There Is Safety in the Dark

My soul is full of longing

For the secret of the sea,

And the heart of the great ocean

Sends a thrilling pulse through me.

—Henry Wadsworth Longfellow, “The Secret of the Sea”

Just a couple miles from the Pacific Ocean where his subjects swim, Chris Lowe directs the Shark Lab at California State University in Long Beach. There, he has led research into the lives of sharks and other fishes for decades. Drawn to sharks by their tenacious record of survival, he summarizes their ancient history as one of adaptation and good fortune. “The advantage that the chondrichthyans [sharks and rays] had was that their physiology enabled them to occupy deeper habitat. Those animals had to evolve all the adaptations necessary for surviving in an environment that was already lacking oxygen, is already really cold, and didn’t have a lot of food. So many of their adaptations—the cartilaginous skeleton, the liver—probably all those features have been retained.”⁴ He underscores the resilience of sharks and rays, adding, “they survived these massive extinction events because those habitats weren’t nearly as changed as the shallow ocean habitats. And then the fact is that maybe life radiated from the deep sea, and as the planet changed again, animals moved into those new habitats and then evolved adaptations for them.”

A suite of successful adaptations has been fundamental to shark survival for several hundred million years. Sharks and rays are most readily distinguished from other fishes by their cartilaginous skeleton. Cartilage is the stiff but flexible substance responsible for the shape of our noses and the scrollwork of our ears. True bones begin as bendable rods of cartilage but are gradually stiffened by the deposition of calcium, a process known as ossification, that forms stony crystals replacing most of the cartilage. Inside bones resides the marrow, a complex factory floor for production of red and white blood cells, and plasma. By contrast, the skeleton of sharks and skates and rays is never fully ossified and possesses no marrow. In most species, the cartilaginous skeleton is suffused with mineral salts less rigid than calcium, creating more flexible structure. While the distinctions may seem subtle, the benefits are enormous. Fishes with cartilaginous skeletons weigh less and are faster than their bone-lugging cousins. More body mass can be dedicated to muscle, resulting in greater speed. The skeleton’s flexibility allows impressive underwater acrobatics: compared with a rigid, torpedo-like tuna, a shark can spin and double back on itself almost like a snake, a boon in battle or when chasing prey through twisting reef canyons. And whereas all fishes must fight the implacable force of sinking, sharks are less dense thanks to their lightweight skeleton, and to their large and fat-rich livers, so they spend less energy staying up in the water column.

There are downsides, of course, as with any innovation. The flexible cartilage of rays and sharks is not as unbreachable as the rigid fortress of a bony fish’s skeleton. Instead of relying on an internal safety cage, sharks evolved external armor not unlike the chain mail worn by knights of old. The skin of sharks, and rays too, is covered not by scales but rather by tiny and extraordinarily hard teeth called dermal denticles. Like the teeth in our own mouths, shark denticles are formed from dentine, one of the hardest compounds animals can manufacture, then capped with enamel, another substance with unparalleled toughness.⁵ The result is a rock-hard spike, its tip pointing tailward, arrayed across the shark’s body with thousands like it to form a nearly impenetrable armor. If you caress a shark, be sure to swipe your hand from head to tail, the skin will be as smooth as velvet; stroke from tail to head and the skin feels as rough as an iron file.

Shark skin also improves hydrodynamic efficiency, enabling sharks to swim faster, because the tiny teeth create micro-vortices that reduce drag.⁶ Its unique form has been mimicked in a diverse range of human uses, from Olympic swimming suits to Navy battleships, all in search of greater speed through water.⁷ The skin of a shark even has powerful anti-microbial properties, as the studded surface impairs bacteria from growing into dangerous mats. Similar manufactured textures are being applied to hospital doorknobs and even catheter tubes to mechanically defeat bacteria and save lives.⁸

One of the most entertaining, albeit grisly, uses of shark skin in history comes from knights of medieval battlefields. As it happens, stabbing Visigoths or Vikings with a broadsword spills a lot of blood, which makes the sword handle dangerously slippery. Some ingenious warrior, probably a fisherman on his day off, discovered that dermal denticles made shark skin one of the greatest anti-slip substances on earth. Thereafter, the best blades that doubloons could buy—in Europe and even Japan—had handles sheathed only with sharkskin leather, and never more did knights nor samurai find themselves embarrassingly empty-handed in battle.

Shark courtship closely resembles a battle, involving a great deal of passion, strenuous physicality, and a lot of biting. Males bite females, many of whom bear nearly ritual-looking scars testifying to their pre-mating ordeal; they withstand the toothy foreplay thanks to skin that can be twice as thick as their suitor’s.⁹ Males also slash at other males in an attempt to establish their reproductive dominance. All this sexual swordplay likely led to sharks evolving their armor of enameled denticles. Once a male has dissuaded his rivals and dentally signaled his intentions to a mate, he wrestles the female into a belly-to-belly position and holds her in place with a clamping bite. Unlike most fish that rely on external fertilization, with eggs and sperm released freely into the water column, sharks manually transfer sperm to the female. In males, the paired pelvic fins (located on the belly, just before the tail) each bear an elongate extension, deeply grooved and rolled like fleshy tubes, which can reliably be used to distinguish gender in the wild. Called claspers, these modified fins are inserted in the female’s cloaca, her combined urogenital opening, and sperm packets are released into the groove. Gyrating like an earthworm on a hot sidewalk, the male generates currents of seawater that flush his sperm into the female; anywhere from six to sixteen months later, depending on the species, eggs are released or live pups are born. In the case of great whites, the newborns may be up to 4 or even 5 feet in length.¹⁰

