfuls of the very surface layer, along with whatever is floating in it. Basking sharks, the other giant filter-feeding shark, do have a snout and cannot feed this way; they simply drive forward with the mouth held open. Whale sharks do that too, but suction feeding is their more common mode, which makes them the filter feeder most specialized on animals living at or just under the sea surface. There is more on how and what they eat in our guide to whale shark feeding.
The very wide mouth has one further side effect: it sets the nares — the openings equivalent to nostrils — remarkably far apart. More on that below.
Do whale sharks have teeth?
Yes. Whale sharks have thousands of teeth, and they are useless for eating. The jaws carry up to 300 rows of teeth running from one corner of the mouth to the other, and each row is up to 16 teeth deep, front to back. Every tooth is tiny, with a single simple teardrop-shaped point angled backward.
They play no part in feeding. What they did do was name the animal: of all the extraordinary features Andrew Smith could have picked in 1828, it was these apparently vestigial teeth that gave the whale shark the genus name Rhincodon, from the Greek for “rasp tooth”. There may have been method in that, because teeth are usually the only part of a shark that fossilizes — tooth shape is how paleontologists recognize Rhincodon in rocks 23 million years old.
Just behind the teeth, in both jaws, sit several velums — flaps of skin anchored across the roof and floor of the mouth. When the shark closes its mouth these act as one-way valves, sealing the mouth and forcing water out through the gills instead of back out the front.
How do whale sharks filter feed?
Water goes in the mouth, food is separated inside the throat, and the water leaves through the gills — but exactly how the food gets separated is still an open question. The filtering is done by 20 filter pads, five upper and five lower each side of the throat, evolved from the gill rakers other sharks use.
Behind the jaws lies a mouth-and-gill chamber so large that it reaches back to the middle of the pectoral fins — perhaps 30% of the body length. The whale shark is, functionally, a swimming mouth. That chamber holds ten gills and the 20 pads, the lower ones bigger than the upper and the rearmost biggest of all.
The pads themselves are not the fine comb of a basking shark or a megamouth shark. They are a flattened, net-like mesh of irregular holes about a millimeter across, held open by cartilage vanes, and the individual pads are linked by a flexible ridge of tissue so the whole basket can expand and contract. Bigger whale sharks also grow denticles — small tooth-like scales — on the pad surfaces, which are thought to protect the mesh underneath; newborns do not have them. The arrangement means water cannot simply spill out of the gills in bulk. Anything that enters the mouth is swallowed, passes through the mesh, or gets coughed back out — and whale sharks do exactly that, backflushing a clogged filter with a motion that looks very much like a cough.
The unresolved part is the separation itself. The leading explanation is crossflow filtration: water is directed along the face of the pads rather than straight through them, so particles are swept toward the back of the throat still in suspension, gathering into a mouthful that can be swallowed, while the water exits sideways over the gills. It works rather like a plankton net towed behind a boat, and it neatly explains why the filters do not clog even in a dense patch of prey. It does not explain everything. Whale sharks eat fish spawn under 0.8 mm across, smaller than their own mesh pores of about 1.2 mm, and crossflow filtration should not catch those. They may use something else again, such as the “ricochet separation” described in manta rays.
There is one more opening: a spiracle, a small hole just behind each eye connecting the inside of the mouth to the outside. In whale sharks it is thought to do very little. Its main observable function is providing a home for small remoras, whose tails can often be seen sticking out of it.
Can whale sharks see well?
Probably not — but their eyes are the most heavily protected of any shark. Whale shark eyes are small for the animal’s size, and their sharpness of vision is likely limited to a few meters.
The front-facing mouth puts the eyes at the front corners of the head, where they are the first thing to hit an obstacle. Two protections have evolved, both apparently unique to the species. Whale sharks have no eyelids and no nictitating membrane (the “third eyelid” many sharks have). Instead, each eyeball is covered in around 2,000 dermal denticles — tooth-like scales with a bramble shape that looks built to resist abrasion. No other shark is known to have scales on the eyeball itself. For anything more serious, a whale shark can retract the eyeball into its head. A cartilage bar links the eye to the side of the braincase, and muscle flexes that bar to pull the eye in and rotate it downward.
A partial downward rotation also happens without retraction, when a shark is suction feeding with its body tilted up past 25°. That helps it see ahead, although a blind spot directly in front of the animal remains.
As for how much it sees: whale shark eyes are small relative to the body, the pupil is circular, and if the retina resembles that of its close relatives it is dominated by rods with a single cone type. That would mean no color vision and no fine detail — an animal that senses light and movement rather than reading a scene, with useful acuity somewhere around three to five meters. What sensitivity they have is tuned unusually far toward blue: the whale shark genome predicts the most “blue-shifted” visual pigment of any known shark, peaking at 478 nm, which fits an animal that dives to nearly 2,000 meters where only blue light remains.
