The Largest Fish on Earth

He doth bestride the narrow world like a Colossus

—William Shakespeare, Julius Caesar

The oceans harbor some of the largest, longest, and most colossal creatures to call our planet home. The blue whale is Earth’s most massive mammal, tipping the proverbial scales at just under 200 tons. Lobsters are the heaviest arthropods, saltwater crocodiles the largest reptiles; the longest animal on the planet is a siphonophore, distant relative to corals and jellyfish, that stretches to more than 150 ribbon-like feet. And whale sharks are undoubtedly the planet’s biggest fish. These titanic creatures can reach 60 feet in length and weigh over 20 tons, three times that of the largest African bull elephant.³ Such superlative creatures thrive in the sea partly because of its salty buoyancy. On land, if an elephant were enlarged to the size of a whale shark, its bones would shatter when it stood: bone strength grows only with the square of an animal’s length, but weight increases as the cube of length, so in goliaths the former is overwhelmed by the latter. Not even evolution can overcome this fundamental limitation of physics.

Giant animals face another fundamental problem the oceans have helped solve, one of thermodynamics. All living bodies generate heat from the workings of every single cell, each humming along like a tiny furnace, and that heat is shed to the outside world only across the skin. Again, cubes battle squares: as animals get larger, the number of cells in their body rises as the cube of length, but the amount of skin only expands as the square. At extreme sizes, an immense animal’s cellular furnaces would smolder with more heat than its skin could dissipate, cooking the poor creature in its own juices. This inescapable constraint of thermodynamics may have set the maximum size of the dinosaurs: any larger and they would have been the evolutionary equivalent of a hot dish. Oceans crack this conundrum because water strips heat away four times faster than air, making you shiver after an hour-long swim in the sea, but saving a blue whale or a whale shark from overheating.

Once the evolutionary doorway to gigantism was nudged open by the sea, a bevy of animals charged right through, since being oversized yields many rewards (if your weak bones and multitudinous furnaces do not get in the way). The giants who call the ocean home—whale sharks and basking sharks, true whales, manta rays, and the peculiar mola molas—take full advantage of the benefits of bulk and are surprisingly similar in behavior, metabolism, and diet.

Most of the total weight of any fish is muscle, although marine giants also pack a lot of fat on their frames, insulation that buffers them from cold but highly productive waters. Their mouths are bigger, so they can swallow enormous swigs of tiny food like plankton. And their metabolism is lower than that of a smaller animal, requiring less nourishment relative to their body size. On land, for example, a diminutive shrew must eat nearly its entire body weight in insects every day to stay alive; a massive elephant, however, would take more than a month to ingest an equivalent amount of food. The shrew, if deprived of sustenance, will starve to death in a matter of days as its frenzied metabolism blazes through meager fat stores. In contrast, elephants, and whale sharks, can travel for months without eating as the slow boil of their metabolism languidly works through fatty energy reserves.

An unhurried metabolism provides other, more internal advantages. Animals with low metabolic rates tend to suffer less tissue damage, and fewer harmful genetic mutations.⁴ These protections might even save you from extinction, if you are a mollusk: paleontological data show that clams and snails that disappeared rapidly from the fossil record had higher metabolic rates than those species that survived.⁵ Of course, having a mellow metabolism also makes you slow-moving, which means you cannot readily escape predators; few predators, however, are willing to tackle a titan ten times their size.

It is plain to see why whale sharks and the other giants of the sea evolved to be so huge. They feed on explosions of plankton which are intermittent and far-flung, their locomotive stamina propels them across the planetary distances that separate those eruptions, they are nearly unassailable by smaller predators, and their titanic bodies offer fat stores and the slow metabolism needed to fuel the long journeys. And yet there is one final benefit of being a giant, one which lasts a very, very long time.

Beneath the arctic waters of the north Atlantic another ocean giant drowsily cruises. Charcoal-grey and cylindrical, she slips forward like a submarine running silent. When prey is encountered, though, she can pick up speed, engulfing fishes like cod and haddock in her cavernous maw. Reaching nearly 20 feet in length, and weighing almost two tons, this Greenland shark (Somniosus microcephalus) belongs to the expressively named family of sleeper sharks. In the summer she favors cold waters down to 6000 feet and rises to the surface only during winter months.⁶ A few of these somnolent leviathans were captured accidentally in 2010 surveys of Greenland’s commercial fisheries. Researchers from Denmark and Norway seized the unprecedented opportunity to determine the age of these enigmatic creatures. By peeling back the lens layers on each shark’s eye, they were able to date the oldest, central layers using radiocarbon techniques. What they found astonished them. The largest Greenland shark’s age was an extraordinary 392 years, making it literally the oldest fish in the sea.⁷ It had been born sometime around 1620, the same year a few score of intrepid (if slightly underprepared) Pilgrims set sail on the Mayflower.

A Mouthful of Motes

Whale sharks and nearly all the ocean giants (apart from Greenland sharks) sustain themselves with helping after helping of the richest broth in the world: plankton. It is astounding, at first blush, that the largest animals on the globe can thrive on such tiny food particles suspended almost invisibly in the ocean. But plankton can be mind-bogglingly abundant, and they replace themselves swiftly. In highly productive oceanic zones, this brisk growth yields tons upon tons of plankton, enough to support pods of great whales and parades of whale sharks. In the fertile waters off Yucatán, Mexico, researchers observed more than 400 of these speckled behemoths feeding in a patch of ocean just 7 square miles in size.⁸ The short-lived gathering of whale sharks revealed something critical about plankton: despite its omnipresent nature, explosions of plankton rich enough to sustain whale sharks erupt in only a couple dozen places on the planet, and there for just brief periods of time.

