This story appears in the October 2012 issue of National Geographic magazine.
In the mangroves off the east coast of Central America, at the edge of the Mesoamerican Reef, the world is divided in two: the above and the below. As we killed the engines and poled the skiff from the hot April sun into the shade of the forest, Will Heyman, my marine biologist companion, and I gazed into the simplicity above. We saw the green crowns of one of the least diverse of all tropical forests, where there is often but a single species of tree, the red mangrove.
Salinity, storm waves, and oxygen-poor mud discourage understory growth in the mangroves, so there was little beneath the canopy for us to see. The occasional orchid. Rarely, a vine. A troop of fiddler crabs guarding holes in the mud. A big mangrove crab low on a trunk. Some insects. A tricolored heron perched on the stilt of a mangrove root.
I leaned over the gunwale to sample the mud around the roots, scooping up sherds of pottery. The mangroves of the Mesoamerican Reef were once at the fringe of the ancient Maya civilization. I contemplated slipping a souvenir into my pocket—with such a lode here, what possible harm? “Strictly catch-and-release,” Heyman said. With a splashing of jettisoned sherds, we poled to another spot. There, in the still water, we witnessed the miracle of the below.
At the waterline the roots in this forest blossom downward, expanding all shaggy-bearded with mats of algae, and slender brittle stars, and boxy starfish, and the little translucent vases of the filter feeders called tunicates—their “tunics” orange or purple or white—and soft corals and oysters and sponges in still more hues. Nothing here goes unadorned.
Mangroves are crucial nurseries. Schools of small fry shift away through the Moorish architecture of arched roots, each school a pale cloud of translucent fish. The palest clouds are hardly there at all, composed of hatchlings no bigger than the smallest mosquito wigglers. These living motes are too small to name. Are they destined for adulthood in a sea grass bed, or coral reef, or open ocean, or right here in the mangroves? Too soon to tell.
And so it goes on Central America’s reef system. Each component of this tripartite world of mangrove, sea grass, and coral reef is itself divided in two: the world above elementally simple, the one below bafflingly complex.
The Mesoamerican Reef system stretches more than 600 miles along the coasts of Mexico, Belize, Guatemala, and Honduras. Its Australian cousin, the Great Barrier Reef, is great indeed at 1,429 miles long—the biggest structure created by living things on this planet. Yet the Mesoamerican Reef, at less than half the length, is in its own way the more remarkable.
The contours of the continental shelf here encouraged the development of an underwater reef platform that begins within a few hundred yards of shore in some places and as much as 20 miles offshore in others. This platform supports a variety of reef types and a profusion of corals unique in the Western Hemisphere. If the Mesoamerican Reef has any advantage over its massive Australian counterpart in the Pacific, it is in this proximity to land and the intimacy of its connection with inshore habitats. Here the provinces of mangrove, sea grass, and coral reef are bound so tightly together by currents, tides, and mutual need that it’s really not possible to tease them apart.
MANGROVES Mesoamerican mangroves form multiple lines of defense for the reef system. The first line is the tall mangrove forest along the coast and up the mouths of tidal rivers. The second line, and sometimes a third and fourth, occur offshore, in places where pointy mangrove seedlings have taken root atop a series of shallow marine ridges. Each clump slowly gathers the makings of an islet under itself. These islets grow into islands—mangrove cays—arranged in linear archipelagoes. The clusters of cays work as screens, benefiting the sea grass by moderating wave action and the coral reef by intercepting silt, fertilizers, and toxins in runoff from land.
The mangroves, in addition to defense, provide mulch. They can shed tons of leaves per acre every year. Fungi and bacteria break down this leaf litter and consume it, then are consumed by tiny worms and crustaceans, which in turn feed small fish, which feed larger fish and birds and crocodiles.
Life wells outward from the mangroves into the sea. At the same time, a living countercurrent flows back in: the eggs, larvae, and sometimes the gravid females of reef creatures that use the mangroves as a nursery. If any fish is emblematic of this life cycle—kindergarten in the mangroves, graduate school on the reef—it is the rainbow parrotfish.
The scientific name for this species is perfect, Scarus guacamaia, from the native Taino huacamayo, “macaw.” The resemblance is eerie: The fish has the parrot beak and the coloration of the blue-and-yellow macaw. The parrotfish starts small in the mangroves, as drab as a sparrow, and ends in full color on the reef, four feet long, the largest herbivorous fish in the Atlantic.
Mangroves are not just a convenience for Scarus guacamaia. They are a necessity. When mangroves are carved away, to make room for tourist venues, for example, the species tends to go locally extinct, with repercussions in all directions. Coevolution has brought the coral reef and its parrotfish into balance; when the horny-beaked herbivores are fished out or otherwise eliminated, the reef declines, its corals overgrown by carpets of the algae the parrotfish normally eat.
John Muir told us what we can expect when humans with their habits begin to unravel a sound ecosystem. “When we try to pick out anything by itself, we find it hitched to everything else in the universe,” he wrote. The parrotfish are a case in point. The Mesoamerican Reef is one section of the universe where the hitches are particularly tight.
SEA GRASS A sea grass bed begins with the sprouting of a pioneer species like shoal grass, with flat, thin leaves; or long and threadlike manatee grass. The pioneer grasses eventually give way to turtle grass, the climax species, which has flat, straplike blades up to two feet long. Of the various species of sea grass identified off Central America, turtle grass is commonest. Like the others, it is an angiosperm, a flowering plant, having solved the problem of pollination underwater—no bees—and having mastered submarine dispersal of fruit, which simply detaches and tumbles away on the current. Sexual reproduction is not a big preoccupation of the species. Most of the time, turtle grass is chaste. Maintenance and expansion of the bed are largely by vegetative reproduction—asexual sprouting from buried stems.
