Introduction
You glide effortlessly through the balmy and crystalline waters of a tropical coral reef, propelled by coordinated flaps of your wing-like fins. Peering overhead, you catch your reflection on the silvery surface: a female blue-chin parrotfish looks back at you. Large in size and sturdy of build, you are merrily painted with alternating bars of yellow and blue. You wield a powerful set of beak-like front teeth, to expertly scrape algae from rocks. Occasionally, you take a vigorous bite of coral, and extract the soft residents by crushing their hard exterior with a constellation of flattop teeth that stud your muscular throat. At night you spin a cocoon of mucus, a sleeping bag of sorts, that protects your scaled surface from nautical parasites and prevents nocturnal predators from picking up your scent as you doze. By day you cruise the reef, your bright colors on full display, illuminated by the ballet of sunshine refracted through the waves. Now and then you take a brief break at a cleaning station, where attentive wrasses and perhaps an industrious shrimp provide the full salon grooming treatment, plucking tiny, bloodsucking parasites and encrusting algae from your scales. Emerging fresh and clean, you detect a scent in the water, a faint but unmistakable odor that signals courtship is underway.
Excitedly, you scurry to a protected area amid boulders and corals and join a handful of other females, each competing for the attention of one dominant male parrotfish. You females far outnumber the solitary male, a large fellow with a pronounced hump on his forehead, who holds court over a sort of harem; this is polygyny at its most pronounced. Diluting his command only slightly are a few inferior males, smaller and less adept, who hang around the margins of the dance floor, hoping to steal a kiss, as it were. Suddenly, in a demonstrative rush, two females turn their chins toward the sky and rocket upward to the surface. Not wanting to miss your chance, you lash your tail and chase exuberantly after them. A few meters beneath the waves, in spectacularly coordinated maneuvers, you and the surging females jettison eggs into the water at the precise moment when the top male knifes through the crowd, releasing a white cloud of sperm. With luck, a few dozen eggs are successfully fertilized, carrying into future generations the genes of several females and the single dominant male. Occasionally, one of the satellite males will streak through the crowd and surreptitiously release his own gametes; now sperm competition in open water determines whose genes fertilize whose eggs.
But your eggs are shed, your die is cast, and the fate of your offspring is now entirely out of your hands, or fins. Exhausted by your efforts, you retire to a still and shady location under a slab of encrusted rock. The coffee break may have saved your life. Because above you, just as you disappear from view, a silent killer slashes into the crowd of spawning fish. An oceanic whitetip shark, speeding onto the reef from dark blue waters nearby, seizes the dominant male with saw-like teeth and drags him into the depths to his death. This brutal abduction from the seraglio leaves the harem without a master, the crowd of stunned females without a mating partner. A void in the hierarchy has been created. Then, something truly amazing happens.
You sense an advantage to be had. If you were to take the place of the dominant male, every offspring of every fertilized egg of every female would carry your genes. No sooner does this revelation dawn than your body begins a remarkable transformation. The color of your skin changes, and you begin to display new hues and strikingly different patterns. Your yellow and turquoise stripes fade, supplanted by brilliant bluish-green on the flanks, lavender markings on your fins, and a spiderweb of bright blue lines around your mouth and eyes. The graceful curve of your forehead begins to swell into a comical bump shape, as though you had been clobbered by a cartoon club. Externally, you are swiftly becoming quite distinct from the other females; soon, you will not recognize your reflection. But internally, an even more shocking change is taking place. Something miraculous is about to happen, something landlubbers can scarcely imagine, yet it is just one of a thousand wondrous mysteries that play out beneath the waves every single day.
You, our protagonist parrotfish, were born a hermaphrodite, possessing the genes and reproductive capabilities of both genders. You began life as one but are about to transform into another. As your forehead swells and the colors of skin and scale transform, your estradiol levels begin to decline, and your testosterone levels climb. As this modified hormone balance courses through the circulatory system, your body begins to respond. Your ovaries shrink and your testes enlarge. Shortly, you lose the capacity to produce eggs, but this loss is offset by a newfound ability to release countless sperm: you have become a male. In light of the gender-swapping of many fish, the terms “female” and “male” now seem ill-fitting. Instead, you have transitioned from an “initial” phase to a “terminal” phase adult. The conversion complete, you are ready to seize control of the harem of other females—initials—as the dominant breeder. Now, when spawning occurs, all the eggs will be fertilized by you, thanks to the hormone-induced activation of your latent sperm-producing powers. All the offspring will carry your genes, as the little larval fishes are swept through the ocean by currents. Today you are a terminal-phase, harem-holding blue-chin parrotfish, and the father of millions.
