A River Runs to It

Every yard of river seemed to hold a rolling fish.

—Zane Grey, Byme-by-Tarpon

While fishes of the abyssal depths must wait, peering skyward in hopes of spying a falling morsel, coastal fishes like tarpon live where food literally pours into the sea. With every rainfall, bugs and leaves, fruits and feces, a diverse bounty of organic particles are rinsed from forests and prairies into nearby streams. Those rivulets gather into rivers that make their way to the world’s great oceans. There, the flow of nitrogen, phosphorus, potassium, and other essential nutrients mixes with salt water and fuels the profusion of life that makes saltwater flats and inlets so productive. Plankton bloom, algae abound, clams and mussels prosper, seagrasses grow lush, and fishes from finger mullets to majestic tarpon happily reap the rewards. But there is no such thing as a free lunch, and those fishes who school in the coastal shallows face risks unknown to open-water species.

While Atlantic tarpon (Megalops atlanticus) hungrily patrol the thin layer of water blanketing saltwater flats—along eastern shores of North and South America, and the west coast of Africa—they are themselves hunted by blacktip sharks and bull sharks (Carcharhinus limbatus and C. leucas). These speedy and rapacious relatives of great whites also pursue Indo-Pacific tarpon (Megalops cyprinoides), the only other species in the tarpon family, who swims the waters of eastern Africa, Central America, and Southeast Asia. When predators attack in the salt flats, there is nowhere for prey to hide, nowhere to escape: in a watery film sandwiched between sky and sand, tarpon have few options. But evolution has helped them develop some unique advantages. A hunted tarpon relies on armor, eyesight, the ability to tolerate conditions few others can even survive, and if all else fails, a turbocharged burst of pace that rockets the sleek fish to safety.

Tarpon evolved more than 140 million years ago and have changed little since: genetic evidence suggests they may be among the most ancestral of all bony fishes, their successful body plan remaining stable for many millennia.² Their bodies are elongated and flattened cylinders with deeply forked tails, adapted for speedy travel. Stretching to 8 feet in length, their flanks are packed with powerful swimming muscles, explaining how they can battle an angler for hours on end. Draped over elegant frames like intricately jointed plates of armor are huge, chrome-plated scales. So tough are those scales that only the sharpest teeth can pierce them, and even strong-jawed predators like sharks may bite at an awkward angle only to find their lunch skid elusively away. Tarpon earned the moniker of silver kings from these gleaming scales, silvery enough that they are used attractively in jewelry and ornamental artwork throughout Central and South America.³

Foremost among a tarpon’s defenses is its ability to thrive in waters that few other fishes can abide. Nearshore environments like salt flats and mangrove inlets are highly variable in salinity, thanks to the uneven drainage of freshwater rivers. Most marine fishes are adapted to a stable oceanic salinity of around 35 parts per million of salt, but on the flats the water can veer from 10 to 50 ppm, thanks to freshwater floods during rainy months or evaporation during dry periods. Tarpon, as well as bonefish (Albula vulpes), bluefish (Pomatomus saltatrix), and ladyfishes (a handful of species, kith and kin to tarpon) are among the rare fishes who can handle such violent swings in saltiness. Larvae of all these species—glass-like ribbons that resemble eels—are extremely sensitive to salinity and must spend their first few months of life in the open ocean, where saltiness is unwavering. But as juveniles they migrate inshore and settle in the salt flats, mangroves, and other shallow coastal waters where their newly developed kidneys allow them to tolerate salinity fluctuations. When coastal water is extremely salty, fish kidneys work overtime to extract salt from the blood and release highly salty urine; when freshwater floods in and salinity plunges, the gills actively pull salt from water, keeping the blood in balance.⁴ Salmon and eels and their migratory ilk are the most remarkable of all, as we shall soon see, capable of transitioning from the salty open ocean to pure freshwater. Tarpon can occasionally venture upstream into fresh rivers, but only briefly, and most stick to coastal areas where they are within their salinity comfort zone. But the majority of oceanic fishes cannot safely enter such waters, and when confronted with salt levels too high or too low, like Goldilocks and the Three Bears’ porridge, they give up and turn back to an ocean that is “just right.” Not unlike science fiction aliens in futuristic space crafts, tarpon swim behind an invisible force field of salinity that blocks many predators from breaching their sanctuary.

More than one force field guards inshore fishes from danger. While salt imbalances affect fishes slowly, and rarely fatally, nearshore waters are protected by a second invisible shield, one that can kill in a matter of minutes. As the sun shines on chartreuse meadows, those seagrasses pump out oxygen, suffusing the shallow waters with the life-giving gas. But come nightfall, photosynthesis ceases, while all manner of living organisms—from zooplankton to fishes to bacteria—continue breathing, slowly using up the accumulated oxygen. By dawn, the water can be nearly anoxic. Any oceanic fish that ventures into such waters is entering a death trap: a few minutes in anoxic waters and gills cease delivering oxygen to muscles and nerves, lethargy and paralysis ensue, and if well-oxygenated pockets cannot be found the fish will die of asphyxiation.

