Introduction The best way to understand and appreciate the natural world that sur- rounds us may be to explore beneath its surface—not by digging, which destroys the very systems we seek to examine, but by looking underwater, which we can do with little or no disruption of the processes we observe. Birth and death, growth and decline, the interaction of plants, animals and minerals, of predators and prey—in short, everything that happens in the familiar environment aboveground also happens below the waves. Because water is transparent, these phenomena should be easier to ob- serve there; yet the environment is so profoundly alien to human beings that venturing down even 40 or 50 feet is the closest almost any of us will come to visiting another planet. A person needn’t be a scuba diver or a snorkeler to explore the fascinat- ing complexity of life beneath the surface. You don’t even have to be able to swim. I prefer to visit this engagingly exotic world in person, listening to the gentle bubbles of my exhalations as I glide lazily along, enveloped in warm liquid. (As a diver, I’m something of a wuss. I never was one for cold-water diving and have a decided preference for the tropics.) But I have become increasingly amazed at the quality and detail captured by the world’s great underwater cinematographers and even by enthusiastic graduate students. The latter’s camera work may waver, but their obvious excitement and their exclamations of “Awesome grouper!” and the like make up for the home-movie visual quality. Life on earth began in the sea four billion years ago.1 Oceans now cover 127,970 square miles of the earth’s surface.2 Combined they contain 300 million trillion gallons. Each drop of seawater holds between 2,500 and 300,000 single-celled organisms—bacteria, viruses, protists, and protozo- ans—taken together, a hundred million times more than the estimated number of stars in the universe.3 2 · Imperiled Reef As oceanographer Sylvia A. Earle put it poetically in her 2009 book The World Is Blue: “Seen from a space shuttle, Earth’s continents seem to be islands floating in a shimmering indigo embrace . .”4 That “shimmering indigo” embraces millions of species. According to the National Oceanic and Atmospheric Administration, only 9 percent of the world’s marine species have been catalogued. One reason may be that 80 percent of the ocean has yet to be explored.5 The roster of species is growing constantly, thanks to the development of genetic sequencing in the latter part of the twentieth century. Earle described the work of marine biologist Richard Pyle, who explored the ocean 330 to 660 feet down, finding 12 to 13 new speciesper hour.6 For all forms of life, scientific taxonomy proceeds from broadest to most specific: domain, kingdom, phylum, class, order, family, genus, spe- cies. In this book, I use common names for the plants and animals I dis- cuss but also provide, at least once, their scientific names, in the form of genus and species—for example: hawksbill sea turtle (Eretmochelys imbri- cate); or, when the common name refers to members of a family, subfam- ily or genus, that more inclusive name—for example: blennies (family: Blenniidae). Some scientists estimate that 25 percent or more of marine species de- pend for at least part of their life cycles on coral reefs,7 although such reefs occupy only 0.1 percent of the oceans’ total surface area and a thin layer at that; the average depth of the world’s seas is 2.5 miles, and the deepest waters reach down seven miles.8 Situated between a foot or two below the surface and about 300 feet down, coral reefs rely on the filtered sunlight that algae and other aquatic plants require for photosynthesis, which in turn fuels the ever-looping chain of life for corals, sponges, crustaceans, eels, marine mammals, and fishes large, small and tiny. Dotting the earth’s shallow seas, coral reefs extend generally to 30 de- grees latitude on either side of the equator. In some areas where waters farther north or south are unusually warm—such as Bermuda, located at 32.3 degrees north but bathed by the Gulf Stream—patch reefs of hard corals survive.9 Coral reefs come in three major varieties: fringe, which lie close to the shore or around islands; atoll, which started as fringe reefs but became more or less circular freestanding structures when their central islands subsided; and barrier, which form along continental shelves and around groups of islands. (When sea levels drop, islets can also emerge from Introduction · 3 now-exposed barrier reefs.10) For the purposes of this book, I decided to focus on one particular reef, the Mesoamerican Barrier Reef, also known as the Great Mayan Barrier Reef, in the Western Caribbean. I chose it for three reasons. First, although coral reefs occupy only 0.1 percent of the surface area of the world’s oceans, as mentioned previously, that was still too much territory for me to cover in my remaining lifetime. Second, I already had a passing acquaintance with this particular stretch of coral; I’d been diving parts of it for decades. Third, all the beauty and drama of the coral ecosystem, and all the forces and follies that imperil coral reefs everywhere, were clearly evident there.11 Resting on a foundation of limestone and hard corals, at 625 miles long the Mesoamerican Barrier Reef measures less than half the length of Aus- tralia’s Great Barrier Reef; yet in natural beauty and ecological interest, it rivals its more famous cousin. Stretching from Isla Contoy, just north of Cozumel, Mexico’s largest Caribbean resort island, to the Bay Islands of Honduras—Roatán, Útila, and Guanaja, plus five smaller islands and 53 cays—the Mesoamerican Barrier Reef is the second-longest continen- tal coral structure on earth.12 As its official name reflects, this underwa- ter paradise is a multinational asset, skirting the coasts of Mexico, Belize, Guatemala, and Honduras, lying between a few hundred yards and 22 miles offshore. Four atolls, rings of islets that started as fringe reefs surrounding a volcano but now rise barely above the surface, belong to the barrier reef system: Chinchorro Banks, Turneffe Atoll, Lighthouse Reef, and Glov- er’s Reef.13 (Encompassing five islets in the southern crescent of the Gulf of Mexico, 60 miles north of the Mexican port of Progresso, Arre- cife Alacránes, or Scorpion Reef, sits more than 200 miles to the west of the northern tip of the Mesoamerican Barrier Reef. Some scientists ar- gue that Scorpion Reef is not technically an atoll because its origin is not volcanic.14) Much has been made about coral reefs in recent years, especially about the perils they face due to global warming. The hard corals that form the foundation for the reef ecosystem are very particular about salinity and temperature. Most species do best in water between 73°F and 84°F with a salt concentration between 32 and 42 parts per thousand.15 If the warmth of the water they inhabit falls below 64°F for more than a few hours, they die. Although some species can tolerate temperatures as high as 104°F for brief periods, most expel their Symbiodiniaceae, the family of algae with 4 · Imperiled Reef which healthy corals live symbiotically (and which lend them their vivid colors), if the water temperature rises above 90°F.16 Called “bleaching,” this visually striking symptom indicates a patch of coral under extreme stress and vulnerable to lethal damage from storms and disease. This book does address bleaching and the other threats to the Meso- american Barrier Reef and its cousins worldwide. Changing acidity of the ocean, nutrient-laden runoff from fertilizer, sediment kicked up by storms, debris and sewage from coastal development, introduction of in- vasive species, overfishing, inept divers, and plain old trash, such as plas- tic soda bottles, all these and more endanger this fragile ecosystem, as well as the other 99.9 percent of the earth’s oceans. Most of these threats result from human activity. Some can be halted or at least curbed rela- tively easily, given broader awareness, targeted economic incentives and international cooperation. The concluding chapters of this book describe specific examples of successful programs and specific ways in which we can act, collectively and in- dividually, to limit and even reverse the damage. But people don’t protect what they don’t value. The purpose of this book is to raise understanding and ap- preciation of one particular, fascinating and fragile eco- system and thus, by exten- sion, of coral reefs through- out the tropical oceans and of the rest of the earth’s envi- ronment everywhere. Healthy coral ecosystem, photo by Ben Phillips..