After they enter the watery world, shark pups face a challenging future. They must avoid being eaten and find food for themselves, all without the slightest gesture of parental care. Studies of the energy used by swimming baby hammerheads, led by Dr. Lowe, calculated that they have only about three weeks to learn to hunt successfully; if they fail in that time, they will starve to death.¹¹ Even before birth, some shark pups must withstand a terrifying mortal challenge. Sand tiger sharks (Carcharias taurus), great whites, and a few other species experience a keen struggle for life within their very mother, a cage match so extreme that the winners even eat the losers.¹² Cannibalism in the womb is yet another example of the physical battles that sharks must endure, throughout their lives, in order to survive. But survive they do. Perhaps honed by their constant confrontations, sharks boast some of the most rapid injury recovery rates in the animal kingdom, and their immune system might just provide an avenue of safety through viral pandemics.

First, let us dispel a widespread myth: sharks are not immune to cancer.¹³ This is a falsehood spread to bolster dangerously dishonest claims that shark cartilage can cure cancer. It does not: you cannot simply pop a capsule full of ground shark bones and beat cancer; to suggest otherwise is immoral, and criminal. But the myth persists and is responsible for gratuitously sending millions of sharks to the grinder every year. Scientists have, however, elucidated at least three compounds from shark immune systems that hold astonishing promise for modern medicine. Squalene, a building block of steroids that was isolated from dogfish sharks in the genus Squalus, is widely used to manufacture adjutants for vaccines. Adjutants are accompanying compounds that increase the body’s ability to respond to a vaccine, ramping up production of the antibodies that constitute the front lines of protection against diseases such as influenza.¹⁴ Fortunately, squalene can be extracted from olive oil and many other sources, which should limit the impact this discovery might otherwise place on wild shark populations.

Closely related is the chemical squalamine, also discovered in dogfish sharks, underappreciated fishes who right now should be lining up for a Nobel Prize in medicine. Squalamine exhibits powerful antiviral activity against a broad range of human pathogens, by slowing the rate at which infected cells replicate the virus.¹⁵ Laboratory research has shown squalamine to be effective against such killers as dengue, hepatitis B, and even yellow fever. The compound exhibits an entirely new class of antiviral effect, completely different from existing therapies, and thus represents a rich field for medical research to mine for new and effective treatments.

Surprisingly, sharks share one exceptional feature of their immune system with a very unlikely, and decidedly nonaquatic animal, the llama. Both carry in their blood a type of antibody so small it has been dubbed a nanobody, and it may be the key to tackling future viral pandemics. Human antibodies are large, complex molecular structures with two binding areas that seek out their target: a recognizable structure on the surface of a virus, for example. To directly manufacture antibodies is a Herculean task: the intricate folding of a human antibody is nearly impossible to replicate in the lab—like building a wooden ship in a bottle, at a microscopic scale—and you have to do it twice, once for each binding area. But nanobodies are short, much simpler to fold into the proper configurations, and possess just one binding area. Because of these advantages, shark and llama nanobodies now serve as a promising scaffolding on which to mount defenses against a gamut of diseases.¹⁶

Human monoclonal antibodies have already been engineered to treat rheumatoid arthritis and Crohn’s disease. But shark nanobodies are the starting blocks for targeting Ebola, cholera, hepatitis B, botulism, and lupus.¹⁷ Nanobodies can even be programmed to deliver minute radiation doses to tumors, killing them before the radiation is safely flushed from the body by our kidneys, another benefit of nanobodies’ small size. Just before the COVID-19 pandemic paralyzed the planet, researchers in 2018 had already developed a nanobody programmed to block binding of the coronavirus MERS (Middle East respiratory syndrome) by targeting its spike protein.¹⁸ This treatment holds promise for custom-designed cures for future coronaviruses. All these immunological discoveries were made possible by sharks’ ability, honed over millions of years of evolution, to overcome bite wounds and viral infections suffered during hunting and courtship.

When sharks enter a pitched battle in the ocean, whether against fishes or sea lions or other prey, they are protected by their skin and armed to the teeth with row upon row of, well, teeth. Shark teeth vary enormously in shape and purpose, with only a few species brandishing the serrated perfect triangles stereotypical of great white sharks. Those teeth are adapted for cutting flesh, which the shark slices by fiercely slashing its head from side to side. The curved daggers of some hammerheads, however, are adapted for piercing and grasping swift and slippery squid. Other hammerheads, and many rays, have broad and flat-topped molars, suited to crushing shellfish and crustaceans. In lined cat sharks, males have daintier front teeth than females, the better with which to nip her in courtship.¹⁹ Tiger sharks (Galeocerdo cuvier) also sport curled, claw-like teeth, which they wield to seize turtles and shatter their shells. Despite relying on jaws of cartilage, sharks can bite with a force that beggars the imagination, possibly a result of an evolutionary arms race with well-armored prey like turtles. The largest great whites ever recorded (more than 7000 muscular pounds) have jaws that can clamp with an estimated two tons of force, equivalent to the weight of a full-grown white rhino.²⁰

Because shark teeth are not anchored to bone, they readily fall out: a typical shark loses about one tooth every week. That they fossilize so well, and litter the ocean floor, explains why we know so much about sharks from several hundred million years ago. These include the famous megalodon (Otodus megalodon), a shark so large it stalked giant whales as prey, measured an estimated 60 feet in length, and sported teeth the size of an axe blade.²¹ But sharks have turned their habit of losing teeth from a dentist’s nightmare into a dream scenario for hunting. When the outermost rows of teeth grow dull, they are replaced by new, sharp teeth from the rows behind, guaranteeing that sharks never enter battle with a dull sword. Shark teeth also are enervated, sending complex signals to the brain about the objects they touch, just like our fingertips. This may explain why sharks will mouth novel objects to find out more about them, just as a person entering a dark room will daintily brush the wall to locate a light switch.