On planet Earth there are some 400,000 miles of shoreline (although if you enjoy the mathematical allure of fractals, then coastlines are infinitely long: the closer you look, the more crenellated they appear). Tarpon colonized these commodious inshore environments early in their evolution and discovered ways of surviving within the force fields of salinity and anoxia. Swimming toward shore, a tarpon is like a mountain climber ascending the slopes of Everest. With each step, there is less oxygen. Alpinists make their movements as efficient as possible; no wasted effort can be allowed. Tarpon are the same, swimming slowly and efficiently, staying within the range of exertion permitted by surrounding oxygen levels. As oxygen declines, they flap their opercula to flush ever more water across the gills. At this stage, mountaineers rely on “pressure breathing,” deep exhalations that flush carbon dioxide from the lungs and allow more oxygen uptake. But eventually, such techniques fail to offset declining oxygen, and the climber must pull another trick out of their bag. Literally. At very high altitudes, all but the most extreme mountaineers will grab a cylinder of compressed oxygen from their backpack, don a face mask, and draw strength one breath at a time from the supplemental bottle. For tarpon, that emergency reservoir of oxygen is constantly at the ready: just a few inches over their dorsal fin, above the surface of the anoxic water, lies an atmosphere saturated with the life-sustaining gas. All you have to do is rise, open your mouth, and take a deep breath.

Tarpon, and quite a few inshore (and river-dwelling) fishes can do just that. They are mouth-breathers, whose upturned mouths swallow a gulp of air that is shunted to their swim bladder. While the epithet of mouth-breather can be hurled at the slow-witted as an insult, for tarpon it is a stroke of genius. As we learned earlier, most fishes inflate and deflate the bladder thanks to a gas gland and the Bohr effect, but in many species it is connected directly to the throat: air can be swallowed into the bladder, or burped from it. In tarpon and other coastal fishes, the bladder works exceptionally well as an air-breathing lung. Surrounding a tarpon’s swim bladder are dense networks of capillaries that absorb oxygen from the inhaled air. Blood vessels are plumbed so the freshly enriched blood flows directly to the heart, which then pumps oxygen to energy-starved muscles throughout the body.⁵ Human lungs are remarkably similar, apart from a few differences in plumbing. Thanks to this innovation a tarpon can breathe either water or air—or both. Under normal conditions, a relaxed tarpon may mouth-breathe only twice in an hour; the bladder contributes a paltry 2 percent of the body’s oxygen.⁶ While swimming, however, more oxygen is needed than gills alone can provide, and that rate kicks up to one breath every four or five minutes.⁷ But in oxygen-starved waters, a cruising tarpon must inhale every single minute or it will grind to an asphyxiated halt: the bladder provides half, or more, of the fish’s oxygen. Flush with a breath of air, the tarpon’s muscles hum despite the energy-sapping water all around.

Warm water holds less oxygen than cold, so a tarpon’s mouth-breathing allows the fish to thrive in balmy shallows where few others can survive. Near dawn, when those waters are the most oxygen-starved, a tarpon holds a decided advantage. It can outswim almost any fish it pursues thanks to the turbo effect of breathing air when those around you are breathing water. If attacked by a predator, a tarpon will gulp a mouthful—up to three-quarters of a gallon at once—to supercharge its muscles and dash to safety.⁸ Oceanic predators who cannot mouth-breathe are sluggish when hunting in low-oxygen waters, and once a tarpon takes flight, they cannot give chase. Thanks to the swim bladder innovation, a tarpon can readily power a high-speed getaway and enjoy an undisputed advantage, as hunter or hunted, in the safe haven of its shallow water home.

To see their prey, tarpon rely on oversized eyes, rods tuned to shallow-water wavelengths, and a reflective mirror behind the retina. These adaptations allow them in the purplish first light of day to see better than their prey. Juveniles, who prefer murky waters, have rod-dominated eyes that see dark blues and greens best, precisely the colors that suffuse the gloom.⁹ When they reach adulthood, however, their sensitivity shifts to purples and blues, which better penetrate clear waters. Adults even develop retina cells that can detect UV light, which is abundant in the crystal shallows found on salt flats. For all these reasons, tarpon fishing must take place at dawn: the fish must mouth-breath and can be spotted rolling through the surface; they can see a fly dropped in front of them; and they fight with all the manic energy that mouth-breathed air can give them.

There is only one compulsion powerful enough to draw tarpon away from the protective safety of their inshore force fields. At around eight years of age, tarpon slip quietly from the flats and enter the deep blue sea: in late spring, the urge to mate beckons irresistibly.¹⁰ Spawning takes place well offshore, where sperm and eggs are released to fertilize, then hatch into tiny larvae. A full-grown female can release as many as 12 million eggs, a prodigious output linked closely to her size: the larger the fish, the more eggs it can carry, hence the largest of tarpons are always females. Eggs hatch into unique larvae, transparent ribbons shaped like a willow leaf, each called a leptocephalus. Larvae rely on invisibility for safety, lack red blood cells that would perilously give them color, and feed on marine snow (like deepwater fishes) that they filter through bristling, fang-like teeth. The exact spawning grounds, somewhere in the open ocean, remain shrouded in mystery, though satellite tags hint at breeding waters off Veracruz, Mexico, nearly a thousand miles from the Florida coast.¹¹ Crystal-clear leptocephali remain in the deep blue for three or four months until kidneys and gills develop and they morph into tiny tarpon. Once they can tolerate erratic salinity, they swim back to shore and settle in the stagnant safety of brackish lagoons and mangrove backwaters. Already, these juveniles have learned to thrust their heads from the water and gulp a breath of air. Now they are back behind the protective force fields of salinity and oxygen, hunting with aplomb on a steady diet of fingerling guppies, mullets, and the like.