The turtle-grass rhizomes, or underground stems, creep horizontally under the sand and are anchored by an extensive root system. Like the mangroves, they trap silt that might otherwise settle on corals.
This is a vital service. Reefbuilding corals require clear water. The basic unit of a coral colony, the tiny coral animal—the polyp—produces most of its food through photosynthesis by algae resident in its tissues. Sedimentation, which screens out sunlight and kills polyps, is one of the principal causes of coral-reef decline worldwide. De-sedimentation by sea grass is a reciprocal service. Turtle grass thrives in calm waters protected from surf and wind-driven currents by the barrier reef; its sequestering of sediments is merely a return of the favor. Just as the coral polyp lives in internal symbiosis with its resident algae, so the coral reef lives in external symbiosis with sea grass.
All in these waters is quid pro quo. The Mesoamerican Reef is an edifice built of mutualisms.
From a boat adrift above it, the turtle grass looks as monotonous as a field of corn or alfalfa. But snorkel the prairie at the level of the grass, your face mask parting the blades, and the detail and diversity jump out. Older, darker blades are encrusted with epiphytes, undersea “mosses” in hundreds of species. Films of algae and bacteria on the blades are food for tiny organisms, which in turn feed shrimp and small fish. Sea grass is a way station, a middle school, for many species hatched in the protection of the mangroves and bound for adulthood on the reef.
Bright schools of small fish hang over the undersea prairie as the current stirs in the grass. Occasionally you flush adult parrotfish and surgeonfish, come in from the reef to feed on the grass itself. Sometimes in the turtle grass, you meet one of those: a foraging green turtle, hawksbill, or loggerhead. Here and there the sea grass prairie is crossed by what appears to be a game trail. The big game here is blimp shaped, weighs a thousand pounds or more, and is distantly related to the elephant: These are the feeding swaths of manatees.
When foraging in turtle grass, a manatee really digs in. Sometimes it starts with a salad of grass-blades, steering those toward its mouth with wafting, gathering motions of its foreflippers. But often it goes straight for the starch in the stems beneath. Rooting these out, it churns up clouds of sediment. The big myopic head lifts from the muck; the huge upper lip, muscular and prehensile, makes a variety of indescribable chewing motions, the broken ends of plants sticking out past the whiskers; and then the manatee is obscured by the mud cloud it has made. A blizzard of detached turtle-grass leaves swirls above the cloud, like confetti on Armistice Day. Burying its face in the mud again, the manatee rejoins its own parade, plowing onward across the prairie.
CORAL REEF From the deck of a skiff on the surface, the reef makes a lovely but minimalist seascape: the white line of surf breaking along the reef front, the turquoise of the reef flat, the royal blue of the open ocean beyond. But like the inshore habitats that it shelters, the rampart of the barrier reef is a world divided.
Adjust your mask, take a breath, and roll overboard. Now the real reef reveals itself: a concentration of life and a spectrum of colors unlike anything up in the world of air. The reef is a teeming city of hard corals, soft corals, fire corals, lace corals, brain corals, staghorn corals, sea whips, sea fans, sea grapes, coralline algae, sponges. Taking refuge everywhere in the alleys of the coral city, or countersunk in holes drilled in the corals themselves, or perched atop the coral heads, are hosts of invertebrates—clams, crabs, shrimps, worms, sea cucumbers—of stunning diversity. Add to these the hundreds of new species scientists identify worldwide each year.
Schooling above the reef are fish painted extravagantly in an array of electric colors we are missing in the upper world. There is an auto-luminescence to the palette here, as if each fish and feather worm came equipped with its own battery to power its stripes, bars, blazes, and dots.
The coral reef, like all reefs across the tropics, is threatened by ocean acidification and warming episodes caused by climate change. Overfishing, coastal development, and the accelerating pace of oil exploration are worrisome too.
But at dusk in springtime, when the moon is full, age-old magic still happens. Near the Silk Cays off southern Belize, thousands of cubera snappers, dog snappers, and mutton snappers come to spawn at a reef called Gladden Spit. They draw platoons of whale sharks, which feast on the eggs, and sometimes marine scientists as well. These sharks, the biggest fish in the sea, are plankton-eaters; Gladden Spit is the first place they have ever been observed eating spawn. The assembly here of snappers, the predators that eat them, and the colossal sharks that feed on the spawn is the most spectacular convocation I have ever witnessed in the ocean.
In scuba gear, 50 feet deep, Heyman and I swam toward a huge ball of spawning cubera snappers, a dark, slow-rotating, cyclonic tower that resolved itself, as we swam close, into thousands of particular fish. From the smooth rotation, tight groups flurried upward to spawn, releasing white clouds of eggs and milt. These coalesced into a great thunderhead, which billowed up to envelop us. For a time we were lost in a sperm-and-egg whiteout. Then a dim gray shape materialized, like Titanic in the fog, and out of the whiteness came the gigantic gape and spread pectoral fins of a feeding whale shark. Then more whale sharks arrived, and finally bottlenose dolphins and bull sharks.
We chased the party until we ran out of air. Surfacing, we inflated our vests and swam backward toward the skiff anchored under the newly risen moon. The full April moon had called snappers here, their spawning timed to catch the high spring tide that would carry their fertilized eggs into the mangroves. The whale sharks were guided in from afar by whatever mysterious cues they use in navigation. Tonight the shallow, tight-knit ecosystems of the Mesoamerican Reef had opened up. When you try to pick out anything by itself, you find it hitched to everything else in the solar system.
We paced ourselves, finning backward toward the bobbing skiff, under the moon that had brought us.
This article was funded in part by the Oak Foundation.