Michael Crosby and Ernst Reese, “A Manual for Monitoring Coral Reefs with Indicator Species: Butterflyfishes as Indicators of Change on Indo-Pacific Reefs.” Office of Ocean and Coastal Resource Management, National Oceanic and Atmospheric Administration, Silver Spring, MD (1996).
22 H. Fukami et al., “Ecological and Genetic Aspects of Reproductive Isolation by Different Spawning Times in Acropora Corals,” Marine Biology 142 (2003): 679–684.
23 Charles Boch et al., “Effects of Light Dynamics on Coral Spawning Synchrony,” Biological Bulletin 220 (2011): 161–173.
24 Jasper de Goeij et al., “Surviving in a Marine Desert: the Sponge Loop Retains Resources Within Coral Reefs,” Science 342, 6 (2013): 108–110.
25 Patrick Lemaire, “Evolutionary Crossroads in Developmental Biology: the Tunicates,” Development 138 (2011): 2143–2152.
Chapter 3:
The Coral Triangle
Huge clouds of damselfishes in hues of gray, yellow, and baby blue pulse in and out of the protection of the field of branching corals that stretches before me. Trevallies, small relatives of the tuna, burst from out of nowhere to try and snag one of these wary but tasty treats. Several blacktip reef sharks slowly cruise above the sharp coral, paying no attention to either myself or their potential prey. I turn my attentions from the reefscape and look down at the smaller animals around me. A pair of ring-tailed cardinalfish, a species-rich group of small reef fishes, is below, the male’s mouth full of rusty orange eggs. The reef is busy. Every glance reveals a new branch from the tree of life. Not just fish and corals, but sea stars, tunicates, whip corals (whip-shaped corals range in length from a few to ten feet and can be solitary or form small bushes), and sailors’ eyeballs—a silvery marble-like alga—all buzzing, all contributing to the cornucopia of life.
Juvenile twoline dottyback emerging from a giant clam. Raja Ampat, West Papua, Indonesia.
Often dubbed the “rain forests of the sea,” coral reefs contain outstanding natural diversity, packed full of vibrant animals that capture the imagination. They accommodate some of the highest densities of animals on earth and more species than any other marine habitat. Although coral reefs are widespread throughout the tropics, they cover only a small area of our planet: a space smaller than the state of Texas, covering a measly 0.1 percent of the Earth’s surface.26 While the structure and composition of coral reefs appear superficially similar regardless of their location, the number of coral reef–associated species varies considerably depending on where you are in the world.
Tropical rain forests and coral reefs are the pinnacles of biodiversity of life on Earth. Rain forests cover an area twenty times that of all the coral reefs, and host more individual species than coral reefs, but they have fewer groups of species at higher taxonomic levels. It is easy to find organisms living side-by-side on a reef which are so ancient that they last shared a common ancestor during the Precambrian era, more than six hundred million years ago. The kind of diversity found on coral reefs is far greater than that found on land, or indeed anywhere else on the planet. Some species found there today date back to millions of years before the appearance of dinosaurs; cnidarians (the group containing jellyfish, corallimorphs, and corals), echinoderms, sponges, and bryozoans (ancient lattice-like invertebrates) are just a few of the better-known ancient phyla common on coral reefs.
It seems improbable that an ecosystem as limited in geographic extent as coral reefs should host as many as several million species. Estimates of the total number of coral reef species worldwide range between six hundred thousand and nine million.27 28 The only area that competes with coral reefs for high marine biodiversity is the deep sea, largely due to the huge extent of deepwater habitat around the globe. Rain forests compare to coral reefs in many ways, where corals and fish become analogous to the forest’s trees and birds. Both of these tropical ecosystems rely on living organisms—trees and corals, respectively—to produce the structurally complex habitat that provides vital food and shelter. Both also shelter many inhabitants that are extremely specialist, with these relationships coevolving over many millennia.
Anemonefish are just one example of a habitat specialist that lives on coral reefs. All anemonefish live exclusively with anemones; some can choose between ten possible anemone hosts, while some can only live with a single anemone species. Such specific habitat requirements allow many species to peacefully coexist with one another, each inhabiting its own specific niche. The number of these habitat specialists seems almost endless. Each type of animal seems to have another that lives with it or on it; in fact, new species are still being discovered, having previously been hidden in the bustle of the reef.
Center of Diversity
Although coral reefs as a whole accommodate huge numbers of species, the number of species that a specific reef harbors depends hugely on where it is located in the world. Just as the Amazon forest is richer than forests in Europe, for example, some reefs are more dynamic than others. Patterns of variety and population of coral reef species around the world aren’t uniform; for instance you could count as many species of fishes in a single Philippine bay as you could in the entire Caribbean.
In the nineteenth century, British biologists Sir Alfred Russel Wallace and Charles Darwin pioneered the study of animals’ distributions on land. However, it wasn’t until the mid-1950s that scientists began to investigate patterns of biodiversity in the oceans. One particular area of outstanding marine biodiversity has since become known by conservationists as the “Coral Triangle.” The area is relatively small in global terms, just 1 percent of the Earth’s surface, but it boasts the world’s richest marine biodiversity.29 It includes parts of the waters of six countries: eastern Indonesia, the Philippines, Sabah (a state of Malaysia), East Timor, Papua New Guinea, and the Solomon Islands.
Approximately 60 percent of all Indian Ocean and Mid- to Western Pacific (Western Indo-Pacific) reef fish species, and 37 percent of all the world’s reef fishes, are found in the Coral Triangle.30 The same area, equal to 1.5 percent of ocean surface, accommodates 76 percent of all known hard coral species.31 Due to a lack of research, it is impossible to estimate the proportion of other reef-living animals that are found within the Coral Triangle, but evidence suggests that they are similarly high. Beyond the six core Coral Triangle countries, Australia, Japan, Taiwan, Palau, Vanuatu, Fiji, New Caledonia, and the Federated States of Micronesia each has at least one thousand reef-associated fish species (those fish that spend part of their lives living on coral reefs) in their waters; while high, their numbers still fall significantly below the numbers of the triangle countries.
A renewed interest in ichthyological research around the Coral Triangle made it the focus of exploration and conservation efforts in the late 1990s. As a result, it has been the source of many of the newer coral reef species discoveries over recent decades. Although the total area of the Coral Triangle is small, there is plenty of habitat variation in it. In fact, by studying coral distributions in the Coral Triangle, scientists have identified sixteen ecoregions, each ecologically distinct from the others.32
The farther you travel from the Coral Triangle, the fewer the number of fishes, corals, and other organisms living on coral reefs you’ll find. This can be mapped as isobars of diversity with their focal point centered on the Coral Triangle. This pattern is generally found both from the equator toward the poles and in westerly or easterly directions. The Red Sea, for example, has