To capture, slash, or crush prey, one first has to find it. Sharks and rays excel at detecting squid and fish and crabs and the like, no matter where they hide. Their eyesight is keen even in very low light, a holdover from life in the dark refuges of the ocean depths. Behind the eye’s light-sensing retina lies a mirrored curtain, the tapetum lucidum, which reflects stray photons back through the retina. While this works brilliantly in the dark, sharks may face the problem of being blinded in well-lit waters by too many reflected photons. Port Jackson sharks, attractively banded denizens of the seafloor who frequent canyons and caves, have solved this problem by developing a type of internal sunglasses. When light levels get too high, pigmented cells packed with melanin spread over the upper layer of the tapetum, cutting down the light; when the shark dives into the depths the pigment cells retreat, in effect doffing the sunglasses.²²

Beyond vision, sharks rely on other acute sensory systems to detect food at great distances, or when it is completely hidden. Sharks have an excellent olfactory system, capable of detecting schools of fish, or a single wounded animal, from miles away and following the scent toward the source. Great white sharks (Carcharodon carcharias), known formally as “white sharks”—one suspects the “great” has been added by movie directors and novelists—have been shown to detect blood at concentrations of just one part per million.²³ While this figure sounds impressive, it is worth pointing out that primates like spider monkeys can smell the odor of a predator at less than one part per billion.²⁴ Still, since sharks possess two pairs of nostrils, with water entering one and exiting the other, they can sniff a lot of water as they swim in search of food. Essentially, you can always find a needle in a haystack if you methodically pass the entire haystack through a needle detector. The pairs, located on opposite sides of the snout, permit detection of the direction from which an odor originates, just as paired ears allow location of sounds. Hammerheads, with their more widely separated nostrils possess even greater directional sensitivity.²⁵

Like salmon, sharks also use their olfactory sense to assess the flavors rolling off nearby shorelines and pouring from river mouths. Leopard sharks (Triakis semifasciata) can navigate toward shore using scent alone, as was demonstrated by biologists from the Scripps Institute of Oceanography. In a simple yet ingenious experiment, the researchers caught leopard sharks in the shallows, plugged the nostrils of half of them, then released them all well offshore. Those sharks with unblocked nostrils swam directly back toward shore, as if following a compass, while the plugged sharks meandered aimlessly, unable to navigate, proving the navigational worth of a keen sense of smell.²⁶ That this research was led by scientist Peter Nosal is an amusing bonus.

Much of a shark’s brain is dedicated to processing sensory inputs, more so than almost all bony fishes.²⁷ For an animal that roams the great, empty spaces of the ocean in search of infrequent food, prey detection is paramount. Chief among their sensory inputs are vision, smell, and the lateral line detection of pressure waves. But sharks and their cousins the skates and rays possess an extraordinary system that can detect a fish’s beating heart, the muscular activity of a crab, and even the magnetic lines of force that encircle the globe. They can sense electricity, and this unique ability is one of the keys to their millions of years of success.

A Shocking Discovery

I see no reason why intelligence may not be transmitted by electricity.

—Samuel Morse, Samuel F. B. Morse, His Letters and Journals

In 1678, the Italian anatomist Stefano Lorenzini announced a finding that would eventually electrify the world of sharks and rays. While dissecting torpedo rays, broad bottom-dwellers with attractive mottled patterns, he found a network of tiny organs clustered in the head, each filled with clear jelly and connected to nerve endings. These little sacs are shaped like a slender Greek urn, or ampulla, to which they owe their modern name: Ampullae of Lorenzini. Before Stefano could deduce their function, however, he became embroiled in the byzantine politics of the Medici family who ruled Tuscany. Just three years after publishing his work on rays, Stefano and his brother Lorenzo were arrested by order of Cosimo III de’ Medici, Grand Duke of Tuscany, and tossed into prison for twenty years. The brothers were suspected of abetting a plot between Cosimo’s wife Marguerite Louise, who had earlier fled the repressive and splenetic duke, and his worryingly popular son Ferdinando. It probably did not help matters that Stefano dedicated his book on torpedo rays to “the Most Serene Ferdinando III, Prince of Tuscany.”²⁸

What Lorenzini discovered before his imprisonment was a network of ampullae, each shaped like a miniature blind alley that opened to the outside world through a tiny pore. These pores dot the front and sides of the heads of rays and sharks and are visible to the naked eye on many species. Lorenzini was unable to deduce the function of the clear gel in the ampullae, however, and it would take almost 300 years before their true role was finally elucidated. In 1960, a zoologist in Birmingham, England proved that the system could detect faint electrical signals, and that these impulses were conducted into the ampullae by the pellucid jelly that Lorenzini had described centuries earlier.²⁹

The picture that has emerged from hundreds of subsequent studies is one of a fully developed electrosensory system, an exceptionally sensitive detector of electrical charges and even of magnetic fields. It appeared more than 400 million years ago, again in the Devonian period, in freshwater lungfishes; since then, it has resurfaced in many other lineages of bony fishes but is present in virtually all rays and skates and sharks.³⁰ The ability to sense electric current would have been a tremendous boon to ancestral sharks who survived Earth’s great extinctions in deep ocean waters where prey can be found only by their smell and their electrical signature.