How well can whale sharks smell?
Almost certainly better than they see, and probably in stereo — though the anatomy has never been properly described. Because the mouth is so wide, a whale shark’s nares sit further apart than in any other shark, including the largest hammerhead.
That matters because wide-set nostrils are exactly what a hammerhead’s flattened head is thought to buy: comparing the smell arriving at each side reveals which direction it came from. A whale shark’s nares are both large and more widely separated than a hammerhead’s — it is simply less obvious, because the head lacks the distinctive hammer shape. On that basis, the book argues, their directional smelling ought to be excellent.
This is an inference from head shape rather than a measurement. The olfactory capsules of a whale shark have only ever been described in passing, as moderately large and spherical, in a study from 1937. No detailed description of the olfactory rosette — the folded sheet of smelling tissue inside the nares — has ever been published for the species. Whale sharks also have small skin extensions beside the nares called barbels, whose function is unknown; in nurse sharks and wobbegongs, barbels appear to sense touch rather than smell.
There is one piece of direct evidence that smell drives feeding. Captive whale sharks began feeding when exposed to dimethyl sulfide — the chemical phytoplankton release when zooplankton graze on them, and the same cue seabirds use to find productive water — and to a filtrate of homogenized krill. It has even been suggested that whale sharks may smell it in the air, since the nares are held clear of the surface while they suction feed.
What other senses do whale sharks use?
The whale shark has the largest inner ear of any animal — and nobody has ever measured its hearing. That gap is typical of the whole sensory picture: the equipment is remarkable, the testing has not been done.
Sharks hear low-frequency sound, roughly 40 to 1,000 Hz, and low sound travels a long way underwater, so it is often the first cue an animal gets that something is happening. A larger inner ear may mean greater sensitivity, which would make sense for an animal that has to find widely scattered patches of food. Off Utila, in Honduras, whale sharks were regularly seen at schools of feeding tuna — noisy, splashing fish, and a source of the tuna spawn the sharks were there to eat.
Two other senses are complete blanks. The lateral line — the row of pressure-sensing organs that lets a fish feel water moving along its body — has never been studied in whale sharks, and neither has the electrosensory system, the pores on a shark’s head that detect the faint electrical fields living animals give off. Taste has not been examined either.
How big is a whale shark’s brain?
Small for the body, with an unusually large cerebellum — the part that governs coordinated movement. Compared with other giant sharks, including the basking, megamouth, and great white sharks, the whale shark shows a marked reduction in relative brain size.
That may not be a story about intelligence so much as one about growth: body size increased enormously without the brain following. Filter feeding is also less cognitively demanding than chasing fast prey, and low maternal investment tends to go with smaller brains across sharks and rays — whale shark embryos live on their yolk sacs, with no placenta-like connection.
Where whale sharks stand out is the cerebellum, which takes up close to 30% of the brain and is deeply folded. Only the basking shark and the thresher sharks come close. The cerebellum handles motor control and the body’s sense of its own position and motion, which fits an animal that ranges across ocean basins, dives to nearly 2,000 meters, and can hang vertically in the water column to feed. The midbrain, associated with vision, is small at about 6.5% — consistent with everything else that says vision is not their main sense. Statistically, in fact, the whale shark’s brain resembles the basking shark’s more than any other shark’s, despite the two being only distantly related: two giant filter feeders arrived at the same brain from different starting points.
All of this rests on very little material: the hemisected brains of two juvenile males, 6 to 6.5 meters long, supplied by an aquarium, plus one preserved newborn imaged in a museum collection by MRI. In the newborn the brain nearly fills the braincase; in the juveniles it sits in a much larger space filled with fluid and a cobweb of connective tissue, whose purpose is unknown. No female brain has ever been described.
How thick is whale shark skin?
Up to 15 centimeters (6 in) — the thickest skin in the animal kingdom. It is also one of the whale shark’s most important adaptations, and not only as armor.
The structure is two-layered: a thin, tough, pigmented outer layer holding the denticles, and beneath it an extraordinarily thick, rubbery, white inner layer of dense collagen. That inner layer does several jobs at once. It provides genuine structural support for an animal with no bones — one likely reason a cartilage skeleton can carry this much body at all. It holds in heat. And it is a formidable defense: a predator biting a whale shark mostly comes away with a mouthful of gristle. The same layer frustrates researchers trying to anchor a satellite tag.
