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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 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 impacting the high diversity of this region.

Caitlin’s dottyback, described in 2008. Cenderawasih Bay, West Papua, Indonesia.

One of the wildest locations on the planet that I have ever visited is unquestionably Cenderawasih Bay on the north coast of Indonesian Papua. Thickly forested mountains tumble into the expansive, still waters. Small villages dot the bay, and visitors must respect the local ownership of these untouched waters and receive permission from the villagers to dive the sites. On my first visit to the location, I was required to have a local national park ranger stay aboard my liveaboard vessel to aid in these exchanges. Whether it was an honest mistake, a village he should have been avoiding, or a miscommunication, I don’t know—but as we tucked into our lunch after the dive, we heard a ruckus outside. We dashed out to discover four bare-chested Papuans brandishing machetes, anger bursting from their eyes. They were livid that we hadn’t asked permission to dive the site; after all, as they saw it, we were effectively trespassing. It took some time to explain the misunderstanding and return calm. Thankfully a cold Coke isn’t easy to come by in these remote corners of the planet and they were somewhat placated by our supply. They had paddled several miles from their village, travelling in a dugout canoe, so we gave them some fuel to express our apologies, since their supply had long since run dry.

Maurine’s demoiselle, described in 2015. Cenderawasih Bay, West Papua, Indonesia.

Cenderawasih Bay is a marine biologist’s dream. It is a rare example of a location that illustrates evolution by isolation. Also known as “allopatric speciation,” Darwin saw this as the most common source of new species creation in terrestrial animals. In the same way that the Galápagos Islands are isolated from the mainland and species evolve there to suit the local conditions, Cenderawasih provides an almost self-contained ecosystem allowing for high levels of endemism. Endemism means that an organism is unique to a defined geographic region, in this case Cenderawasih Bay; although, it would also be true to say that these animals are endemic to Indonesia. The huge bay covering almost one and a half million hectares was isolated from the Pacific Ocean when large landmasses drifted across its mouth between two and five million years ago.37 These landmasses may not have created a complete physical barrier, but they were significant enough to alter currents in and out of the bay. Without this flow through, populations inside became separated from those outside the bay and over time adapted to the conditions they found themselves in within the bay.

Female ornate angelfish. Cenderawasih Bay, West Papua, Indonesia.

Diving in Cenderawasih Bay is a strange experience. While the reefs are ostensibly the same as many in Raja Ampat to the west, the inhabitants are largely alien. Bright blue and white Price’s damselfish, pink and yellow Caitlin’s dottybacks, and beige and yellow Maurine’s demoiselles are all ubiquitous here, but found nowhere else on Earth. Farther down the reef slope, the stunning Cenderawasih fairy wrasse inhabits rich coral areas at sixty feet below the surface. The shocking yellow streak and black blotches on the male’s side allow divers to spot the beauties from a fair distance away. It’s amazing that these fish remained in obscurity until 2006 when scientists first explored the bay. Previously, none of these fish were known to Western science. There are at least fourteen species of reef fishes known only from the bay, which is many more than you would expect from such a small area, with many sure to join them as it is explored further.

Male ornate angelfish. Cenderawasih Bay, West Papua, Indonesia.

While exploring the limits of recreational diving at around ninety feet, another quirk of the bay is revealed. The topographical structure of the bay means there are limited areas where the reef gently slopes into the abyss; instead, shallow slopes give way quickly to deeper drop-offs. Over the course of several ice ages, where the water level drops and subsequently rises thousands of years later, the shallow coral reef habitat was repeatedly lost and many species went locally extinct. When sea levels rose again, the lack of connectivity with areas outside the bay meant that the vacant niches couldn’t be filled by migrants. It seems that deepwater species instead seized the opportunity to fill some of these niches and moved up into the shallows. As a result, while diving in Cenderawasih it is possible to see fishes such as the alluring ornate angelfish, Randall’s anthias, and the Burgess butterflyfish in water much shallower than they are found in elsewhere in the world.

Male Randall’s anthias. Cenderawasih Bay, West Papua, Indonesia

Burgess butterflyfish. Cenderawasih Bay, West Papua, Indonesia.