All animals produce faint electrical fields, since all muscle movements are triggered by an electrical signal passed down a nerve.³¹ This phenomenon was ghoulishly discovered in 1780 by Luigi Galvani, an Italian physicist who found he could make a dead frog’s leg twitch by applying an electric current.³² One imagines that dinner parties in eighteenth-century Italy were quite the droll affair. A contemporary scientist, Alessandro Volta, helped clarify that the frog’s body itself was generating current, the travel of electricity between two poles with different charges; as proof, he invented the world’s first battery. Both men’s names are preserved in our contemporary lexicon as the words “galvanize” and “volt.” Muscular contractions for movement, breathing, and even heartbeats generate an electrical field around the animal, akin to a faint glow. Even at rest, individual cells harbor a slight charge differential, like Volta’s battery, a charge that is detectable as an electric field.

In rays and sharks, electrical currents travel from the surrounding water into the ampullae through the skin pores and propagate through the clear jelly that conducts electricity better than any substance known in the animal kingdom.³³ At the end of ampullae are rows of sensory cells that detect minute voltage differentials. The system is so finely tuned that it can detect the equivalent current of a single AA battery passing through two poles separated by 1000 miles, the distance between Australia and New Zealand.³⁴ White sharks, some of the most sophisticated hunters on Earth, can respond to charges as infinitesimal as one millionth of a volt.³⁵ No matter how bafflingly a prey fish might be camouflaged, it cannot fool the shark’s electrosensory system. Even tiny crabs buried in sand or silt can be detected with ease by rays and skates in search of a meal. Gliding over the bottom, a ray acts like a living metal detector, its wide snout homing in on the pings from hidden crustaceans, tube worms, clams, and other invertebrates which it unearths like treasure.

Rays and their various kite-shaped relatives—stingrays, skates, electric rays, and more—are uniquely suited to an existence near the ocean floor, where most make their living. They evolved from ancestral sharks around 200 million years ago, most likely from bottom-dwelling species similar to today’s angel shark and wobbegongs, broad and flattened sharks we will shortly meet. Since the open ocean is often empty of prey, it is natural that some cartilaginous fishes would eventually specialize on the rich bounty found atop, and within, the sea’s bottom sediments. Rays and their kin exhibit distinct forms of locomotion, hunting, defense, and even breathing, all honed over millions of years of cruising the seafloor.

Marine sediment accumulated on the bottom can be exceptionally fine, like corn starch, and is readily swirled into a murky cloud by the slightest flick of a tail or downward beat of fins. In such a fog of sediment, prey escape with ease, and predators can lurk dangerously. At some point in geological time, the ancestors of rays switched from powering their swimming with alternating thrusts of their tail to flaps of their pectoral fins. Tails concomitantly shrank, becoming a balancing organ rather than an oar. Over time, the broadened fins advanced from flapping up and down like bird wings to tracing a rippling motion that pushes water backward and the ray forward. Like a sidewinder in the desert, the undulating motion of their fins propels them ahead at a stately pace while barely disturbing the bottom. To avoid fouling their gills with inhaled sediments, rays possess a special pair of openings atop their head called spiracles that draw unsullied water from above. While most sharks have knife-like teeth specialized for capturing swift and slippery prey, many rays sport broad molars, the better to crush the armor of their favored crustaceans and mollusks. Rays find these buried creatures with their electrosensory system, but they differ from open-water sharks in sporting greater numbers of pores linked to their ampullae,³⁶ and an enhanced ability to divine their prey’s exact location.³⁷

Food detection is not the only capacity endowed by Lorenzini’s ampullae. While rays, skates, and sharks rely on this system to find fish, squid, crabs, and other prey, their ancient ability to sense electric fields also served as an evolutionary launching pad for a host of novel abilities. Rays and sharks can detect Earth’s magnetic field as they swim through it, a feat that humans could not replicate until lodestone compasses were invented in China just 2000 years ago.³⁸ They use this information to help navigate hundreds of miles of empty water and can even read the speed and the course of oceanic currents, since seawater moving through the planet’s geomagnetic field generates a unique electrical signal of its own.³⁹

It should come as no surprise that electrosensory systems can be used for communication. Skates are graceful creatures, closely related to stingrays, who prefer nearshore waters and are active mostly at night, when vision is limited. Skate tails are packed with twin sausage-shaped organs that can generate a weak electric current; moreover, the frequency of the current can be finely tuned, like the pitch of a trombone.⁴⁰ Their ampullae of Lorenzini permit them to discriminate specific frequencies, something sharks cannot do, allowing them to hear the full range of trombone tones.⁴¹ Skates almost certainly use these tones to converse amongst themselves: the frequency ranges of different species do not overlap,⁴² adult skates emit their signature frequency only when they are sexually mature,⁴³ and they emit more signals when they travel in pairs than when alone, as if they were conversing or courting.⁴⁴

Returning to the torpedo rays studied by Lorenzini, one finds an evolutionary advance that takes the electrical signaling of skates one giant leap beyond mere communication. Torpedo rays carry electro-generating organs much like skates, but they are larger, more powerful, and located atop the head near their eyes. Each organ is composed of thousands of tiny plates, arranged like a honeycomb, that convert chemical energy obtained from the ray’s food into electrical current. The organs closely resemble a battery and can accumulate the charge the ray produces until it is summoned in battle. When unleashed, the current can pack an impressive wallop. Some electric rays deliver jolts of more than 200 volts at 15 amps or more, the equivalent of tossing a toaster into a European bathtub (do not try this at home).⁴⁵ Chris Lowe, from whom we heard earlier, found that Pacific electric rays (Tetronarce californica) use moderate warning pulses to deter predators, but blasts of higher voltage to stun and even kill prey: those attacking bouts resembled a lightning drum roll of more than 1200 jolts each lasting just a few milliseconds.⁴⁶ Ancient Greeks and Romans were aware of the anesthetic effects of the lower voltage discharge from a torpedo ray: they prescribed light shocks to alleviate pains as diverse as headaches, toothaches, arthritis, gout, and even childbirth.⁴⁷ The very name “torpedo” is derived from a Greek word meaning to numb or stun. In modern times, torpedo rays are even being studied as a potential source of power, though the idea of hooking your cell phone charger to a tank full of electric rays seems more than a little preposterous.⁴⁸