Algae flourish where nutrient-rich and well-oxygenated waters are brought to the surface, and join with sunlight to supercharge phytoplankton photosynthesis, growth, and reproduction. The Humboldt current drives a powerful upwelling off Peru, which almost never flags; the Benguela current propels another alongside the southern tip of Africa. But elsewhere, upwellings are influenced by seasonal winds, and plankton growth booms only when conditions are ideal. In the warm tropical waters surrounding Indonesia and the Philippines, and in the Timor Sea which separates those island nations from Australia, the appearance of rich phytoplankton broth is heralded by the departing monsoons. Cloudy skies give way to warm sunlight in the early months of each year, and by spring the soup is ready. Soon whale sharks, manta rays, and other giants flock to the buffet.

To feed on miniscule phytoplankton and miniature zooplankton, petite jellyfish, diminutive fish larvae, and other tiny tidbits, these oceanic titans must ingest, filter, and expel massive quantities of water. A whale shark’s cavernous mouth can inhale 2600 gallons of water every minute.⁹ At that rate it could process an entire Olympic swimming pool—660,000 gallons in all—in just four hours. Each hour the whale shark strains up to 6 pounds of plankton from all that seawater, a prodigious harvest considering the miniature size of each morsel. For a human equivalent of this feat, try walking through a snowstorm with your mouth open until you have gulped enough snowflakes to fill a glass of water.

Whale sharks, basking sharks, and manta rays use filter pads to separate suspended particles from the water, before it flows over the delicate gills. The filtered food is then directed to the esophagus and swallowed. Filter pads are long, slender ribs of cartilage, packed into dense ranks, each rib covered by fine whiskers. Like the filaments of a dust broom, every whisker traps a few particles: the more hairs, the more particles caught. In whale sharks, the slender ribs are cross-linked, the whole apparatus arranged like a basket deep within the funnel-shaped mouth. This reticulated basket traps particles larger than about one millimeter, which then are swallowed. Small particles eventually become stuck, however, clogging the sieve. Whale sharks and other plankton eaters routinely “cough” to back-flush the filter basket and expel the jammed particles. Filter-feeding manta rays (whom we will shortly meet) have evolved an even more advanced technique that never requires back-flushing, so elegant that engineers have been inspired to apply it to intractable industrial problems.

Filter feeding allows ocean creatures to subsist on the smallest of foods, but it does have its drawbacks. First, it is inherently nonselective. Anything large enough to be trapped by the filter basket will be eaten. For millions of years on planet Earth this was not a risky proposition: the occasional gulp of stinging jellyfish might have caused a stomach ache, nothing more. In the last century, however, people have been dumping plastic waste into the environment, and one 2010 study estimated that some 14 million tons of plastic reached the ocean in that year.¹⁰ By 2016, the United States was producing more plastic waste than any other nation, some 280 indigestible pounds per person every year.¹¹ Thanks to this disappointing facet of humanity’s so-called progress, whale sharks feeding today on a soup of plankton will inevitably consume obscene quantities of plastic. In 2018, a whale shark was found stranded on a beach in the Philippines, and it died shortly thereafter. The autopsy revealed plastic particles wedged in its gills and shreds of plastic crowding its stomach.¹²

A second drawback is that filter feeding is a drag—a hydrodynamic drag, that is. Fishes who rely on filter feeding cannot travel rapidly when forcing thousands of gallons of water through a tiny mesh; the effect is like hauling an open parachute through the water. Fortunately, whale sharks can alter the shape and orientation of the filters to allow water to pass through them more easily, a trick they employ when swimming to cross great distances. Once they reach an area with bountiful plankton, they fan open the filters and slow to a speed that maximizes their sieving efficiency, about 2 miles per hour.¹³ At this speed they expend less energy swimming but must still battle drag. Ultimately, if the calories they gain from feeding on plankton fall below the calories burned by swimming, they face an energetic deficit that sets the minimum density of plankton on which they can afford to feed. When spasmodic plankton blooms subside, whale sharks and their ilk are energetically forced to abandon feeding, lest they starve, and embark on a long journey to another plankton hot spot. In some cases, those journeys can lead them halfway around the world.

Cruise Control

I am tormented with an everlasting itch for things remote. I love to sail forbidden seas and land on barbarous coasts.

—Herman Melville, Moby-Dick

In September of 2011, off the Pacific coast of Panama, biologists from the Smithsonian Tropical Research Institute spotted the fins of three whale sharks.¹⁴ The team leapt into action, and using a modified spear they attached satellite tracking tags to the back of each shark, just behind the dorsal fin. All three were females, and one, nicknamed Anne, would set a world record. Over the next 841 days, Anne’s tag delivered positional fixes that revealed a shark on a mission. After deep dives near the Panamanian coast, she swam to the Costa Rican island of Cocos, then to the famed shark gathering grounds in the northern Galápagos Islands. Moving on, she cruised west through deep blue waters to Hawaii, halfway across the world’s largest ocean. When the transmitter’s batteries finally expired two years later, she had made it all the way to the western rim of the Pacific amid the Mariana Islands. All told, Anne had completed a journey of 12,500 miles, smashing all previous records.¹⁵ While some have questioned the validity of this particular voyage, more recent satellite studies have confirmed tracks of 4500¹⁶ and 4000 miles over considerably shorter stretches of time.¹⁷

The travel logs of Anne and other adult whale sharks suggest they have discovered the marine equivalent of a moving sidewalk across the vast Pacific. The North Equatorial Current, a stream of surface water gushing westward a few hundred miles north of the equator, offers a highway linking productive feeding grounds off Central and South America to the warm, soupy waters of Indonesia and the Philippines. Because the current slackens during winter and early spring, however, the one-way trip can take as long as two years. And that, precisely, is why plankton feeders like whales and mantas and whale sharks are united in being so massive: they need a gargantuan gas tank to fuel the journey on this long aquatic highway. If you were on a road trip with a whale shark, she could drive tirelessly all day and all night and never stop for a snack.