Comfortably ensconced behind the force fields of salt flats and shallow backwaters, a few other species have evolved the physiology needed to live alongside tarpon. Mullet (a family of several dozen species, not the haircut) share a tarpon’s propensity to mouth breath, and along with snook (a smaller family) will readily move into freshwater rivers or even hyper-saline lagoons.¹² In tropical regions, Atlantic bonefish (Albula vulpes) and their relatives hunt over mudflats and seagrass beds. Bonefish resemble tarpon in their large silvery scales, sleek torpedo-shaped body, and dorsal and pelvic fins set well back toward the tail. Cagey adversaries, a bonefish will put up a blockbuster fight if it can be hooked, and as a sport fish they are widely sought by anglers. Unlike tarpon, however, their mouths are distinctly downturned, better suited for plucking crustaceans from seagrass meadows and unearthing mollusks buried in silt than for gulping smaller fishes from behind as do tarpon. Despite their down-at-the-mouth appearance, bonefish readily roll at the surface to gasp air and use their swim bladder as an ersatz lung. They also share with their shallow water neighbors the habit of spawning in deep seas, and transparent leptocephalus larvae. Bonefish swim in tremendous schools numbering in the hundreds or thousands, though the very largest individuals eschew a crowd, preferring instead a solitary lifestyle.¹³ This may be a reflection of one of their hunting techniques: relying on the tides to bring them to rich feeding grounds. When the tide is incoming, bonefish will swim up tidal creeks to places large predators rarely reach, and there they feast on buried and encrusting prey. When the tide turns, rather than risk stranding they ride the outgoing tide down the creek to deeper waters.¹⁴ Thanks to their peripatetic lifestyle, wandering bonefish carry nutrients from rich inshore feeding grounds out to deeper waters, linking these separate ecosystems into a single food network.¹⁵

Tangled Up in Blue

In the year 1291, the Venetian explorer Marco Polo left the employ of Kublai Khan (son of Genghis) and embarked on a four-year journey to reach his native shores. Having lived in Asia for nearly a quarter-century, his accounts of China and the Far East were some of the first descriptions of those exotic lands ever read by Europeans. During his circuitous return voyage, he explored Indonesia by sea and recorded landing at the city of Palembang, in what is now South Sumatra. There he found mangroves skirting the settlement, providing a protective fringe of forest against the battering served up by monsoon storms. Each tree was rooted on marshy sediment but propped up by dozens of pole-like roots plunging into the sea, like a conference of stilt-walkers jostling together at the beach. A veritable thicket of slender, crisscrossed columns, the roots offer bountiful space for algae, sponges, and mollusks to grow and also create sheltered nurseries for juvenile fishes. Over time, those still waters accumulate sediment, and new land is born from the ocean’s sands. If Marco Polo had arrived today, he could never have landed his ship at Palembang, for one simple reason: it is no longer located on the coast. Mangrove trees, tiptoeing seaward on their slender roots, have steadily moved the shoreline over the intervening 700 years and now the city is an astonishing 30 miles inland.¹⁶ At a rate of nearly 250 feet per year, the forest has contrived a land grab from the ocean, building habitat where once there was only water.

Thriving along the world’s equatorial shores, mangrove forests fringe nearly a half-million miles of coastline and cover an area larger than the state of Illinois or New York, about 60,000 square miles. They achieve their legendary growth rates thanks to hyperactive photosynthesis fueled by tropical sunshine and lavish inputs of nutrients from rivers draining to the sea. Mangroves expend energy every minute of every day, however, fighting a silent killer: salt water. All mangroves block the uptake of salt with tight membranes between cells, actively pump pure water into their tissues, and shed any salt that breaches those defenses through specialized leaf glands. Despite the energy costs, mangroves are incredibly successful wherever blazing sunshine and salty shoreline are found together: more than thirty tree species have adopted the salt life.¹⁷

Just as corals build invaluable physical structure in an empty ocean, so too do mangroves weave new architecture on formerly featureless coastlines. Spreading branches, sprawling roots, and captured sand are home to a host of organisms: sloths, monkeys, deer, and even tigers roam these tangled jungles, while seabirds like pelicans and frigates nest in dense colonies amid the leaves. Below the waterline, a thicket of roots is encrusted with algae and all manner of clams, sponges, anemones, and crustaceans, offering a veritable smorgasbord to visitors from stingrays to sea turtles and even manatees. An impressive diversity of fishes—particularly juveniles—tuck themselves into the safe harbor of interlocking roots, like children playing under the dining table, where they feed contentedly while growing into adulthood. Barracudas, snappers, sharks, mullets, parrotfishes, pufferfishes, and even seahorses all enjoy the invaluable nursery of mangrove roots; in the hyper-diverse western Pacific, more than 200 species of fishes can be found in these sheltered waters.¹⁸

Other fishes visit mangroves intermittently, swimming in with the tide like bonefish, or window-shopping like a great barracuda (Sphyraena barracuda) until it surges like a glittering missile into the roots for a morsel. In their daily wanderings, snappers and jacks and others may visit half a dozen mangrove islands, browse a seagrass meadow, and swim out to open water to nab a young mullet or two. Both predators and herbivores connect mangroves, seagrass beds, and coral reefs like marine conveyor belts delivering suitcases of nutrients. Coral reefs tend to occur near mangroves, and seagrass meadows are often found between the two. Each system benefits from the proximity of the other. Corals enjoy a boost to their already impressive native productivity from abundant nutrients that pour out of mangrove forests: some reef corals can thank the forest for as much as a third of the carbon they use to grow.¹⁹