Mysterious and Mirrored Waters
All that we do is touched with ocean, yet we remain on the shore of what we know.
—Richard Purdy Wilbur, For Dudley
The ocean is more mirror than lens. It reflects back to us our aspirations and dreams, our struggles, our failures and victories. It reveals souls to poets, portrays sunsets for painters, and echoes the stars floating overhead. Yet scarcely do we see what lies beneath the surface. One can gaze upon the sea for hours, marvel at glimmering lights dancing on the ripples, at the moon suspended amid flecked foam. We can observe patiently, penetratingly, but never catch so much as a glimpse of the life below the waves. We barely know the fish that see those same waves, those same stars, that same moonrise, but all from below.
Thousands upon thousands of species call the sea home. Indeed, a single drop of seawater can contain more than a million organisms. Most are tiny algae and other invertebrates, but every drop can also transport hundreds of microscopic larvae that will grow into fishes, crabs, squids, and more. Fishes are astoundingly diverse, with 33,700 described species comprising half of all vertebrates on planet Earth.1 They live in the sunlit shallows and in the black depths. They hunt in open waters and over the continental shelf. They hug the bottom or fly above the surface, squeeze into crevices, bury in sand, or flash through the great three-dimensional watery space that covers most of our planet. There are as many surprising strategies for capturing prey as there are for eluding predators. There are as many extraordinary exceptions as there are rules. And there are as many entertaining tales as there are fish in the sea.
Still, as land-dwelling humans, we are largely blind to the anthology of stories whose main characters are marine fishes. For us, fishes are restricted to tanks in an aquarium, or digital representations in popular films, or breaded fillets. If you asked your sister or brother to draw a pollock, or a haddock, could they? Given that most people have probably eaten pollock, and two-thirds of the planet is aquatic, it is shocking how little we know about fish.
A minority of us, albeit a sizable one, enjoy sport fishing in lakes, streams, and the sea. Anglers know quite a bit more about the lives of the fish they seek with hook, line, and lure. Ask anyone who fly fishes, they command a detailed knowledge of the insects that are hatching, the fish that feed on them, and how to trick them into taking a manufactured fly instead of the genuine article. But the wet feet and waders and bass boats only get anglers slightly closer to fish; we remain forever above the surface. We only infrequently spy our quarry before they are on the line. Those fleeting peeks through murky waters or tumbling rivulets afford limited glimpses into the lives of fish. We know not how they court, where they shelter at night, why they choose one pool over another, nor how they perceive their world. While there is much we do understand, there is far more that eludes our ken.
Even fewer of us are fortunate to enter the sea with a snorkel or scuba tank, where we can briefly observe fishes in their natural environment. Our gear is like a temporary backstage pass to the world behind the curtain of the waves. Divers slip beneath the surface, where they can watch fish with the same intensity that bird-watchers apply to their feathered friends. Spellbinding behaviors are revealed: a moray lurching from its narrow cave to nip at prey; eagle rays swatting the sandy bottom as though unfurling picnic blankets, flushing sediment to reveal hidden crabs; a richly colored damselfish tenaciously guarding a patch of eggs glued to a small rock face.
Amid all the diversity, the colors, the movement and sound, however, it can be easy to forget just how limited are our underwater experiences. People rarely spend more than an hour beneath the waves before returning to the surface. Almost all diving is done by day; even on a rare night dive, your perspicacity is limited by the strength of your flashlight. Most scuba and snorkel trips are near the shores of continents or islands, so we rarely venture into the great open waters. We also explore only the shallowest fraction of the ocean: depth limits for recreational diving are around 150 feet, while the average depth of the ocean is over 12,000 feet. Consider, then, the enormity of what we do not see, compared with the infinitesimal sliver of marine life into which we peer. The dark depths, the open ocean, the seamounts and undersea ridges, the abyssal trenches, all are hidden from us, as are the creatures who therein reside.