The final area of the Bird’s Head trio is Triton Bay. Located southeast of Raja Ampat on the south coast of Papua, this area has been revealed as another region rich in endemic species. In 2006 scientists mounted an exploratory expedition and found many new and indigenous species, such as Jamal’s dottyback, Nursalim flasher wrasses, and a unique walking shark. The leading theory that most likely explains the high levels of endemism around Triton Bay relates to the two large freshwater rivers that flow into the ocean to the north and south of the bay. In a similar way to the landmass-blocked mouth of Cenderawasih, the freshwater from the rivers acts as a barrier to the marine organisms in the bay. Trapped within by these walls of unsuitable habitat, the animals within evolved to suit local conditions.

Jamal’s dottyback, described in 2007. Triton Bay, West Papua, Indonesia.

The walking sharks, or epaulette sharks, are an intriguing group found around the coast of New Guinea and northern Australia. As their common name suggests, their preferred method of locomotion is walking rather than swimming. They use adapted pectoral fins to crawl around in the reef shallows to hunt for their prey. As they live in shallow water and won’t swim across deep water or unsuitable habitat, they can easily become cut off. As a result, at least six species of walking shark have evolved around the coastline of New Guinea.38 Each of the three areas of the Bird’s Head has a unique species of walking shark and there are another three in Papua New Guinea to the east. It is not beyond the realms of possibility that more species may be discovered along the New Guinean coast as scientists explore it further.

Raja Ampat walking shark. Raja Ampat, West Papua, Indonesia.

Milne Bay walking shark, described in 2010. Milne Bay, Papua New Guinea.

I have made some of my most unexpected discoveries and observations in the Bird’s Head region and experienced the bounty of the coral reef in its full splendor. I have been surrounded by such thick schools of fish that I could not even glimpse my dive buddy a few feet away. I have seen the biggest fish in the sea, the enormous whale shark, in both Cenderawasih and Triton Bays and one of the world’s smallest fish, Satomi’s pygmy seahorse, in Raja Ampat. This is truly a special corner of the world.

Male Galapagos pike blenny displaying. San Cristobal, Galapagos Islands.

Outside the Coral Triangle

Travelling outside the Coral Triangle, and to the periphery of the ocean’s coral realm, reveals reefs with their own special assemblages of creatures. When compared to the Coral Triangle these reefs can seem relatively poor in terms of species richness, but each of the different area adds to the overall richness of coral reefs globally. The limited coral reefs of the Galápagos Islands, for instance, have one-tenth the number of species as their counterparts in the Coral Triangle, but these peripheral reef locations tend to have many unique and indigenous organisms. Almost 20 percent of the marine life in Galápagos is found nowhere else on Earth. The areas with the highest numbers of such endemics are found at various isolated eastern Pacific islands, as well as Baja California, a Mexican state just south of California; Hawaii; Galápagos; the Red Sea; and Oman.39 Despite their high proportions of indigenous species, these areas all have far fewer total numbers of species than the Coral Triangle, which is blessed with both amazingly high biodiversity and many endemic species.

Red Sea anthias. Egypt.

The conspicuous angelfish is found only on the subtropical reefs of central east Australia, New Caledonia, and east to Lord Howe Island. Lord Howe Island, Australia.

Marine iguana feeding on algae. Fernandina Island, Galapagos Islands.

The more I have explored coral reefs around the world, the better my appreciation of the world’s varied reefs and how each has its own unique defining attributes. For instance, clouds of bright orange anthias fish, exuberant coral growth, and impossibly blue waters are instantly recognizable as a Red Sea reef. The Red Sea is an area of extreme contrasts. The average rainfall there is less than one centimeter per year, and barely a plant grows on the exceptionally arid land beside it. But the parched and dusty land spills abruptly into the azure sea, which bustles with life more colorful than a painter’s palette. Rather than being hindered by the desolate terrestrial landscape, the coral reefs of the Red Sea are shaped by the conditions above. The dazzling blue water is a result of the very limited rainfall, and without freshwater influx hindering their growth, the corals fringe around the land’s contours almost exactly.

Red Sea longnose filefish. Egypt.