Seagrass beds hold an intermediary position in the chain of food delivery. As much as half the organic material fertilizing these undersea plants is supplied by mangrove forests; conversely, some 20 percent of the organic carbon found among mangrove roots originates in seagrass meadows.²⁰ Captivatingly named turtlegrass, eelgrass, manatee grass, and some sixty other species flourish in warm, sunny waters—including seas far from the tropics—and their meadows are some of the most productive ecosystems on Earth (and the most threatened), capturing and storing an estimated 7 trillion (with a t!) tons of carbon.²¹ Each year they turn over as much as three-quarters of that productivity, feeding fishes and invertebrates, and shedding bits of leaves and stems.²² Corals in turn can take up that organic matter, literally munching on the nitrogen-rich grass clippings swept their way by the outgoing tide.²³ Flowing water conveys lunch to the reef, one particle at a time. But the best food delivery comes in larger packages, and fishes make the best nutrient baggage handlers in the sea.

Sharks and jacks, grunts, snappers, and more perform nocturnal migrations between mangroves and coral reefs, feeding among the roots before returning to the reef.²⁴ Their midnight snacks in the mangroves eventually rain down on the reef in the form of fish feces, providing a boost of fertilizer for the entire coral reef community. In the Gulf of Mexico, gag groupers (Myctereoperca microlepis) forage in seagrass meadows before ambling to distant reefs, sometimes crossing 50 miles of open water to get there.²⁵ As much as a quarter of the grouper’s muscle is built from prey harvested in the seagrass: if he is eaten by a shark while visiting the reef, that toothy predator is indirectly eating seagrass as well, even while hunting amid the far-flung corals. On the other side of the planet, off the coast of Queensland, Australia, mangrove feeding subsidizes the mottled spinefoot (Siganus fuscescens), a type of rabbitfish. These modest-sized herbivores graze on reefs and help keep algae from overtaking the corals, but mangroves contribute 40 percent or more of their daily harvest of calories. Were it not for mangroves, these little lawnmowers might run out of fuel before fully trimming the reef.

Back in the Caribbean, noisy grunts begin their lives in seagrass beds, but as they grow they seek the safety of mangroves. If none can be found, bluestriped grunts (Haemulon sciurus) will move to coral reefs, but they suffer much higher predation in those uncomfortably exposed sites: where mangroves are present, their numbers can be twenty-five times greater than where such hideouts are lacking.²⁶ Commercially important fishes like yellowtail snappers (Ocyurus chrysurus) double in size and abundance when mangroves are located near their reefs, a benefit to the reef and to local fishing villages. Gentle grazing giants like rainbow parrotfishes (Scarus guacamaia), the largest parrotfish in the Caribbean, make their homes on coral reefs but are absolutely dependent on mangroves as nurseries.²⁷ So reliant are they on aquatic kindergartens amid the roots that, in Central American sites where mangroves have been deforested, these marvelous polychromatic fishes disappear altogether.

Submerged mangrove roots are teeming with life, more than enough for hungry fingerlings to get their fill every day. But just above the waterline, the stilt-like roots are crawling with every imaginable critter the tropics can serve up. Tree frogs clamber and croak, spiders amble and spin, snails glide noiselessly, lizards scuttle and pounce, beetles march about, millipedes, crabs, ants, and hundreds more stroll from pillar to post, just inches above the sea surface. If only a fish could reach out of the water and wield a stick or flyswatter to knock down some of the cornucopia hanging tantalizingly overhead. One small fish, however, has solved this conundrum. It uses neither sticks nor swatters, but rather its own version of a water pistol: it fires an aquatic bow-and-arrow with the unerring accuracy of Robin Hood.

Seven species of archerfishes frequent mangrove-lined estuaries and billabongs from Sri Lanka to northern Australia. They are closely related, all members of the genus Toxotes, which fittingly is the Greek word for “bowman.” Modest in size, archerfishes rarely exceed a foot in length and resemble small perches with dark brown bars or blotches and conspicuously pointy snouts. Living in the backwaters of mangrove forests, they encounter a lot of floating debris overhead: bits of moss, fallen spiders, the occasional dead beetle. These they pluck from beneath, jabbing their snout to the surface and sucking in the free meal. But somewhere in the distant past, an ancestor had a novel idea, or perhaps just an accidental hiccup, and evolution sprang into action like a geyser. Archerfishes began spitting drops of water at prey and were so successful that their mouth evolved into a precision water pistol. The roof of the mouth has a distinct slot running toward the lips, while the tongue is uniquely shaped to shove water along the groove. Combined, the two mouth parts can blast a jet of water beyond ten times the fish’s body length.²⁸