Waves of Diversity
Fishes were among the first vertebrates—animals with backbones—to appear on Earth, half a billion years ago. They survived each of the planet’s five cataclysmic extinction events and multiplied in diversity during the rich years that followed. Many of the fishes we know today descend from recognizable ancestors in the Devonian, a geologic period that began over 400 million years ago and was characterized by an equable climate, both terrestrial and marine. In the sandy and silty bottoms of warm tropical seas, the so-called Age of Fish had begun.
From primitive scaled creatures with stubby, fleshy appendages arose a lineage of fish that possessed true fins, stiffened with bony spines. Three pairs articulated to the body: pectoral (the parrotfish’s wings) from the flanks; pelvic and anal fins emerged aft, from the belly. Another fin, the dorsal, topped the back like a sail; a caudal fin tipped the muscular tail to provide thrust. Fish bearing such fins were magnificently successful at exploiting complex undersea environments. Across the breadth and depth of the oceans new species bloomed at an astonishing rate, giving birth to many of the fish families swimming the seas today. Hot spots of diversity erupted in coral reefs and rocky submarine peaks, those places with complex topography that nurture rich communities of algae and invertebrates. The explosion of fishes found its epicenter in the island-speckled western Pacific, where the ocean’s great age, its stability, and a galaxy of shallow environments offer unrivaled conditions for biodiversity.
Fishes live together, not alone, and are engaged in a constant evolutionary ballet with one another. Schools of sardine and herring developed intricate choreography to feed, and to avoid being eaten. Meanwhile, mackerel and tuna and other speedy predators adopted creative responses to hack this group defense and snatch a mouthful. Fishes help maintain their homes, like clownfish who fertilize an anemone abode, or humbugs who fan oxygen over the coral that shelters them. Colleagues as dissimilar as a grouper and an eel work together closely, flushing prey from narrow tunnels and engulfing them in a huge mouth. Little fishes enthusiastically groom larger fish; huge fishes tow petite companions across oceans. One fish even strikes up a touching relationship with a blind shrimp, who lacks sight but boasts a weapon powerful enough to defend them both. Such relationships, each a dance of coevolution, built diversity upon diversity as watery eons rolled through our planet’s seas.
While the continents were undergoing geological upheaval and climatic turmoil, the oceans remained comparatively stable. Water heats and cools more slowly than air and is buffered—though not completely—from drastic fluctuations in temperature, acidity, and more. Rising and falling sea levels alternately drowned and desiccated coastal regions, rains drenched interior forests or left them as deserts, glaciers advanced like bulldozers, but oceans were left largely unaffected. During millennia of stability, fishes were free to adapt ever more exquisitely to their environments. Still, global extinction spasm sporadically rocked the oceans, sometimes wiping out as much as 90 percent of all species. And yet, the marine realm retained enough organisms in the wake of those decimating events to rebuild its diversity swiftly, often in just a few million years.
Our seas are exceptionally resilient, quick to recover diversity, ecological function, abundance, and productivity, if given the chance. This resiliency is at the heart of modern ocean conservation strategies. On islands where excessive fishing has devastated reefs and fish populations, the creation of protected “no-take” zones has proved immensely successful. With heartwarming speed, fish can recover to previous levels: total abundance surges, average size increases, and reproductive potential swells exponentially. These protected areas more than pay for themselves, through the spillover of juvenile and adult fish who boost productivity in neighboring marine sites. Fishers who initially opposed marine protected areas swiftly become converts, their staunchest supporters and most strident defenders. The recuperative power of the seas, their ability to heal themselves if insults like plastics and pollution are reduced, if excessive fishing is scaled back, is a reason for profound optimism. For every challenge that confronts oceans, proven solutions exist, and evidence abounds that the marine world, a vital contributor to life on this largely blue planet, can be kept healthy and productive.
The seas cover about two-thirds of the surface of the Earth, but because they are three-dimensional and deep, they represent 99 percent of all habitat on the planet. Imagine rolling together all the rainforests, marshes, deserts, pine woods, grasslands, deciduous forests, and every other terrestrial ecosystem into a ball; that sphere packed with plants and animals would be dwarfed by the colossus of water that is the marine realm. Immense storehouses of abundance and extravagant productivity, the oceans harbor some 2 billion tons of fishes, six times the weight of all the people on Earth. But fish abundance is not evenly distributed. The relatively unproductive centers of oceans are sparsely populated, while marine abundance flourishes in waters that are cold and rich in nutrients: over continental shelves where rivers discharge, in the oxygen-rich polar seas, and at the delivery ends of deepwater conveyer belts known as upwellings. These places are home to colossal fish schools that sustain the planet’s biggest fisheries, whose nets and lines procure 100 million tons of seafood every year.