Most rays and skates cannot discharge the voltage of a torpedo ray and usually rely on camouflage for defense. Many species are speckled, spotted, or marbled, and some even change color to match the overall shade of the bottom. They have a habit of settling on the sediment, then shimmying until a light dusting of sand covers their body and obscures their outline. With eyes jutting only slightly from the top of the head, a ray buried in this fashion is almost impossible to detect, at least visually. But they can be unmasked by the electrosensory array of a hammerhead, and they must have another defense to survive the inevitable attack.

If you have ever skipped gleefully from the baking sands of a sunny beach into shallow ocean waters, you might have been taking a considerable risk. In certain locales, stingrays repose in those selfsame shallows, basking in the warm water while resting between nocturnal forays. Were you to unwittingly plant your bare foot atop a dozing stingray, you might impale yourself on the ray’s most fearsome defense: a cluster of spines as tough as teeth and tipped with an agonizing venom. Daggerlike in form, the spines arise from atop the tail and are modified dermal denticles, made from the same material that armors the skin. Each spine bears numerous backward-pointing serrations, making it all but impossible to remove from a wound, and each is wrapped in venom-producing cells that secrete complex toxins into a groove running down the spine’s midline.⁴⁹

If you are unlucky enough to be injected, the toxins immediately begin torturing your body, adding insult to the injury of being stabbed with a spine the size of a letter opener. Stingray venom is a complex cocktail of painful chemicals. Galectins attach themselves to tissues near the wound and induce cell death, triggering excruciating pain: tormented cells literally scream into your nerve endings as they perish one by one.⁵⁰ The stingray’s chemistry set may include peroxiredoxin-6, an enzyme also found in jellyfish, snakes, and scorpions which prevents blood clotting and thus wound healing, provokes necrosis (tissue death, such as in gangrene), and can even inflict brain damage. Side effects include nausea, trembling, and a high temperature; the latter symptom explains why a gathering of these animals is collectively termed a fever of stingrays (like a murder of crows, or parliament of owls). While numerous myths circulate about treating stingray punctures, including the dubious recommendation of applying fresh urine, clinical research has shown that simply immersing the wound in hot water (not boiling), plus a couple aspirins, is sufficient to alleviate a victim’s suffering in just half an hour.⁵¹ The best treatment is prevention: when wading into shallows, use the “stingray shuffle” instead of lifting your feet and setting them down.

The Mermaid’s Purse

With more than 600 kinds of rays flapping through today’s oceans, their diversity exceeds that of sharks by about 100 species. Their sprawling variety, however, can be clustered into a few distinct groups: stingrays, skates, torpedo rays, pelagic (open water) rays, and those well-known giants the manta rays. Stingrays are largely, but not entirely, distinguished by their excruciating venom. A few other rays can deliver similar toxins, however, and most rays in the sea have at least one tail spine, whether toxic or not. Like skates and torpedo rays, the wing-like pectoral fins are so enlarged that they entirely engulf the head and fuse in front of it. A stingray’s body resembles a diamond-shaped or circular pancake, like a broad sun hat, with head and body distinguished only as a thickened portion in the center. Stingrays also give birth to live young, perhaps up to a dozen in a brood. In truth, the offspring hatch from eggs while still inside their mother, then feed on the yolks plus protein-rich uterine fluid she provides, before being born.⁵² This mode of reproduction, intermediate between egg-laying (ovipary) and true live-birth (vivipary), is known as ovovivipary, a word that would warm the tiles of any Scrabble lover’s heart were there three rather than two Vs in the game.

Skates are superficially similar to stingrays. Their pectoral fins also envelop the head, although their tail is usually shorter and slightly thicker compared with the stingray’s slender whip. Some, like the descriptively named little skate (Leucoraja erinacea), use the tips of their small, rearward pelvic fins to walk on the seafloor; researchers revealed that the same genes responsible for the skate’s stroll also code for mammalian ambling, illuminating just how little genetic distance can separate unrelated animals.⁵³ They frequently have thorn-like scales down their back, and occasionally a single spine arises from their tail, but neither are charged with venom. Skates tend to live in cooler waters, while stingrays prefer warmer seas. But the most delightful distinction is in their reproduction.