As any marksman will tell you, hitting the bullseye does not rely simply on the power of the bow. Archerfishes have developed a suite of skills that improve their odds. They boast exceptional eyesight capable of spotting a target despite confounding camouflage. The retina of each eye is packed with paired color-sensing cones specifically tuned to the brown background afforded by silt-covered mangrove roots. Those cone pairs are known as “offset detectors,” a duo that emphasizes contrast: a light-colored bug basking on a slightly darker leaf will be highlighted, appearing almost to glow.²⁹ In the lower, rearward part of the retina—where light from overhead targets strikes—the cones are so tightly packed that an archerfish’s visual acuity is the highest reported for freshwater fishes and compares favorably to terrestrial animals. Even once the prey is seen in high-contrast detail, archerfishes must still compensate for light refraction. Living underwater yet viewing objects above the surface, these clever fishes account for the distortion of light as it crosses from air to water. They achieve this feat by merging estimated distances between themselves, the surface, and the target, and they can accurately separate prey size and distance and even map every detail of its movement in three-dimensions.³⁰

All these assessments, a complex set of calculations that would detain all but the sharpest of mathematicians, take place in as little as 100 milliseconds before the archerfish looses its bolt of water.³¹ Once the unfortunate insect or spider is super-soaked and knocked from its perch, it tumbles into the water. Even then, the archerfish’s trajectory calculator is tracing the complex descent. Gravity pulls downward while the water jet pushes horizontally; combined, these forces drag the insect along what is known as a ballistic path, like a punted football falling beyond goalposts. To most animals, mapping the landing zone would be nearly impossible, but the diminutive archerfish—computations completed—sprints ahead and intercepts the bug precisely where it crashes into the drink.

If another archerfish observes such ballistic proceedings, it too can puzzle out where the insect will land and surges forward, straight as a laser, to the precise spot where it can catch the prey; triangulation, it seems, is not a feat performed only by cartographers.³² Experiments meticulously conducted by Stefan Schuster at the University of Erlangen-Nürnberg in Germany unveiled something even more astonishing: captive archerfish, who in the wild hunt in small troops, watch each other’s archery efforts and actually learn from them.³³ In Dr. Schuster’s words, “observers can ‘change their viewpoint,’ mapping the perceived shooting characteristics of a distant team member into angles and target distances.” Thanks to a bit of studying, subordinate fishes picked up several distinct hunting strategies for nailing bugs on the run. In one, known as “leading,” the fish estimated the speed and direction of a moving insect and aimed its shot ahead, like a quarterback throwing a pass to a sprinting wide receiver. But they also learned another approach, “turn and fire,” in which the fish rotates its body while blasting a jet, matching the prey’s speed and hitting it on the nose (or antennae). Clearly, when aiming for a moving target in a thicket of mangrove roots, it helps to apprentice under an experienced master.

A Submarine Forest

To look into such a pool is to behold a dark forest, its foliage like the leaves of palm trees, the heavy stalks of the kelps also curiously like the trunks of palms.

—Rachel Carson, The Edge of the Sea

Tropical sunshine, throbbing down like a fiery plumb bob onto blistered shores, is transformed into something cool, angular, and demure in the world’s temperate latitudes. Move away from the equator, and mangrove forests begin to fade. They simply cannot harvest enough energy to sustain the unrelenting battle against salt. Coastlines, shorn of their protective mangrove roots, are pummeled by waves that strip sediment from shorelines like giant backhoes, gnawing at the base of rocky cliffs. Great boulders tumble into the surf. Yet still a forest survives beneath the waves, one that takes up the duty of mangroves to dampen the ferocity of the sea and provide a safe haven for fishes who need a little tranquility and security.³⁴ Anchored firmly to the bottom many yards below, a swaying kelp stretches upward to the light. Though not a true plant, these brownish-green algae bear numerous similarities to their distant terrestrial cousins (taxonomically speaking, more like great, great, great, great nephews). A holdfast grips the seafloor, taking the place of roots; a sinuous stipe spirals toward the sun like a trunk; flat and spreading blades, identical to leaves, absorb light to power photosynthesis. One kelp grows next to another, and another, forming tangled forests that cover hundreds of acres.³⁵ So successful are these algae that more than 130 species of kelps dominate a quarter of the planet’s coastlines and provide structure and habitat for an utterly unique marine community.³⁶

Bob Steneck, a marine biologist from the University of Maine, has been studying algae and kelp for more than fifty years. He got his aquatic start early in life. “I was always a water baby,” he says.³⁷ “I started scuba diving around ten years old, before there were any certification programs, actually. In 1972 I was asked by a Smithsonian curator who was living on his trimaran sailboat if I wanted to join his research team, and you can imagine that was not a hard choice for me to make! So I spent ’73 and ’74 living on a 41-foot trimaran doing research.” After two years at sea, his knowledge of the marine realm was encyclopedic. “I was a sponge. I could identify virtually all marine organisms by 1973, every angiosperm, alga, fish, invertebrate, I had it all under my belt.” Soon he was drawn to the complexities of underwater algae, including kelp. Over his exceptionally productive career he has studied kelp groves around the globe and remains entranced by the experience of diving within them. “I go way back, working on coral reefs all over the world, frankly, but there’s nothing like diving through a kelp forest. You are basically swimming through a forest … In places like California and Tasmania where I’ve worked, and in the Aleutians, you’re going through these towering kelp forests, and it’s very impressive. In some places the water clarity is terrific, and in a Macrocystis [giant kelp] bed these things can be 300 feet long. It really is unlike anything else.”