Charles Darwin, in his earthshaking study of Galápagos mockingbirds and finches, discovered that isolation of one island’s birds from another’s induced the birds to evolve into distinct species. The same principle applies to many fishes, especially small ones. Species endemic to the Atlantic Ocean differ significantly from their relatives in the Pacific because of the isolation imposed by distance, their weak swimming, and a modest obstacle called Central America. Whale sharks, however, present quite a different picture. Thanks to their marathon transoceanic cruises, a whale shark can visit all the oceans on Earth in its lifetime, estimated to be as long as 130 years.¹⁸ Their unparalleled mobility means whale sharks are among the least isolated organisms on the planet, polar opposites of Galápagos finches. For this reason, whale sharks belong to a single species (Rhincodon typus), and because of their genetic mixing they form a single population, with only faint distinctions between sharks plying Atlantic waters and those favoring the Indian and Pacific Oceans.¹⁹ But from a conservation perspective, their planetary voyages convert isolated dangers in a single region, like an illicit fishery, into jeopardy for all whale sharks everywhere. Protection of the planet’s largest fish requires cooperation across all oceans, and all continents.

Jennifer Schmidt has studied whale sharks at the Shark Research Institute for more than twenty years. From her very first encounter in the waters of the Philippines, she was captivated: “This massive thing, it looked like it was the size of an aircraft carrier, just passed right beneath me.”²⁰ Her research applies genetic techniques to shed light on whale shark movement and reproduction dynamics, and to steer their conservation. She laments that the threats faced by whale sharks are manifold. “We believe that fishing, or poaching—depending on the rules of the country—is a huge player, both intentional and accidental. Commercial factory fishing probably takes a great toll; much of that is unregulated in international waters, so it’s hard to say how much.” Whale sharks regrettably command high prices in Asian seafood markets, traded covertly because of their endangered status, where the gentle giants are dubbed tofu shark because of the soft texture of their meat.²¹ Majestic whale shark fins are also sold, after the owner has been dispatched, as trophies to be exhibited like macabre ornaments in emporiums touting shark fin soup on their menus. Hunting of this sensitive species, easily captured and slow to reach reproductive maturity (at 20–25 years of age), has decimated their numbers. Estimates for population declines range from 40 percent around Ningaloo reef²² to 80 percent in the waters off Mozambique.²³

Dr. Schmidt continues to tick off threats: “We think boat strikes probably take a big toll, particularly from large container ships. I mean, we tend to forget how many ships cross virtually every part of the ocean all of the time, and whale sharks are on the surface.” To serve a swelling global economy, the number of oceanic shipping vessels has quadrupled since the 1950s;²⁴ by 2015 they were transporting more than 10 billion tons of freight per year.²⁵ “And I’m sure habitat change takes a toll. The ocean is warming; patterns of food—plankton—are almost certainly changing.” Earth’s seas are absorbing more carbon from our changing atmosphere, making their waters more acidic, which impacts shell-forming plankton, among other life. As evidence, researchers surveying such creatures on routes first sampled in the late 1800s found that shell thickness had plummeted by three-quarters in every single specimen.²⁶ Last but not least, Schmidt cites the impact of marine pollution, such as the oil spilled by the Deepwater Horizon platform into the Gulf of Mexico in 2010.²⁷ “We have a lot of whale sharks off that coast, and the track, the oil plume, went straight through their habitat.” Her summation was stark: “It’s bad to be a surface-feeding animal in an oil spill.”

The collective impact of all these threats is maddeningly difficult to assess. Most of the time, whale sharks are solitary creatures, and the ocean is indeed a very large place. Apart from individuals who wash up on beaches or are reported as bycatch (such honesty is distressingly scarce) or happen to bump into a scientific exploration, we know very little about how these giants live in the deep blue waters of the open ocean. To illustrate just how rare such firsthand encounters have been, consider that by 1985—a century and a half after this enormous animal was first described by science—whale sharks had been observed in the wild only 320 times. But all that changed with an amazing discovery: in a handful of tropical locations, juvenile whale sharks gather in dense aggregations, shedding their solitary ways and joining crowds that can number in the dozens or hundreds.

Congregation Stations

Since congregations of whale sharks were first found at Australia’s Ningaloo reef and a few remote scuba locations, the number of known aggregation sites has swelled to more than two dozen worldwide. Like a scene from a city park in summer, throngs of juveniles and a smattering of adults gather to socialize and gorge themselves on eruptions from the marine equivalent of a picnic basket: a nearshore upwelling. Whale sharks are remarkably faithful to such locations—one individual nicknamed Stumpy has returned to Ningaloo for over twenty years—a fidelity that finally allows marine scientists to study elusive creatures whose biology is shrouded in mystery as murky as the waters they favor.²⁸ And those locations are generating more than just data. Their popularity has spawned a whole new industry, one that might just save these gentle giants from the dangers our civilization has been throwing at them.