If you have ever happened upon a mermaid’s purse in the wrack line of an ocean’s shore, you have found the egg case of a skate. The fancifully termed purse is leathery and rectangular, about the size of a deck of cards, with wiry tendrils or horns springing from the corners. The center often reveals a modest bulge, where the egg resided until a tiny skate hatched and slipped out of the purse to its seafaring future. Amazingly, a skate’s electrosensory system is already switched on even in the purse-dwelling embryos. Clever lab experiments have shown that an embryonic skate only nine weeks old will instantly freeze when the electric field of a predator is applied, halting even its breathing until the threat has passed.⁵⁴

Rising from the ocean bottom, some rays have retained the capacity of their shark ancestors to swim commandingly in open waters, flapping their way across great distances. Pelagic rays are more deep-bodied than skates and stingrays, and their pectoral fins are more pointed and wing-like. Diagnostically, a pelagic ray’s pectoral fins do not meet in the front, leaving the head quite visible and distinct; the pectorals spread outward just at or behind the spiracle openings. A fine example is the spotted eagle ray (Aetobatus narinari), huge dark-bodied creatures decorated with a constellation of light speckles who swim through the water with all the fluidity and grace of their aquiline namesake. Their tail is long and whip-like and usually bears defensive but nonvenomous spines near its base. Their head terminates in a flattened snout, somewhat resembling a duck’s bill, which they wave over the bottom when searching for food, in true metal detector fashion. These 500-pound creatures frequently catapult into the air from tropical seas, attaining heights that any pole vaulter would envy, before crashing back to the waves with a characteristic belly-flop slap. Some hypothesize the powerful slap is performed to dislodge parasites; others suspect the displays are a form of mating rituals, like college students cannonballing into a pool to impress one another.

Closely related to eagle rays are gentle creatures known pastorally as cownose rays for the snub muzzle that projects Holstein-like from the front of their head. A deep gold or mustard color, these rays congregate in shallow lagoons where they browse the sandy bottom in search of mollusks and other invertebrates. Cownose rays are notably smaller than eagle rays, and perhaps for this reason they carry venom in their single defensive spine. Like their larger relatives they frequently take to the open water and also engage in acrobatic, somersaulting leaps into the air. They have been known to undertake prolonged winter migrations, in massive schools of 10,000 rays or more, from Florida to the Yucatan peninsula or the coast of South America (this from no less an expert than University of Virginia scientist Dr. Ray).⁵⁵ Intriguingly, females from one tagged population departed Chesapeake Bay in July and swam to Florida, while the male rays headed northward, spending a few late summer months foraging off New England before ending the bachelor outing and veering south to rejoin the females.⁵⁶

From Sandy Bottom to Salty Blue

If everyone were cast in the same mould, there would be no such thing as beauty.

—Charles Darwin, The Descent of Man

Not all sharks fit the classic description of the shriek-inducing villains in blockbuster films: a muscular torpedo sporting triangular fins and a gaping maw bristling with razor-sharp teeth. Nearly 500 species of sharks span an impressive range of shapes and sizes, preferred prey, favored ocean zones, and behaviors. Some eschew open waters, preferring like stingrays a life near the bottom. These include the delightfully named carpet sharks, who earned their moniker thanks to striking patterns of alternating dark and light blotches that call to mind a Persian rug. Many carpet sharks also have a habit like their namesake of lying on the ocean floor. Other examples are the wobbegongs of Australia and Indonesia, moderate-sized sharks so decorated with frill and filigree and tendrils that some species have earned names like ornate (Orectolobus ornatus), floral banded (O. floridus), and tasselled (Eucrossorhinus dasypogon). The word wobbegong itself is believed to arise from an Australian Indigenous term meaning “shaggy beard.” Their tasseled margins and complex spotting patterns endow these gorgeous animals with superb camouflage, invaluable as they lie in wait amid undulating tufts of seagrass and algae, poised to ambush unsuspecting prey.

Another carpet shark, though they have traded the wobbegong’s flamboyant patterns for a matronly tan uniform, are the widespread and docile nurse sharks. Their downward-pointing mouths are adapted to preying on lobsters, stingrays, and other bottom-dwellers, whom they detect with two catfish-like fleshy barbels hanging from their upper lip. Like many sharks, they have a vigorously muscular throat which, when expanded rapidly, sucks in water and hapless prey. That sucking, combined with a soft-edged mouth that looks almost pursed, may be the origin of their name, as they can resemble a nursing baby. An intermediate relative is the diminutive epaulette shark, 3–4 feet in length, which combines the downward mouth and barbels of nurse sharks with the extravagant splotching of wobbegongs. Well adapted to life among the rocks and algae of the bottom, they frequently walk on their front fins, scampering between coral heads rather than swimming. Evolution has granted them longer and more flexible fins to improve their locomotion, and a higher tolerance to oxygen deprivation, probably an adaptation to life in tide pools that become anoxic at low tide. Incredibly, one epaulette species (Hemiscyllium ocellatum) can even walk on land: it scuttles on its belly and four fins, entirely out of the water, from one tide pool to the next in search of food.⁵⁷

An environment as rich and diverse in prey and habitat complexity as the ocean bottom unsurprisingly yields an equally rich and diverse spectrum of shark species. Alongside the angel and carpet sharks are the bullhead sharks, named for the high forehead and sloping face that give them a slightly bovine head shape. These include several species of well-defended residents of rocky bottoms, including the Port Jackson shark (they of the retina-dimming sunglasses) and horn sharks. Both are handsomely marked: the back and head of a Port Jackson is draped with dark stripes and swirls, while horn sharks are irregularly spattered with small dark blotches. Bristling with thorny defenses that would make a stingray envious, Port Jackson and horn sharks bear spurs that jut menacingly from the leading edge of both dorsal fins. Like stingray spines, the spurs help defend these docile sharks against attack from above while they lie in wait for prey. Bottom-dwelling prey are topmost among these nocturnal species’ diets, a preference manifested by their small and downward-facing mouths, and the predominance of molar-like teeth adapted for chewing tough crab shells and the like. Port Jackson sharks exhibit a novel solution to the challenge of breathing near the seafloor where silty water brought in through the mouth can foul delicate gills: they draw water in through the first gill slit instead, pumping it across the gills, and then out through the remaining four slits.⁵⁸ This elevates them into a select club of sharks who can eat and breathe at the same time.