Of course, meandering underwater through tangled ropes of giant algae carries its risks. Unflappable while scuba diving, Dr. Steneck nonetheless is keenly aware of the challenges posed by kelp. “I was working in the Aleutians with two German colleagues. I’m working away, and one of the younger guys came and he told me to come, that he needed a hand. The senior German scientist had started getting tangled in the kelp. He started rotating and he got tangled even more. When I came over to him there was this tower of kelp with two fins sticking out of the bottom. You couldn’t even see him. So all I could do was take out my dive knife to cut off all the fronds. I found where his head was and I just gave him the thumbs-up, you’re going up now.”

For the kelp, growing in water has distinct benefits over a life on land. Freed from gravity, kelp can reach towering heights with very little in the way of mechanical support (though their stalks are loaded with unique, elongated cells that spiral upward, giving them an elasticity that terrestrial trees would envy). Many species suspend themselves from hollow bladders pumped full of buoyant gases that stud the upper branches and leaves. Thanks to these inflatable bladders, the accurately named giant kelp can reach heights exceeding all but the tallest of redwood forests. Swaying in cool temperate waters, kelp enjoy a continuous bath of rich nutrients that supercharges their growth. Around the globe, these oversized marine algae capture a staggering 150 teragrams of so-called “blue carbon” every year (1 teragram is 1 trillion grams, or 1 million tons, or a tad more than the weight of San Francisco’s Golden Gate Bridge).³⁸ That harvest enables them to sustain an entire ecosystem, pinched between the cobalt-colored depths and the pounding surf, with a nutritious algae salad. Steneck reveals an unseen side of the forest’s bounty: “A lot of photosynthesis leaks out of kelp. It’s actually very common for all algae. Maybe as much as 40 percent of the photosynthates leak out of the kelp, and heterotrophic [food-eating] bacteria take advantage of that … and mussel growth rates in the vicinity of kelp are much greater than where there are no kelp.”³⁹ So lavish is kelp productivity that each year literally tons of nutrients are shipped by currents to the continental slope, and even the deep sea.⁴⁰ Kelp can sustain extraordinary abundance and diversity, Steneck summarizes, because “you’ve got structure, you’ve got particulate food, you’ve got algal detritus, and so you actually have quite an advantage.”

Within these dense and tangled jungles, coastal fishes find a sanctuary that protects them from the vicissitudes of wave and tide, offers safe refuge and protected breeding grounds, and teems with delectable life. The first morsels to set up residence on holdfast, stipe, and blade are diverse colonies of invertebrates. Towing their protective homes with them like pop-up campers to a national park, snails, clams, crabs, and amphipods (marine versions of a garden roly-poly) soon constellate the towering alga. In Norwegian waters more than 80,000 individuals were found on a single giant kelp, within a forest that harbored nearly 250 different species.⁴¹ “All of the hitchhikers on the sides of the kelp,” Steneck explains, “they too get out of the benthic [bottom] boundary layer; that means there’s greater water flow—water flow is one of the bigger drivers of nutrient availability.” Where there are invertebrates in abundance, there are invertebrate-eaters, and fishes of all sorts are drawn to kelp like hungry students to free pizza. In Ireland, over 300 species of fishes and invertebrates were found in surveys of just four kelp forests.⁴² Worldwide, thousands of fish, invertebrate, and marine mammal species call kelp home.⁴³ While traversing Tierra del Fuego in Chile, Charles Darwin marveled at the diversity of fish that could be found in kelp, and its resilience in the face of oceanic adversity. “I know few things more surprising than to see this plant [actually an alga, even Darwin gets it wrong now and then] growing and flourishing amidst those great breakers of the western ocean, which no mass of rock, let it be ever so hard, can long resist … The number of living creatures of all Orders, whose existence intimately depends on the kelp, is wonderful. Amidst the leaves of this plant [!] numerous species of fish live, which nowhere else could find food or shelter.”⁴⁴

Just as fishes depend on kelp, so too do kelp groves rely on fishes. The seemingly robust forest actually teeters on a delicate ecological see-saw, and fishes lean heavily on the scales. Off America’s west coast, kelp forests are cruised by California sheephead (Semicossyphus pulcher), bulky wrasses painted attractively with broad bands of midnight blue and sunset orange (the scientific name pays homage to their pulchritude, while their blunt head and white chin inspire their ovine common name). These burly carnivores hunt urchins, lobsters, crabs, and other critters that scuttle between the dense holdfasts. Like many fishes of cold waters, they grow slowly, breed late (at age seven or eight), and enjoy long lives of fifty years or more.⁴⁵ Powerful jaws are equipped with strong canines honed by evolution to pierce the tough shells of their prey. If the urchin or lobster is too well armored, sheephead may resort to a method blacksmiths might use to crack walnuts: they choose a rock as an anvil and slam their prey upon it until it shatters, a rare example of tool use by a fish.⁴⁶ Confusingly, a similarly named species inhabits the opposite coast: Atlantic sheepshead (Archosargus probatocephalus, occasionally dubbed “the fish with human teeth” for their broad front incisors) are robust fish who also crush invertebrates found while prowling nearshore waters, though they live far from kelp forests and are as yet unacquainted with tools.