Whale shark tourism has boomed in the past two decades. Many aggregation sites are found in shallow waters near tropical shores, where paddling in turquoise waters with the largest fish in the world has swelled in popularity. In the Maldives, some 75,000 whale shark tourists visiting a single atoll contributed more than US$9 million to the local economy in 2013.²⁹ Four years later, a similar study in Indonesia’s Cenderawasih Bay National Park showed the industry netted over $10 million; based on these local figures, one economic analysis calculated whale shark tourism injects some $2.6 billion annually to the nation. As with predatory sharks, the value of a single whale shark over its entire life span has been estimated at ten times its worth as a one-off fisheries catch.³⁰ Tourism is not a panacea, however, and under-regulated sites risk disturbing or injuring whale sharks when too many swimmers, boats, and motors clog the waters. Conservationists today are focused on enforcing codes of conduct that permit rewarding experiences for visitors but safeguard the star attractions.

Much of what scientists like Jennifer Schmidt have discovered about whale sharks comes from observations at these popular aggregations. Even citizens can contribute; conservation-minded tourists upload underwater photos to identification databases, from which more than 12,000 separate individuals have been identified. Schmidt counsels caution, however, when generalizing the discoveries made in these aggregations, where adults are less often seen, and some two-thirds of congregants are males. She notes wryly, “I compare it to an alien landing on Earth and ending up at an all-boys high school, and taking that to be the sum of what humanity is.” Many mysteries remain: where adults travel after leaving aggregations, where they mate, or even give birth, are still completely unknown.

Whale sharks seen at aggregation sites sometimes bear the unmistakable signs of boat collisions, the result of oceanic voyages that regularly cross major shipping lanes. Researchers working in the Maldives, however, discovered something amazing. When reviewing multiple photographs of the same whale shark, its identity confirmed by the fingerprint-like pattern of its spots, they were able to trace wound healing rates in individuals with visible injuries. They estimated that even severe gashes achieved nearly complete closure after just one or two months, with one animal showing 50 percent recovery in just four days.³¹

How have whale sharks evolved their swift healing rates? Partly thanks to their toothy ancestors who evolved to withstand biting and clawing by flailing prey. But a filter-feeder’s habit of eating near the surface may have also contributed. In areas where currents bring productive waters together, these creatures collide with all sorts of marine flotsam: logs, sticks, dead fish, jellyfish, coconuts, and more. Whale shark eyes, for example, show an adaptation unique among fishes: they are covered by the same tooth-hard denticles that armor the skin of predatory sharks. These scales armor the entire eye, even the iris, and shield it from damage by floating objects. Further protection is granted by the shark’s ability to retract its eye almost fully into the head, using an exaggerated eye-roll reminiscent of a flamboyantly exasperated teenager.

The sites where whale sharks aggregate are remarkably similar: warm and shallow seas, adjacent to deep water, with steeply sloping bottoms leading to an abrupt drop-off. This trifecta of conditions virtually guarantees upwellings of nutrient-rich water that fertilize plankton in the balmy and sunlit shallows. Whale sharks cruise happily and open-mouthed through the surface waters to feed, but during the day plankton dive to escape visual predators and hide in the shadowy depths, taking an escape elevator to the basement bunker. Fortunately, whale sharks enjoy a suite of adaptations allowing them to follow their food wherever it may go.

Near the surface, a whale shark’s armored eyes can readily see dense clouds of soupy plankton. Their retinas are packed with both rods and cones, like our eyes, but this duplex design is uncommon among fishes. Rods work well in dim light; cones provide color vision in brighter light. In whale sharks the color receptors are tuned to match the broad spectrum available near the surface, their favored depth, whereas in most fishes they are calibrated for the heavily blue-shifted colors characteristic of deeper waters.³² But when the sun arcs high in the sky, and whale sharks dive after plummeting plankton, they take advantage of their unique duplex retina by switching off cones and allowing rods to take over. Like donning a pair of night-vision goggles, they can now target swirls of plankton even in deep, dark waters.

To reach the tasty depths, a hungry whale shark takes advantage of his negative buoyancy and enters a glide descent.³³ Sinking in a controlled fashion, he steers himself forward with the pectoral fins but holds his tail motionless to save energy. Whale sharks may glide to depths of 6000 feet or more, where the temperature hovers just above freezing.³⁴ Once at the nadir of the dive, our aquanaut opens his mouth to feed and gently uses his tail to propel himself slowly forward. The bracing cold penetrates his thick skin, gnawing remorselessly at his warmth. Icy water flushes over gills rich with blood vessels that carry the chill deep within his body. But whale sharks show an unusual arrangement of musculature: a huge mass of white muscle, poorly enervated by blood vessels, lies in the core of their body, surrounded by highly vascularized red muscle.³⁵ Chilled blood mostly reaches only the red muscle, while the white muscle core remains warm and isolated, functioning like a battery for storing heat. This arrangement inverts the blueprint of warm-blooded tuna, in which the core is composed of red muscle that generates metabolic heat through vigorous swimming.