Pushing off the ocean floor, you enter the open waters where the more widely known shark species are found. Pelagic predatory sharks, whether they patrol the rocky outcroppings of reef heads or cruise the wide open spaces of the ocean, fall into one of two types: requiems and mackerels. Requiems are a shark family that epitomizes our classic vision of a shark. Streamlined and long, head slightly pointed and fearsomely toothed, back drably colored but belly pale, they are built more than anything for hunting at great speed. The family includes tiger, blue, lemon, whitetip, blacktip, bull, and silky; the latter may be the most abundant sharks in the sea. They are among the most numerous of the open water sharks and include several species implicated in attacks on people, such as tiger and bull sharks (Carcharhinus leucas). Bull sharks have even been known to swim up freshwater rivers, and one was reported (in 1937) as far inland as Alton, Illinois, near St. Louis, a distance of some 1500 miles of Mississippi mud from the Gulf of Mexico.⁵⁹ All requiem sharks give birth to live young, with most species supplying nutrients to their developing embryos through a placental connection, just as in humans. As with many larger sharks, females require several years to reach reproductive age, and often a few more years will pass between successive pregnancies.

Tiger sharks, named for the tiger-like stripes that prominently mark juveniles, and for the attacking velocity of adults, take years to reach their full size of 12–15 feet. They are the largest of the requiems and may live to twenty-five years of age, reaching sexual maturity around the age of nine.⁶⁰ Females mate only once every three years and carry their young for fifteen or sixteen months before birth.⁶¹ The math is simple: an average female tiger shark will reproduce just six times in her entire life, one of the reasons why this magnificent species has declined so precipitously. Tiger sharks are also famous for their eating habits which are anything but finicky: stomachs of captured adults have held everything from snappers, seals, seabirds, and turtles to license plates, tires, dead cows, bags of coal, a tom-tom drum, and even the head of a crocodile.⁶²

Where sharks mate, and give birth, is critically important to the survival of their young, and thus the perpetuation of the species. Blue sharks (Prionace glauca), beautiful and willowy requiems stretching to 10 graceful feet in length, undertake monumental migrations between mating, birthing, and feeding areas. Tagging studies have revealed that Atlantic blues make routine trans-oceanic crossings between mating waters near Brazil, feeding areas off North America, and birthing wards in the warm waters west of Spain and Morocco.⁶³ To get the timing of birth just right, when a migration of 10,000 miles can separate courtship and birth, many females delay implantation, holding live sperm in their bodies until they trigger fertilization at precisely the right time to ensure their young will be born in just the right place.

In California, leopard sharks may have stumbled onto a thermal solution to the challenge of scheduling birthdays. Female leopards (not true requiems, but in a sister family) were observed by Chris Lowe and colleagues to gather in shallow waters during the day. In a narrow band of sun-warmed water along the shore, “females will aggregate in large numbers in this thermal envelope.”⁶⁴ Reposing in the balmy shallows raises their body temperature and boosts their metabolism by as much as 17 percent. Ultrasound imaging revealed most of the lounging females were carrying young, and Lowe believes the temperature boost should accelerate embryonic development, leading to earlier births. Similar behavior has been seen in round stingrays (Urobatis halleri), who gather to enjoy warm estuaries in California and even bask in the heated water discharged from power plants. It is possible that both animals are speeding up gestation so that their young are born at the precise moment when food will be more abundant for the hungry infants.

The other distinctive group of large, oceanic sharks are the muscular hunters known as mackerel sharks. These powerful and athletic species include the famed white sharks, easily recognized by their brawny body and blunt nose, and the more streamlined, fleet-finned makos. Among the fastest fishes in the sea, shortfin makos (Isurus oxyrinchus) have been clocked at speeds over 30 miles per hour, with one estimate putting their top speed at an eye-popping 60 mph.⁶⁵ They reach such speeds by cranking up the thermostat on their swimming muscles by as much as 20 degrees; the effect is like engaging a futuristic warp drive that propels them into hyper speed.⁶⁶ White sharks also have this ability, though they cannot challenge a mako for pure speed. Instead, their thermal advantage allows white sharks to hunt at the thermocline—the upper limit of the ocean’s deep and frigid waters—diving repeatedly below the boundary to capture unsuspecting or torpid fish.⁶⁷ Like marlin and swordfish, white sharks also possess networks of warm-blood arteries that wrap their eyes, enhancing their vision while hunting in the shadowy depths. In makos, a special vein passes through their red swimming muscles, picking up heat and transporting it to their head, where it warms the brain.⁶⁸ Thermal measurements of both mako and white sharks revealed their stomachs also can be 10 or 12 °F warmer than the surrounding water, supercharging gut enzymes and accelerating digestion.⁶⁹

Blacktip reef sharks (in the requiem family) routinely linger in warm, shallow sites on coral reefs, basking before making hunting forays into cooler open waters. This behavioral thermoregulation—also practiced by lizards on sunlit rocks—allows them to maintain their body 2 or 3 degrees above the seawater temperature.⁷⁰ Blacktips retain this heat as they chase prey fishes down into deeper water: both predator and prey cool as they dive, but because the shark is slightly warmer when the battle commences, it maintains a decided advantage in speed and acceleration until its jaws close on the chilly and sluggish prey. Once again, science gifts us an amusing alignment of researcher and research, as the lead investigator on the blacktip study was none other than Dr. Conrad Speed.