California sheephead are particularly fond of urchins, and this predilection is a boon to kelp, because a mob of urchins is the alga’s worst nightmare. When sheephead and other urchin-eaters disappear—from either overfishing or changing oceanic conditions—urchin populations usually erupt. Gangs of urchins gnaw ravenously at kelp holdfasts, chewing through the trunk until the entire kelp floats free, and perishes. If unchecked, these spiny buzzsaws can deforest acre after acre of kelp, leaving behind an urchin barren: bare, lifeless sand devoid of structure and food where once there was abundance. A famous example of such cascading effects occurred in Alaska at the turn of the last century, when hunters virtually exterminated sea otters for their luxurious pelts. Otters feast extravagantly on urchins, and when the fur trade decimated their populations in the late 1800s, the urchins exploded and thousands of square miles of kelp were wiped out.⁴⁷ Only after decades of intensive conservation efforts, particularly the Marine Mammal Protection Act of 1972, have otter colonies been restored in parts of their former range, where urchins now are kept in check and kelp can once more thrive.

On the coast of Maine, Bob Steneck’s early studies documented wild swings in the enduring tussle between fish, kelp, and urchins. Where once only a narrow fringe of kelp persisted after heavy fishing unleashed years of unbridled urchin deforestation, intense harvest of sea urchins for Japanese culinary markets shifted the balance dramatically. “Without a doubt, kelp were the first to respond, and remarkably fast. I have videos that I took at my [barren] study site in 1993, and by 1995 it was 100 percent kelp. You know, for people who think about natural history broadly, the very possibility of going from a completely deforested to a completely forested state in just two years is just mind-boggling.”⁴⁸ These days, however, urchins seem to have the upper spine, as their populations are booming worldwide.⁴⁹ “The bottom line is that the global rise of sea urchins, I think, follows on the tail of the global collapse of coastal groundfish, that is, fish that feed off the benthos.”

In Californian waters, this saga played itself out all too clearly. In the early 1980s a commercial fishery began targeting sheephead. A decade later sport fishers added their hooks to the siege, and combined catches soared to more than a half-million pounds per year.⁵⁰ When numbers declined, size-based regulations were put in place, causing a perverse consequence. Sheephead, like most wrasses (portrayed in the coral reef chapters) change sex from female to male when large, harem-holding males disappear. Imposing a minimum catch size meant that anglers took mostly males, who then were replaced by smaller females. The remaining pint-sized females produced far fewer eggs, and the population’s ability to rebuild after fishing was hampered even further.⁵¹ Today, the sheephead is officially listed as a declining and vulnerable species. Urchins, as their fishy predator’s abundance slipped, were unleashed and began decimating kelp forests. To make matters worse, the biggest sheephead are the only ones who can chew through large urchins, and large urchins—like large fishes—release more eggs, a biological rule that placed sheephead and urchin populations on opposite trajectories: the former plummeting while the latter boomed.⁵² Kelp forests were savaged as a result, clearcut into urchin barrens at an alarming rate. Fortunately, the state has since established a number of Marine Protected Areas where sheephead can grow safely to their accustomed larger size and greater reproductive potential. Thanks to the spillover effect, ecosystems outside the reserves benefit from reproduction inside. Protected and plentiful larvae, juveniles, and adults drift and swim across boundaries, boosting their own populations and knocking back those of kelp-nipping urchins.

Smaller grazers also contribute to keeping kelp healthy, by nibbling encrusting invertebrates and calcified algae off the branches and blades. Among these helpful cleaners is the kelp perch (Brachyistius frenatus), a diminutive and rhombus-shaped fish swathed in glittering, brass-colored scales. They pluck all manner of encumbrances from the kelp blades, akin to cleaning moss from solar panels, a scouring that rejuvenates the growth of the algae.⁵³ Clinging snails, gnawing isopods and amphipods, blanketing hydroids, burrowing worms, these nuisance critters are favored entrées on the kelp perch’s menu. In addition to keeping the kelp clean, these little perches also serve as fish groomers, an activity more commonly seen in coral reef cleaning stations. Garibaldis and kelp basses, stunning opaleyes, brawny halfmoons, and many more kelp fishes rise to grooming salons near the surface and solicit a de-parasitizing session from these nimble valets. Not a true perch, kelp perches belong to the curious surfperch family (aka seaperch), a group of fishy oddballs because they give birth to live young. Males internally impregnate females, using a swollen part of their anal fin to transfer sperm. Embryos develop wholly within the female, after some delay, and feed on some of mom’s tissues as they develop, emerging as perfect miniatures of their parent.

Kelp rockfish (Sebastes atrovirens) is another pint-sized denizen of the topmost blades, who sway with the waves and conceal themselves among the leaves as youngsters. Irregular, dark brown blotches on a background of dingy yellow only add to their camouflage. They also adopt curious postures while lolling amid the kelp. Marine biologist and self-styled humorist Milton S. Love bemusedly reported, “among the rockfishes, kelps seem to be among the least concerned about remaining upright, as you can see them lying sideways or even upside down, often with their lacy pectoral fins extended and lazily undulating in the current.”⁵⁴ Presumably this attitude, or lack thereof, allows them to appear more kelp-like—and less fish-like—as they grow out of their awkward (and risky) teenage years.