Despite the appeal of abundant plankton, cold soon saps muscle power and compels our deep-water diner to return to the sunny surface and replenish his lost body heat. On the ascent, his swimming angle is twice as steep as was the descending glide path, and he travels more slowly, at about half the speed of the dive.³⁶ Thus the round trip is highly asymmetrical, pairing a long and swift glide to depth with a short but deliberate swim back to the balmy shallows, like trudging up the stairs at a water park after a gleeful slide to the splash pool below. Compared with horizontal swimming, this asymmetrical glide and climb technique saves swimming energy, maximizes the time spent feeding, and yields a welcome savings of nearly 30 percent in overall foraging efficiency.³⁷

When our whale shark reaches the surface, he triggers another special adaptation: a surge of blood flows to his gills, speeding absorption of much-needed warmth. This phenomenon, also seen in deep-diving blue sharks, supercharges the reheating process and recharges his thermal battery of white muscle. Researchers also discovered that whale sharks spend more time surface-basking after colder and deeper dives, showing that behavioral and physiological adaptations unite to meet the challenges of foraging in deep, cold waters.³⁸

A similar strategy is employed by the appropriately named basking sharks (Cetorhinus maximus). These 50-foot cousins of whale sharks strain zooplankton from boreal surface waters where great oceanic fronts meet and boost productivity.³⁹ They visit such regions only during the summer, when they can simultaneously feed and bask in the summer sunshine. In winter, they undertake great migrations toward equatorial seas, off the coast of Brazil and elsewhere. Upon arrival they eschew the hot surface layer and instead spend their days foraging in comfortably cool waters 2000 or 3000 feet below the waves. Incredibly, they may remain at these depths, without surfacing, for up to five months at a time.⁴⁰ Basking sharks most likely have slower metabolisms than their tropical cousins the whale sharks, permitting them to forage for such lengths in the cool depths. Or perhaps they are just escaping the frenetic jangle of samba music pulsing from Brazilian beaches.

A Whopping Wet Blanket

Round about 66 million years ago, a cataclysm on Earth wiped clean the slate of life. But its aftermath heralded the arrival of most modern fishes. Evidence suggests a colossal meteor strike off the Yucatán Peninsula blasted enough debris into the atmosphere to trigger a fatal shift in planetary climate. Dust storms dimmed the sun for centuries, plant life collapsed, dinosaurs went extinct, and oceans turned to acid. Fossil records show that more than three-quarters of Earth’s plants and animals perished, altering marine ecosystems so dramatically that a million years would pass before they could recover.⁴¹

During the planetary calamity, the deep sea provided a refuge where bottom-dwelling rays persisted, munching away on the few shellfish that survived. Above, a race of huge plankton-eating fishes known as pachycormids disappeared in the paleontological blink of an eye.⁴² As the choking dust settled, continents and oceans embarked on recoveries, and marine productivity surged. Plankton turned the seas emerald green, thanks to nutrient runoff from now-verdant lands, and the extinction of pachycormid planktivores. On one fateful day, it is tempting to imagine, somewhere deep in the sea a curious ray swam up from the abyss into this veritable pea soup of plankton. Its jaws open wide, the ray suddenly found its mouth full of delicious and nutritious algae soup. In that moment, around 50 or 55 million years ago, a new race of filter-feeders was born.⁴³

However it happened, a branch of the eagle ray family tree evolved gradually to specialize on plankton. The mouth position shifted from downward, where shellfish are found, to forward-facing; its opening became larger, welcoming more water and plankton. To counter the tremendous force of pushing open-mouthed through water, struts that had formerly reinforced jawbones for chewing hard shells were repurposed.⁴⁴ Pelvic fins developed stubby outgrowths, like rubbery butter knives, that helped channel even more plankton into the mouth. But in that bountiful broth, which offered pathways to new kinds of life, there lurked new forms of death.

As these novel rays developed a taste for plankton, toothy predators began returning to the oceans. One of these, the fearsome megalodon shark (Otodus megalodon), dominated the oceans for some 20 million years, savaging all but the largest marine animals.⁴⁵ At the same time an ancestor of the modern sperm whale, nicknamed Melville’s leviathan (Livyatan melvillei), was attacking squid and other large prey with foot-long teeth.⁴⁶ These animals co-occur in fossil deposits in Peru, testament perhaps to an evolutionary race to gigantism in which each relied on its enormous size for defense against the other. In the face of these terrifying and massive titans, placid plankton-eating rays had only one option, evolutionarily speaking: join the race to enormity.

Today’s manta rays are the culmination of this evolutionary saga. Their newfound bulk afforded them some measure of protection from giant predators, filled an ecological niche vacated by extinct planktivores, and granted the diverse benefits of gigantism. What were once modest-sized, shell-eating, bottom-dwelling rays evolved after the meteor strike into the filter-feeding colossi of today. Giant manta rays (Mobula birostris) are built like flying carpets, and broad enough to transport your entire family, and a couple camels for good measure. “Manta” derives from the Spanish word for shawl or blanket, and what a whopping blanket they are. The largest mantas can weigh 4400 pounds and boast a 23-foot wingspan, capable of blanketing six king-sized beds.⁴⁷

Manta rays and whale sharks have much in common and often frequent similar locations, like gourmands favoring the same obscure restaurants. Both are supremely efficient feeders, cruising slowly with enormous mouths agape, filtering a meal of plankton with highly modified sieves. Mantas, however, swim with bird-like flapping of their expansive pectoral fins, the largest animal on earth to use this form of locomotion. Reinforced by an interwoven skeletal mesh, those powerful yet rubbery fins propel the animal with a hydrodynamic efficiency that rivals tuna and mackerel, albeit at lower speeds. Each aquatic flap is asymmetrical, the wings reaching much higher on the upstroke, pausing briefly while the ray glides effortlessly, then driving shallowly downward to complete the cycle. Superimposed on this stroke is an oblique undulation that twists the fin, further increasing power output. Their novel swimming motion is so economical that a team of scientists studying manta kinetics applied their findings to a unique invention.⁴⁸ They built a wholly new type of remotely operated underwater vehicle, the MantaBot, whose silicon wings beat in a convincing mimicry of the real creature. The team leader’s name, Dr. Frank Fish, is a droll bonus; one presumes he also speaks most forthrightly about his chosen subject.