One unique example of warm-blooded mackerel sharks is the uniquely sculpted thresher, whose extraordinary upper tail lobe resembles the curved blade of a scythe and can comprise half the shark’s total length. The asymmetry of a thresher’s tail generates lift, as the upper lobe pushes the tail down slightly, raising the head as the shark rotates around its center of mass. Tail-generated lift is particularly beneficial for slow swimmers, and all but the fastest sharks have the stereotypically longer upper tail lobe. As we have seen, threshers also wield their tails as hunting weapons. Hungry threshers will methodically circle schooling sardines, herding them into compact balls. When the sardines are perfectly packed, the leathery strap of a tail is whipped at the school so violently that dissolved gasses can bubble out of the water from the force of the blow.⁷¹ Stunned sardines then make easy targets when the threshers, often working in pairs, slice into the school to snatch their dazed and disoriented prey.

Twenty-Thousand Teeth under the Sea

I could never write Jaws today. I could never demonize an animal … that we may, if we don’t change our destructive behaviors, extinguish from the face of the earth.

—Peter Benchley, introduction to Jaws

People have a schizophrenic relationship with large, powerful predators. A grizzly bear spotted across Yellowstone Valley is thrilling, but one pawing at your sausage-filled cooler is terror incarnate. Safaris in the Serengeti are a roaring success if a lion or cheetah can be photographed, but few on the tour bus would volunteer to step down and stroll about the plains. Sharks are no different. We revel in their unchallenged mastery of the ocean realm and are electrified by their speed, their agility, even their savagery, but we harbor an unfounded terror cynically amplified by film and television. Our fear gives us license to turn a blind eye as more than 100 million sharks are annually slaughtered by people.⁷² We justify our willful blindness by muttering that sharks kill four or five swimmers every year, but this justification rings hollow when we contemplate that dogs, our most beloved animal companions, will in that same year savage 400 or 500 people to death.⁷³ Other comparisons are less chilling, more amusing. For example, in a given year more people will die from teetering vending machines falling on them than from shark attack.

Despite our deep-seated dread of sharks, it is sharks who have more to fear from humans. Skates and rays, too, are swept up in a whirlwind of nets and baited long-lines that threaten their very existence. Studies tracking the precipitous declines are sobering. Assessment of twenty-one pelagic species of sharks and rays commonly targeted by fisheries found that all of them, every single species, is now under threat of extinction.⁷⁴ In a recent global investigation researchers placed blame on fisheries that directly target sharks, and those indirectly taking sharks and rays as bycatch, for rendering nearly one-third of all ray and shark species on Earth vulnerable to extinction.⁷⁵ A parallel review estimated that populations of sharks and rays have sunk by 71 percent in the last fifty years, a precipitous decline due to an eighteenfold rise in fishing pressure.⁷⁶

Foremost among the threats to sharks is soaring demand for shark fins, an industry founded on the barbaric practice of catching sharks on the high seas, slicing off their dorsal and pectoral fins, even their tails, and dumping the dismembered body back into the water to die. A key ingredient in a popular Asian soup, the fins add neither taste nor color, but rather a peculiar gelatinous texture. As with any high value wildlife product, much of the trade is unreported, unregulated, and downright illegal. Estimates that valiantly incorporated not just reported fisheries catches but also data from the fin trade itself suggest that between 30 and 52 million sharks are abducted by the fin industry every year;⁷⁷ the Humane Society International puts that number at a ghastly 72 million.⁷⁸

Making matters worse, for them, sharks are slow to reproduce, with some species waiting decades before mating. Ingenious new research—based on uptake of anomalous carbon atoms circulating the atmosphere since early atomic bomb tests—revealed that some female white sharks do not reach reproductive maturity until their thirty-third year.⁷⁹ The same study estimated that the largest male white sharks could live to an astonishing age of seventy-three years. As with many species that reproduce late in life, like gorillas and whales, lengthy delays before sexual maturity will effectively doom shark species to rapid extinction in the face of even moderate fishing pressure.

The only option is to mimic the approach applied to save rhinos, elephants, lions, and gorillas: establish safe havens where hunting is illegal and severely prosecuted, and ensure that long-term benefits like tourism revenue pass directly to local residents.⁸⁰ An uplifting example is offered by the Micronesian island of Palau, which in 2009 banned all shark fishing in its expansive territorial waters, inspired by the conviction that live sharks are worth far more to tourists than dead sharks are to commercial fishers. Soon after the ban, shark tourism was generating US$18 million annually for the island nation, about 8 percent of its gross domestic product.⁸¹ Elsewhere, shark watchers contribute $25 million every year to Australia’s economy, and a staggering $42 million to the island of Fiji;⁸² globally, shark tourism generates a tidal wave of revenue reaching $314 million every year.⁸³ By 2017 the number of shark sanctuaries worldwide had swelled to fifteen, covering a combined 3 percent of global oceans, and dramatically reducing shark mortality within their bounds.⁸⁴ Fortunately, white sharks appear to be making a comeback in shark sanctuaries, and in the waters off California where their numbers have risen every year since 1994 when they were placed under state protection.⁸⁵ And in 2022, the US House of Representatives agreed to ban all shark fin trade within the nation.⁸⁶

If shark sanctuaries can be vigorously defended and ambitiously expanded, these magnificent predators will endure. Generations of nature lovers will have the opportunity to view them in the wild, assuming they can survive pre-snorkeling encounters with ruthless vending machines. If weddings and other celebrations can find an alternative to shark fin soup, we can reel humanity back from the barbarism of finning. And if people can learn that few sharks resemble the implacable killers of movie screens and television spectacles, that most are docile, fascinating, and unique creatures like wobbegongs and whale sharks, they will embrace the wonder of sharing our planet with animals who have called the oceans home for 420 million years.