Feeding a Planet
Perhaps I should not have been a fisherman, he thought. But that was the thing that I was born for.
—Ernest Hemingway, The Old Man and the Sea
Since the first human strode across a dune to view the great sea, men and women have hunted fish. Our ancestors used lines, poles, hooks, spears, weirs, and nets to capture the silent animals that slipped stealthily through the waters. To this day, fish provide some 3 billion people on Earth with a substantial portion of their daily protein.2 The bounty of the oceans granted meat that could be eaten directly, or smoked, salted, or dried for consumption during long, cold winter months. Advanced civilizations, such as Indigenous Americans of the Pacific Northwest, developed rich cultures bursting with art, storytelling, weavings, and dances all made possible by ready access to protein: breathtaking runs of millions of salmon. Coastal communities around the globe have subsisted on ample harvest of shellfish, crabs, barnacles, and other intertidal animals that could be plucked without resorting to a boat. Cultures on South America’s coast invented an advanced monetary system using coins carved from the wine-colored shells of Spondylus, a local mollusk. In the Far East, algae have been gathered or farmed for millennia, providing valuable calories and essential minerals and vitamins.
Formerly, the prodigiously productive oceans met our needs with barely a shrug. Fish and crab and clam populations rebounded as fast as we could harvest them. Our population and our demand for seafood have grown, however, and fisheries have intensified: larger ships, longer voyages, bigger nets, stronger lines, an industrialized approach to a noble art. There is room still for the sport fisher, the hobbyist, and the coastal communities that ply their trade in boats of wood and fiberglass, risking their lives on the pummeling waves to bring a meal to families and neighbors. But massive and heavily equipped industrial operations are fishing the seas so thoroughly that hook-and-line small timers are being driven out, heading for home with boats nearly empty.
In response, the world has taken a dramatic step forward, rearing immense harvests of farmed fish in aquaculture facilities based on land and in the ocean. Tremendous financial and scientific investment has been directed to this effort. The yield from all aquaculture operations, both freshwater and marine species, now equals the total production of global fisheries. While capture of fish upon the ocean waves is sure to continue, there is a pleasing echo of history in humankind’s evolution away from hunting wild game and toward the stewardship of animals reared in farms. If managed well, aquaculture can alleviate pressure on wild fish populations while satisfying global demand for seafood. We can quite literally have our fishes and eat them too.
Setting Our Course
The sea, once it casts its spell, holds one in its net of wonder forever.
—Jacques Cousteau, Life and Death in a Coral Sea
This book is an exploration of the sublime spectacle of exotic behaviors and curious adaptations exhibited by saltwater fishes. Thanks to nature programs and magazines, zoos and ecotourism, we know plenty about elephants, giraffes, chimpanzees, toucans, and tortoises. We have learned some of their fascinating life history stories: box tortoises pull into their shells for defense, elephants march for miles to communal graveyards, chimpanzees can use tools and learn sign language. These animals have become familiar to us. But that familiarity does not extend to the denizens of the oceans. We can hardly recognize a frogfish, describe the development of a flounder, or comprehend the size of a full-grown manta ray. Fishes are a mystery as deep as the sea. Did you know that marlins have an exotic heating organ to warm their eyes, by as much as 25 degrees, so they can see better while hunting in cold waters? Or that eels can use a compass, but only on a one-way journey? In the chapters ahead we will embark on an underwater safari, to discover some of these spellbinding and awe-inspiring creatures of the saltwater world.
The diversity of marine fish species is bewildering, and we cannot attempt to cover them all. Instead, this book divides the ocean into broad zones: the open ocean, the seafloor, and the shore. In each, chapters portray groups of fishes that behave in similar ways, such as open water predators like tuna and swordfish, or that live in particular environments like coral reefs, seamounts, or the deep abyss. Each chapter focuses on what makes those fishes unique, on their body shape and arrangement of fins, and how those forms represent finely tuned adaptations to their lifestyles and habitat. For example, tuna are streamlined, torpedo-shaped, and their fins fold into narrow slots so perfectly they lie absolutely flush with the body surface. These are adaptions to a life of pursuit hunting: efficiently cruising hundreds of miles of water in search of prey, then rocketing into schools of smaller fish to feed. In contrast, fishes that live on coral reefs typically are flattened from side to side, like a dinner plate stood on end, to permit slipping into narrow crevices for safety. Their fins may be long, or broad, or tipped with defensive spines, since they do not rely on them to swim great distances; their mouths commonly are projected forward on a long snout, the better to pluck tiny invertebrates from rock faces. Marine fishes present an enchanting array of forms, and each provides clues to the fish’s life story, environment, and evolutionary history.