Manta rays and their close relatives are instantly distinctive, thanks to their catamaran-shaped head. Projecting forward from either side of the mouth are twin paddles called cephalic (head) fins. These arise embryonically as lobes of the pectoral fins, a developmental process spurred by genes (like Hoxa13 and Hand2) that also promote the appearance of claspers in male sharks, another set of modified fins.⁴⁹ Those two genes even played a key role in the evolution of terrestrial animals like ourselves, spurring the fin-to-limb transformation that allowed our ancestors to crawl from the sea.⁵⁰ In the gigantic oceanic mantas and reef mantas (Mobula alfredi), cephalic fins are robust and oar-shaped, while in eight modest-sized kin the smaller and more pointed protuberances resemble horns, earning this group the nickname of devil rays. Their purpose, however, is more dietetic than diabolical: the paddles stretch forward and curl downward, forming a funnel that channels vast volumes of plankton-rich water into the mouth. Once engulfed, the green chowder is passed through gill rakers, cartilaginous arches lying atop the large gills, that sieve a meal of plankton from the water using an innovative approach which is today inspiring novel engineering solutions to an age-old problem.

In a typical filter—imagine a kitchen sieve straining a pot of watery rice—the filter pores eventually get clogged, and water can no longer run through. Most filter-feeding animals will backflush their filters with a forcible cough of water, rinsing the pores clear but losing a plateful of food in the process. Whale sharks cough once every 7 or 8 minutes when feeding.⁵¹ But mantas never do, a fact that caught the attention of marine scientists and mechanical engineers. In sieve-filtering fishes like whale sharks, water passes perpendicularly into the filter, and through tiny pores that trap all larger particles. But scientists discovered that particles smaller than the pores were still being filtered and swallowed by mantas, yet no caking or clogging of the filter occurred. How was this possible? No filtration system invented by engineers could achieve such results.

It turns out that plankton particles in mantas are skittering across the top of a filter that resembles a washboard: tightly packed rows of miniature speedbumps, with pores at the bottom of each valley. As edible particles stream along, they collide with the first bump and are bounced upward into the flow; at the same time, a little water drains through the pore. As the process repeats, particles ricochet off successive bumps, eddying in the moving water until a dense cloud of particles has accumulated. Amazed engineers dubbed this innovation “ricochet separation” and are hotly pursing its application to industrial filtration applications, where cleaning of filters is a tedious and expensive chore.⁵² For mantas, the innovation means that more plankton are filtered more efficiently, since smaller particles can be captured, and there are no delays for coughing. Pushing water over a washboard rather than through tiny pores also imposes less drag, so manta rays burn fewer calories while swimming through their soup. Unfortunately for giant rays, their filters have caught the eye of more than just engineers. Gill rakers are prized in traditional Chinese medicine as an alleged cure for ailments from acne to cancer (no evidence whatsoever supports these claims), and surging demand has devastated ray populations around the world.⁵³ Their decline is heartbreaking in part because of a growing understanding that mantas are highly intelligent and remarkably social.

Nestled between their cephalic fins, manta rays have one of the largest brains ever measured in a fish.⁵⁴ This discovery was somewhat surprising as filter feeding would not appear, superficially, to be intellectually challenging: just open your mouth and swim. But manta rays assail clouds of plankton in much more intricate performances, engaging in choreography befitting a ballet company. Feeding mantas will swim head-to-tail like a conga line, or perform repeated somersaults to corral plankton, or swirl into a coordinated cyclone, compacting helpless plankton in the center before diving through the storm’s eye to feed. Giant manta cyclones may even draw cold, nutrient-rich water up from the depths to fertilize surface plankton. Engineers (who really are starting to owe a professional debt to manta rays) have applied these social feeding techniques to optimization algorithms, mathematically guided searches for solutions to convoluted problems like aiming solar panel arrays on partly cloudy days.⁵⁵ Impressively, the manta-inspired formulas outperform existing optimizers in nearly all cases.

Despite being most commonly seen at the surface, mantas are known to feed at considerable depths. In New Caledonia, reef mantas equipped with satellite tags regularly dove to below 1000 feet (occasionally to 2200 feet), almost always at night, and lingered there for up to 10 minutes in a U-shaped dive profile that suggests a foraging pattern.⁵⁶ Off the coast of Ecuador, isotope analysis revealed that giant mantas habitually feed in deeper waters, rather than on surface plankton.⁵⁷ This biochemistry technique compares the relative proportion of carbon and nitrogen isotopes—molecular variants with slightly different weights—in manta tissues against their preponderance in plankton at various depths. The isotopic signature gradually accumulated in muscle and fat can reveal lifelong foraging patterns. After a feeding plunge, mantas warm themselves at the surface, basking like whale sharks. Their large brains benefit from having heating blankets like those in marlin and swordfish, dense networks of blood vessels that keep the brain warm and functioning acutely to plan and execute the next dive.⁵⁸

