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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 significant coral structures but noticeably lower hard coral diversity, with just 240 species in the northern section compared to six hundred or so in the Coral Triangle.33 Red Sea fish life, too, although abundant, is much less diverse. Likewise, Hawaii has coral reefs, but the number of species that inhabit them is much lower than in the Coral Triangle. These patterns of coral reef biodiversity show us that the Coral Triangle has the richest reefs. Further inspection shows that within the Coral Triangle there is one area that stands out as an epicenter of biodiversity.

      A picturesque coral garden. Raja Ampat, West Papua, Indonesia.

      Heart of the Coral Triangle

In the remote western part of the island of New Guinea, one of the most continually rewarding areas for new species discovery remained obscured from the rest of the world until the mid-1990s. Dutch explorers named the peninsula Vogelkop, meaning “bird’s head,” due to its shape, and while today its formal name is Doberai, it’s often called the Bird’s Head Peninsula. It consists of three main areas: Cenderawasih Bay to the east, Raja Ampat to the west, and Triton Bay to the south. This region is the true heart of the Coral Triangle and the epicenter for global marine biodiversity. Of the Coral Triangle’s 605 hard coral species, 574 of them have been recorded here; some reefs support 280 species per hectare.34 By comparison, there are around one hundred hard coral species in the entire western Atlantic and Caribbean. Raja Ampat, to the west of the Bird’s Head, officially has the world’s highest coral diversity with 553 species present.35 This is incomparable across the world’s oceans and makes the area hugely important in terms of conservation.

      Europeans first explored Raja Ampat in the 1800s. It is in this region that Sir Alfred Russel Wallace spent time collecting bird skins he could sell to fund his research expedition. Scientists of Wallace’s era were preoccupied with species distributions on land and largely ignored the oceans. However, in the early to mid-1800s when Europeans, including Wallace, were exploring Bird’s Head, they noted and named many common reef fishes, including widespread Indo-Pacific species such as bluefin trevally, blacktip reef sharks, and semicircular angelfish. After this initial flurry of activity, the outside world largely forgot the peninsula. This twist of fate was probably its saving grace. When modern scientists began to explore the area centuries later, they found a wilderness full of rich, pristine reefs and previously unknown species.

Two ichthyologists who have made a particularly huge contribution to the documentation of the fishes in Bird’s Head are doctors Gerry Allen and Mark Erdmann. I had the pleasure of diving with the dynamic pair in 2013 as they continued their exhaustive inventory of the Bird’s Head reefs. Dr. Allen made some of the first modern underwater scientific observations of Raja Ampat in 1998 and has returned many times in subsequent years. By 2009, when they published their Check List of species, they had recorded 1,511 reef fishes.36 They have now passed the eighteen hundred species mark and are still counting. Raja Ampat is bathed by various currents that pass through the Indonesian archipelago and into the Pacific, bringing nutrient-rich waters through the area. In addition, Raja Ampat has a huge variety of habitat types, which seems to encourage a greater number of species to inhabit them, positively

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