Yet another disguise artist, giant kelpfish (Heterostichus rostratus) relies even more heavily on superbly matched and shifting colors (though the marketing department must have been on vacation when their dowdy name was handed out). More elongate than the previous rockfish and surfperch, they sport a pointy nose and extended, fringe-like dorsal and anal fins that superficially resemble an eel’s. Green, red, and brown morphs, each splotched or barred or striped, can all be found. Colors can be changed quickly, to match the background of their particular patch of kelp, but the different shades also hint at gender and age. Males tend to be brown (and live among brown-hued algae), while females mostly favor green or red (and reside in like-colored districts); juveniles are green or greenish-brown.⁵⁵ Patterning can enhance any camouflage by breaking up outlines, so the bedecked fish fades into kelp fronds rather than presenting a recognizable silhouette. Among giant kelpfish, females typically are barred (lines running vertically, from back to belly), or plain-colored, while males, in contrast, seem to prefer stripes (head to tail) or blotchy mottling. But both sexes can shift at will: when males or females find themselves in wide-bladed giant kelp, they adopt stripes; in finer algae they shift to barring or mottling. Camouflage begins, however, long before adulthood. When female giant kelp lay eggs (unlike surfperch, they are oviparous, not viviparous), even the clutch of sticky little eggs is pre-colored brown or red to match the alga that serves as crib and cradle.

Why does this diverse community of fishes work so hard to breed, and remain invisible, within stands of kelp? Because the forest is also prowled by fearsome predators, hungry and sharp-eyed, always on the lookout for inattentive or unconcealed prey. Giant sea bass (Stereolepis gigas) and kelp bass (Paralabrax clathratus) inhabit kelp groves as juveniles, plucking crustaceans and mollusks from the branches and the seafloor below, but they mature into fierce, large-bodied, big-mouthed hunters of fish. Kelp bass are speckled with pale blotches on top of greenish brown (marketing team still on holiday, though some people supportively call them calico bass), but they can change colors quickly to match their surroundings. They spawn in aggregations of several hundred individuals, where white-checkered males court females with a variety of dance moves and fin bumps. Their ample reproductive output (spawning continues throughout summer) allows them to withstand modest fishing pressure, and today they are targets of a thriving artisanal fishery in southern California. As adults, they eat just about anything they can suck into their gaping maws, including kelp perch, rockfish, and others trying to hide among the fronds.

Giant sea bass are named adequately, if uncreatively, since they frequently reach 500 pounds, with record-setters tilting the scales beyond even 800 pounds.⁵⁶ Large males make powerful booming sounds, especially when disturbed, adding to the crackling of shrimp and the rest of the undersea orchestra. And for a time, they were disturbed incessantly, by a tenacious fishery intent on catching every last one of these giants: if you are a fish, it is most unfortunate to be both large and delicious. Fortunately, after it tumbled onto lists of critically endangered species, capturing this giant has been banned, and the population has made a remarkable recovery, particularly in the protected waters of Catalina Island.⁵⁷ When not being fished, they pursue a life that takes them in and out of coastal kelp forests. There, although they unmenacingly accept routine cleanings by kelp perch and even juvenile sheephead and kelp bass, they are formidable predators when appetite overtakes hygiene. Over their seventy-plus-year life span they will gulp lobsters, squid, crabs, skates, and just about any kind of fish they can find.⁵⁸ Their unmatched size makes them apex predators in their environment, and nearly every species that scuttles or swims is on the menu. Except one, that is, a fish who wields a unique and inflammatory defense against being swallowed.

Swell sharks (Cephaloscyllium ventriosum) are compact fish, just a yard in length, whose tan bodies are beautifully draped with irregular blotches of ochre sprinkled with speckles of white and black. By night they swim amid ropes of kelp, hunting for fishes and the occasional crab or mussel. Some are even bioluminescent, a curious phenomenon that may aid their camouflage or foster communication during nightly raids. By day they snooze in social clusters while camping around the bases of the kelp stalks, where their unique patterning helps hide them from predators. But predators do find them, larger sharks and terrifyingly oversized bass, and suddenly the swell sharks’ lack of size and speed become distinct disadvantages. Discovered, they dart into the kelp forest, desperately trying to outdistance their hunters. But if caught, they have one more trick up their sleeve. As a last resort a swell shark will guzzle mouthfuls of seawater (or air, if near the surface) into a distensible stomach. As far back as 1947, the renowned marine biologist Eugenia Clark was cutting her teeth on shark dissections and noted that swell sharks actually have two stomachs. “The stomach of sharks is divided into two parts—the cardiac stomach, which follows the oesophagus, and the pyloric stomach, which follows the cardiac stomach.”⁵⁹ Filled like a party balloon, the foremost stomach bulges until the swell shark nearly doubles in size, rendering it too big to be swallowed. “As in the puffer fishes, air or water is gulped into the stomach, inflating the belly region of the fish … [and] the cardiac stomach swells out in all directions.” To make even less of a meal of themselves, a threatened swell shark will bite its own tail, latching on tightly and curling its body into a U-shape, and a rather bloated U at that. Confronted with a swollen balloon that can no longer be swallowed, the perplexed predator swims away for other less-inflated prey.

Thanks to all these specialists, kelp forests are incredibly diverse (especially those in the Pacific) and packed with uniquely adapted species found nowhere else. Like rainforests, in their delicate balance they cultivate biodiversity. Perhaps it is the kelp forest’s ability to buffer the changing conditions that batter coastlines—erratic temperature, light, salinity, and wave force—that makes them most attractive to fishes. It can be challenging to thrive near the shore, where change is the only constant. Still, a few fishes who swim in the sea are ready to confront variety, and even use wildly changing conditions to their advantage. To do so, they will swim halfway across an ocean and confound the world’s brightest minds for centuries.