In the shallows where divers can observe them, mantas routinely socialize with each other. Individual rays can recognize one another by variations in their distinctive patterning theme: the black backs of reef and giant manta rays are marked with broad strokes of white chevron-shaped clouds behind the eyes and stripes pointing to the wingtips, and their white bellies usually display black speckles. These patterns can change, often dramatically, during feeding or intense social exchanges.⁵⁹ Extreme variations occasionally occur, like the bright pink reef manta ray discovered in Australian waters and nicknamed Inspector Clouseau after the bumbling detective of Pink Panther films. On the reef, mantas gather into distinct groups, respond to one another by wagging and curling their cephalic fins, and show complex, lasting bonds.⁶⁰ Females prefer the company of other females, and teenagers hang around their own age group (as any parent can confirm); males, in contrast, show few lasting associations and congregate indiscriminately at the richest of feeding sites.⁶¹ In the flamboyant coral reefs of Raja Ampat, Indonesia, rays spend hours at cleaning stations where small fishes scour the bodies of larger animals, plucking parasites and exfoliating algae. Reef mantas gather in large numbers and remain faithful to a particular spa, jostling for space even when alternate sites are available quite close by.

Sociality is particularly inflamed when mantas reach the age of reproduction. In this, however, they are less like fish and more like humans. Mantas are long-lived creatures, delay mating until they are nearly full sized, bear live young but infrequently, and give birth to only a single offspring at a time. Reef mantas, for example, may live for forty years and reach sexual maturity only around their tenth year.⁶² Gestation can require twelve months or more, and females will usually delay up to three additional years before mating again. Courtship is intensely social, with receptive females leading multiple suitors on a “mating train,” a vigorous dash through open water punctuated by somersaults that are mimicked by the males. This underwater steeplechase helps the female select the most vigorous male. Once a worthy partner has outlasted all rivals, he swims atop the female, seizes her pectoral fin in his mouth (oddly, it is almost always the left fin), jostles her body until his belly presses against hers, then uses his clasper fins to manually inseminate her.⁶³ During gestation the developing baby ray is bathed by uterine milk that provides nutrition and oxygen. By the time she or he is born, a manta ray pup is enormous, up to half the size of the mother.

Across the marine giants, a full range of reproductive strategies are on display, revealing how many pathways there are to evolutionary success. Manta rays emphasize the survival of a single offspring, investing as much energy as they can in the pup. Whale shark females carry 200 or even 300 eggs and give birth to tiny youngsters. Incredibly, fertilization of these embryos can be staggered in the mother, with some offspring born early while later-fertilized embryos remain unborn for weeks or months.⁶⁴ Females may achieve this assembly-line approach by storing sperm, then releasing it for intermittent fertilizations, something seen in blue sharks over periods as long as twelve months.⁶⁵ Staggered birthing is literally the opposite of placing all your eggs in one basket: by spreading their young across the ocean like a necklace of pearls, female whale sharks increase the chances that at least one or two find themselves in a perfect nursery, predator poor and nutrient rich. If winning the lottery of reproduction is your aim, then playing a lot of tickets can be the best plan, precisely the strategy pursued by superlative ocean sunfishes.

The Greatest and the Slightest

An egg is always an adventure: it may be different.

—Oscar Wilde, The Wit and Humor of Oscar Wilde

When a prickly little ball was sieved from Australian waters in 2017, a fish larva resembling a tiny balloon studded with transparent pyramids, its identity remained a mystery. Unlike mantas and whale sharks, most fishes release their eggs to hatch in open water rather than developing inside the mother. For each miniscule larva it must be quite a shock to emerge from a cozy egg, set abruptly adrift in an empty ocean to survive all by itself. As soon as their yolk is exhausted, they must rely on nascent eyes and barely formed fins to find and pursue food, until they reach their adult form. Identifying these underdeveloped larvae, which look maddeningly different from their parents, can be an equally intimidating task. But after three diligent years, researchers at the Australian Museum solved the enigma of the “conspicuous, rotund, and distinctly spiky” creature who measured just one-tenth of an inch across.⁶⁶ It was the newly hatched offspring of a bump-head sunfish (Mola alexandrini), the heaviest fish in the world.⁶⁷ Had it not been netted by science, its destiny would have been to attain an adult mass of more than 6000 pounds, a flabbergasting 60 millionfold increase over its birth weight.⁶⁸ Even more astonishing was the discovery that an adult female can carry up to 300 million eggs, making it the most fecund fish in the world, yet another sunfish superlative.⁶⁹

Sunfish are peculiar in just about every way imaginable. Their scientific name, Mola, is Latin for millstone, an admirable description of a weighty fish shaped improbably like a giant disc. They have only two fins, one projecting toward the surface, the other toward the seafloor. Instead of a tail, they sport a truncated, fleshy lobe that serves as a rudder. Like a millstone, their body is entirely rigid: vertebrae and ribs have been severely reduced over 10 million years of evolution, and today all propulsion is provided by the two fins. They are surprisingly powerful, however, permitting sunfish to swim against ocean currents and travel at speeds that rival cruising marlins and sharks.⁷⁰ Only one other kind of animal swims as they do, and it is not even fish, but a penguin. Both sunfish and penguins flap their paired fins in unison, generating lift that is converted to forward momentum by rotating the fin angle, more like a bird winging through the sky than a rowboat thrust across a lake.

