National Geographic : 2012 Oct
require clear water. The basic unit of a coral colony, the tiny coral animal---the polyp---pro- duces 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 exter- nal symbiosis with sea grass. All in these waters is quid pro quo. e Meso- american Reef is an edi ce built of mutualisms. From a boat adri above it, the turtle grass looks as monotonous as a eld 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 sh. 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 sh hang over the un- dersea prairie as the current stirs in the grass. Occasionally you ush adult parrot sh and sur- geon sh, 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, hawks- bill, or loggerhead. Here and there the sea grass prairie is crossed by what appears to be a game trail. e big game here is blimp shaped, weighs a thousand pounds or more, and is distantly related to the elephant: ese are the feeding swaths of manatees. When foraging in turtle grass, a manatee re- ally digs in. Sometimes it starts with a salad of grass-blades, steering those toward its mouth with wa ing, gathering motions of its fore ip- pers. But o en it goes straight for the starch in the stems beneath. Rooting these out, it churns up clouds of sediment. e big myopic head li s from the muck; the huge upper lip, muscular and prehensile, makes a variety of indescrib- able 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 Armi- stice 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 ski on the surface, the reef makes a lovely but minimalist sea- scape: the white line of surf breaking along the reef front, the turquoise of the reef at, 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. e reef is a teeming city of hard corals, so corals, re 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 stun- ning diversity. Add to these the hundreds of new species scientists identify worldwide each year. Schooling above the reef are sh painted ex- travagantly in an array of electric colors we are missing in the upper world. ere is an auto- luminescence to the palette here, as if each sh and feather worm came equipped with its own battery to power its stripes, bars, blazes, and dots. e coral reef, like all reefs across the tropics, is threatened by ocean acidi cation and warming episodes caused by climate change. Over shing, Ken Brower contributed to Ocean, a 2010 National Geographic special edition. For another of Brian Skerry's photos, see e Moment, page 145. is article was funded in part by the Oak Foundation. HERE THE PROVINCES OF MANGROVE, SEA GRASS, AND CORAL REEF ARE BOUND SO TIGHTLY TOGETHER BY MUTUAL NEED THAT IT'S REALLY NOT POSSIBLE TO TEASE THEM APART.