What I most enjoy about diving the Red Sea is that 13 percent of its fishes are found nowhere else on earth.40 Over the past several million years, as sea levels have fluctuated, the Red Sea has been cut off by a land barrier at its southern reaches. It is one of the saltiest bodies of water on Earth and its high salinity drives the evolution and adaptation of its inhabitants. Huge clouds of tiny reddish anthias fish are endemic to the region; striking sunrise dottybacks and vivid orange and green spotted longnose filefish that putter about feeding on coral polyps are also indigenous.

Red Sea mimic blenny. Egypt.

Black-­line fang blenny. Egypt.

Another fascinating example of evolution that exists in the Red Sea is the venomous fang blenny of the genus Meiacanthus, whose relatives have been found throughout the world’s reefs. They are quick to bite and potential predators know to avoid them, so they can largely go about their business on the reef without fear of attack. They hold a privileged position and have attracted other envious fishes wishing to exploit the situation. The Red Sea’s fang blenny is an endemic species and has a unique coloration of a blue head, black stripe, and yellow rear half of the body. Red Sea predators know to avoid these colors, so it makes sense that their mimics would need to keep up and copy these colors if they’re to successfully pull the wool over their would-be predators’ eyes.41 A harmless blenny of the genus Ecsenius mimics the venomous fang blenny, allowing it to feed on algae out in the open during the day without fear of predators. The Plagiotremus blenny also exploits the Meiacanthus, mimicking the fang blenny’s coloration to get close to fish that they themselves prey on. Predators avoid the Plagiotremus and its prey species do not fear the Meiacanthus, so they don’t worry about it being near them, allowing the mimic to approach extraordinarily closely and take a bite of a scale. I find it fascinating that because the model Meiacanthus is different in the Red Sea, all its mimics have had to evolve to continue to benefit from mimicking it.

Subtropical Reefs

Keen to experience another type of reef system, I visited the remote subtropical reefs of the Izu Islands some 180 miles south of Tokyo, in Japan. Unlike the calm and tranquil diving around the sheltered islands of Indonesia, these high-latitude reefs tend to suffer more adverse conditions. To reach the entry point, I had to rappel in full dive gear down a steep ramp into the ocean, toward the crashing waves below. Holding a rope in one hand and my hefty camera in the other, I wondered if I should have just visited the stunning temples of Kyoto and given this a pass. Once I sank safely under the surface, the clear and surprisingly tranquil water revealed a beautiful and peaceful sloping topography. There were very few hard corals, with algae more dominant than would be expected for a tropical coral reef, but lots of sponges, gorgonians, and soft corals.

Wrought-­iron and Japanese butterflyfishes. Hachijō-­jima, Japan.

Many quirky creatures call these Japanese reefs home. The yuzen, or wrought-iron butterflyfish, endemic to Japan’s Izu and Bonin islands, is stunning with its black-and-white body and a tail of bright daffodil yellow. During my dives, I came across the Japanese swallowtail angelfish, Sakura anthias, Japanese eeltail catfish, and Yatabe blenny—all indigenous to the region. Unexpectedly, I also came across a Bargibant’s pygmy seahorse clinging to a gorgonian coral at 110 feet. This find extended the recorded geographic range for the species several hundred miles north from the coral reefs of Okinawa. This vagrant seahorse’s presence in these northern reefs implies some of the processes that have resulted in Japan’s unique marine life.

Sakura anthias. Izu Peninsula, Japan.

Japanese eeltail catfish, described in 2008, and bigscale soldierfish. Hachijō-­jima, Japan.

Huge ocean-scale currents flow across the Pacific Ocean and split as they strike Australasia. The South Pacific Gyre pushes water across the Pacific in a counterclockwise motion and hits New Guinea, becoming the East Australian Current as it flows southward, down toward Tasmania. The North Pacific gyre heads up toward Japan as the Kuroshio Current. It pushes water from the equator northward toward Japan and is the Pacific’s largest current. This current has a significant impact on marine ecosystems, bringing warm water and tropical fishes to where you might not expect them in Japan’s northern reaches. This northward flow also has the effect of creating a barrier to fish trying to migrate in a southerly direction. Since they are effectively isolated in Japanese waters, with the current as a barrier to them moving south and cold polar waters in the north, they have evolved into unique forms. As a result, Japanese reefs have many endemic marine species that exist nowhere else on Earth.