Form and function are inextricably linked, in fish as in many things. We will explore how fish behaviors are facilitated by body shape, fin placement, skin coloration, and internal morphology. Parrotfish—your sexually flexible alter ego of a few pages ago—largely subsist on a diet of stony corals and hard, calcium-rich algae. To harvest these encrusting food sources, their front teeth are sturdy, broad, and protrude almost like a beaver’s. With them a parrotfish can take a bite of coral, itself a colony of soft polyps surrounded by a protective cement rampart. But like a baseball player with a mouth full of sunflower seeds, they must crush the cement to release the polyps before swallowing them. The interior of a parrotfish’s mouth is studded with bizarre, molar-like teeth, which even extend down into a muscular throat. Those mouth molars and muscular pharynx pulverize the cement skeleton, releasing the soft, nutritious polyps for digestion. Rich in alabaster-colored calcium carbonate, the cement dust is then excreted onto the reef: parrotfish excrement is a principal component of the gorgeous snow-white sand of tropical beaches.
Because the oceans are such an alien environment to us, the adaptations and behaviors found there may be astounding to land dwellers. Fish are free from the size limits imposed on terrestrial animals by gravity—little wonder that the largest animals on Earth, great whales and giant squid, are exclusively marine. The oceans are deeper than Mount Everest is tall, and curious fish forms have evolved in the abyssal depths where they must adapt to the crushing pressure, extreme cold, and acute scarcity of food. Water holds far less oxygen than air, but sound travels faster and farther: damselfish and toadfish communicate habitually with whistles and chirps. Light penetrates only the first few hundred feet of seawater, and certain colors like red drop out almost immediately; however, chemical signals disperse superbly in water, so many deep-sea creatures are blind to imagery but communicate by pheromones. Each of these adaptations opens a window into the environment in which the fishes live and evolved.
Most fishes exhibit life history characteristics that place special constraints on the fisheries that pursue them, and the diners who eat them. While certain species are fast growing, like anchovies, and can be caught in great numbers, they undergo mysterious population fluctuations that befuddle agencies tasked with setting catch targets. Others, like orange roughy, are exceptionally slow growing and late to reach breeding age.3 While early catches were bountiful, now restraint is required to ensure young fish can safely grow and reproduce. Large predators like swordfish and tuna have a disagreeable tendency to accumulate mercury in their muscles.4 Coldwater fishes like cod and pollock can be spectacularly abundant, but they are sensitive to minute fluctuations of temperature and food availability. If overfished too dramatically, as unfolded in the northwest Atlantic, their populations can cross an invisible tipping point, never to return. Without knowing the life history of each fish, fishers cannot successfully catch them, but neither can the public contribute intelligently to their lasting stewardship.
The final chapter will dive into the challenges facing global fisheries. We will investigate the role of aquaculture in meeting demand for seafood, survey the current state of sustainable fisheries, and explore the swelling tide of ocean conservation. There are a great many successes to relate, in both marine conservation and in fisheries management. We should be buoyed by the breadth of proven solutions, and the abundance of data demonstrating their effectiveness. The island nation of Palau, in just one example, calculated the value of a live shark to ecotourism at $2 million, dwarfing the $100 a dead shark might yield the fishing industry.5 Palau elected to ban shark fishing within its expansive national waters, benefiting not only the marine realm but also diverse local businesses and the underwater tourists who sustain them.
The unseen world of marine fishes may be unfamiliar to us, but their lives are extraordinary, surprising, even arresting. The oceans are full of diverse stories and spectacles, and you are about to enter the theater where they unfold. To truly understand the fishes of the sea, to learn about their forms and fins, their changing colors, the exhausting migrations they undertake, and the ways they greet and eat one another, is to enter a watery and wondrous world that is as spellbinding as a Shakespearean drama. Take a seat; the curtain, or at least the tide, is about to rise.