Closely related to the bump-headed sunfish is the equally spectacular ocean sunfish (Mola mola; the fish so nice they named it twice); a third species, the hoodwinker sunfish (Mola tecta) was described only in 2014.⁷¹ All three molas are massive, with ocean sunfishes reaching 9 (or possibly 13) feet across. Their skin, covered in multi-cusped, jagged scales, is gritty and rough. Beneath the skin, however, lies a thick layer of rubbery gelatin that can account for more than 40 percent of its total body weight.⁷² Researchers surmise the layer helps insulate the fish during deep dives for food, a habit it shares with mantas and whale sharks. It also helps them reach and return from these depths, as its low density renders the fish neutrally buoyant. The gelatin may be acquired from one of their favored foods, jellyfish, a preference that has earned molas the title of gelativore, or jelly-eater.

After sunfishes dive, they bask on the surface, turning sideways until they lie completely flat just inches below the waves, catching rays until they warm again for another plunge. The longer the dive, the longer they sunbathe, suggesting that basking is a behavioral mechanism for thermoregulation after long forays into deep, cold waters.⁷³ All the ocean giants rely on behavior changes to maintain their body thermostats, save one. Opah (Lampris guttatus), a large disc-shaped species superficially similar to the molas, was discovered in 2015 to be the world’s first truly endothermic fish.⁷⁴ These open-water giants, tipping the scales at some 600 pounds, can sustain internal temperatures 8 degrees above that of the water surrounding them, the heat generated by dark red pectoral swimming muscles.⁷⁵

Drifting back to the basking molas, several researchers suggest their surface loafing may be an attempt to encourage cleaning of parasites that grow copiously on their large frames. Seagulls have commonly been observed visiting lounging sunfish to pluck crustaceans and other pests from their upturned flank, as if from a crumb-strewn picnic table. The common name of sunfish stems from this basking behavior, as most early encounters occurred when seafarers stumbled upon them during tanning sessions (one imagines a startled fish hastily reaching for a towel to cover her modesty). When lolling at the surface, an enormous fin may occasionally flop into view, provoking anxious but deceived beachgoers to report an imminent shark attack. In southern Massachusetts a flood of such calls to 911 prompted emergency services to respond forcefully via text: “The sunfish is doing normal sunfish activities. It’s swimming. It is not stranded or suffering. The sunfish is FINE … PLEASE STOP CALLING THE POLICE DEPARTMENT ABOUT THIS SUNFISH!!”⁷⁶

Unfortunately for sunfishes, their habits of luxuriating at the surface and feeding (like basking sharks) at oceanic fronts where productivity is rich, put them on a collision course with the world’s fisheries. Though rarely the explicit target, they are trapped and killed by indiscriminate gillnets with shocking frequency. In the Mediterranean during the early 1990s, ocean sunfish composed between 70 and 93 percent of the total catch by Spanish drifting gillnets.⁷⁷ A decade later, more than 30,000 sunfish were still captured every year by the Moroccan gillnetting fleet.⁷⁸ Even in well-managed waters like California, where swordfish fisheries also rely on gillnets, nearly a third of all bycatch (nontarget captures) hauled on board were ocean sunfishes, their discarded bodies preposterously outweighing the total catch of the swordfishes.⁷⁹

The decline of ocean giants like sunfishes, mantas, and whale sharks may have cascading consequences for their oceans. Filter feeders and jellyfish specialists have long been thought to control, through predation, the abundance of jellies and their ilk. With the decline of these giants, the swift reproduction of their prey threatens to overwhelm the sea, with disastrous costs. Jellyfish are notorious predators on fish larvae, and the overabundance of the former can smother the latter’s survival. Numerous studies around the world have signaled increases in jellyfish abundance.⁸⁰ Some evidence suggests we are merely on the upswing end of a twenty-year cycle driven by regular oscillations in ocean conditions.⁸¹ But too many human-driven factors have been implicated in the surge to ignore our own impact: global warming, increasing nutrient loads, ocean acidification, and overfishing of filter-feeders are all proven to provoke blooms of jellyfish.⁸² In one assessment of combined human impact on oceans, jellyfish outbreaks had plagued six of the top ten impacted coastal regions.⁸³ In crisis, however, opportunity can be found. People have been eating jellies for centuries, and today nearly a half-million tons are consumed annually in Southeast Asia alone.⁸⁴ Japanese food scientists have even engineered a way to squeeze the water from these gelatinous creatures, rendering them into a rubbery and edible patty. So you might double-check the fine print next time someone offers you a peanut butter and jelly sandwich in Tokyo.

Just as the absence of ocean giants can lead to an unraveling of the marine realm, their protection can yield tremendous benefits, beyond the massive value of their tourism appeal. We now know that large marine creatures like whales and seals, but also sharks and mantas and whale sharks, transport nutrients from the open ocean to nearshore reefs. After feeding in the depths, all these animals return to the surface where they defecate and release valuable nutrients that sustain healthy reef communities. Fancifully termed “the whale pump,” one estimate suggests that whales alone recycle more nitrogen to the surface waters of the Gulf of Maine than the combined input of all the region’s rivers.⁸⁵ In Pacific coral reefs, more than three-quarters of the nitrogen released by grey sharks was brought in from open-ocean feeding forays.⁸⁶ And the fidelity shown by manta rays to specific coral sites, where they return for routine spa treatments, underscores just how much investing in local conservation efforts can provide lasting returns for coral reef ecosystems, and for the people whose livelihoods depend on them.⁸⁷