Whale shark skin is not just passive plating. Wounds close remarkably fast, with the rate depending on the injury: shallow abrasions reach 90% closure in around three weeks, deeper cuts take closer to seven. That is useful well beyond the biology, because it lets researchers tell a fresh boat strike from old scarring in a photo-identification record — the difference between a threat happening now and one that happened years ago.
The skin is also a habitat. A global survey of the whale shark skin microbiome that we co-authored sampled sharks from populations around the world and found that, although the mix of microbes differs between oceans, the communities converge on a similar overall architecture. It is a whole layer of whale shark biology the textbook does not cover — the animal’s outermost surface is a living community, not just plating.
Why do whale sharks have spots?
Nobody knows for certain. The leading explanations are camouflage-related: countershading, breaking up the animal’s outline, and possibly protection from ultraviolet light. None has been tested directly.
The pattern itself is worth a closer look. Ahead of the pectoral fins the spots are irregular; behind them they settle into orderly alternating rows of stripes and spots. The whale shark’s relatives offer one clue. The zebra shark, among its closest living relatives, has both the skin ridges and a spotted pattern. But its spots are dark on a light background — the reverse of a whale shark’s white-on-gray. The tawny nurse shark, also a close relative, has neither ridges nor spots.
One thing about the spots is beyond doubt, and it changed the study of the species. Each pattern is unique to the individual and stable for life, so a decent photograph works as well as a tag. The matching is done by software adapted from astronomy, originally written to match star fields, and more than 17,000 whale sharks have now been identified this way on Sharkbook.ai, the global database our team helps curate. Almost everything known about how long whale sharks live, where they go, and how their numbers are changing rests on those photographs — see our guides to whale shark movements and lifespan.
Are whale sharks warm-blooded?
No, but they are very good at staying warm. A whale shark’s body temperature tracks the water it is in, yet it changes so slowly that a shark can carry surface heat down into the cold deep.
The one direct study of body temperature found no sign that muscle activity warms the animal — and the red muscle, which generates heat in genuinely warm-bodied fishes like tuna, sits close to the skin in whale sharks rather than deep in the body core. What that study did find is that whale sharks have the lowest heat transfer coefficient of any fish yet measured: they gain and lose heat more slowly than any other fish tested. Sheer size does the work, helped by that thick insulating skin.
Biologists call this strategy gigantothermy, and whale sharks share it with leatherback turtles and, in all likelihood, the sauropod dinosaurs. In practical terms, a whale shark can warm up in tropical surface water, stay functional for hours hundreds of meters down in water a few degrees above freezing, then return to the surface to warm up again. It is one reason a tropical fish can feed in the deep sea.
Being built this way ties the animal tightly to water temperature, which is why warming oceans are a physiological problem and not only a habitat one. An energy-budget study we co-authored projects that warming will raise a whale shark’s energy demand while shrinking the water that can meet it. Our guide to whale shark threats takes that further.
What are a whale shark’s closest relatives?
Whale sharks are carpet sharks — the group that also contains the bottom-dwelling wobbegongs and nurse sharks. They are the only species in their genus, and the only living genus in their family, so there is nothing much like them alive.
Genetic work puts the whale shark’s closest living relatives as the short-tail nurse shark and the zebra shark, with the tawny nurse shark and the nurse shark slightly more distant again. All are modest-sized, mostly seafloor animals; the largest, the tawny nurse shark, reaches around 4 meters, less than a quarter of the biggest whale sharks. Whale sharks are the group’s only open-ocean plankton feeder, and their family has no other giants in the fossil record either — the whale shark’s size is not a leftover from some earlier dynasty of huge carpet sharks. There is more on how they compare with other ocean giants in our guide to whale shark size.
The differences run deeper than size. Those relatives either lay eggs or bear a few dozen live pups; whale sharks carry around 300 embryos at once — see our guide to whale shark reproduction.
Why it matters for conservation
A body this specialized has limits. Whale sharks feed at the surface, which is where boats are; their thick skin heals well but cannot stop a propeller; their slow, plankton-powered lives leave little room for extra losses. The IUCN Red List lists whale sharks as Endangered, and knowing how the animal is built is what turns a sighting into evidence — a healing wound that dates a strike, a spot pattern that identifies a returning individual, a dive profile that shows which waters need protecting.
You can support that work directly by adopting a whale shark.
Further Reading:
Whale Sharks: Biology, Ecology, and Conservation is the first scientific textbook on whale sharks, co-authored by MMF co-founder and Principal Scientist Dr. Simon J Pierce.
More on the species: whale shark facts, our whale shark research program, and where to swim with whale sharks.