Bargibant’s pygmy seahorse. Bangka Island, Indonesia.

Why the Triangle

There is no doubt that Japan has fascinating reef inhabitants, but the total number of species I encountered on the trip was dramatically lower than I could have seen on a comparable number of dives in Indonesia. There is some debate about the reason for high biodiversity in the Coral Triangle, but there are three leading theories to explain why global patterns of marine richness center in this part of the world.42 43

The first theory to explain the existence of the Coral Triangle as a center of biodiversity suggests that the region is a species factory, with many new species being created there. These go on to boost the overall diversity of the area compared to any other. The southern Coral Triangle has experienced geological instability for at least the past thirty-eight million years.44 Diving along Indonesia’s southern Lesser Sunda island chain, where Bali, Komodo, Flores, and the Alor Islands group are located, illustrates this point. It is not uncommon to see three smoking volcanoes on the horizon as you descend on a dive. One of the largest volcanic eruptions in recorded history, of Mount Tambora, took place here in 1815. The eruption was so big that it caused crops to fail across the world, and the darkened skies reportedly inspired the creation of Frankenstein and Count Dracula. The continually changing geography of the region is thought to have driven the evolution of new species by separating populations from one another and changing the local conditions to which they are exposed.

Given the many millions of years over which evolutionary processes occur, we must remember that we are seeing just one snapshot of the planet as it is today. A map of the Coral Triangle would have looked very different two million years ago. Present-day examples of Cenderawasih and Triton Bays are likely to have been played out many times throughout the geological history of the Coral Triangle. After evolutionary processes have cast their spell on a given bay or stretch of coastline, the new species that evolve there may eventually spread out to the wider Coral Triangle, complementing and enhancing the diversity of the region.

Yatabe blenny. Izu Peninsula, Japan.

Although evolution by isolation does occur in the marine environment (and I have highlighted several examples here), it is much less common than on land. Sir Alfred Russel Wallace described a theoretical boundary line, Wallace’s line, that cuts through Indonesia, and explains the transition between Asian and Australian fauna. To the west of the line sit the large islands of Sumatra, Java, and Borneo, which during ice ages would have been joined to the Asian landmass. Tigers, orangutans, rhinos, and monkeys roam in these Asian forests. To the east of the line they are noticeably absent, replaced instead by various animals we tend to consider as Australian natives, like marsupials and cockatoos, as well as eucalypt trees. The island of Sulawesi sits just to the east of the line, but accommodates an intriguing mix of Asian macaque monkeys, tarsiers, and pigs, as well as Australian cockatoos and cuscus marsupials. The island has a mixed geological origin with parts that have drifted from Asia and others antipodean in origin, but biogeographers continue to argue over the island’s faunal origins.

Underwater, Wallace’s line has been much less significant in understanding geographical distributions of species. The walking sharks of New Guinea and Australia are limited to unbroken areas of suitable habitat, which explains their distribution, but the vast majority of fishes are not so constrained, with a surprising number of reef fish species found on coral reefs all the way from East Africa to the mid-Pacific. Dispersal across a huge distance was made possible by their larvae, which spend weeks or months floating in ocean currents, allowing them to reach even the most remote reefs.

The pajama cardinalfish is a striking reef fish with red eyes, yellow head, black waistband, and pajama-like spotted rear half. These fish are found all the way from Java in western Indonesia to Tonga in the mid-Pacific. Like almost all cardinalfishes, they are paternal mouthbrooders; the male broods fertilized eggs in his mouth until they hatch and the resultant fry, or juvenile fish, float off in ocean currents. In the case of the pajama cardinal, the young spend a relatively extended twenty-four days floating in the water column before settling on a reef. This allows ocean currents to carry them relatively far afield, hence their wide geographic distribution.

Pajama cardinalfish. Wakatobi, Sulawesi, Indonesia.

Weedy cardinalfish brooding eggs. Lembeh Strait, Sulawesi, Indonesia.

A superficially similar cardinalfish offers a good illustrative example to demonstrate the importance of larval pelagic (open sea) duration in a species’ distribution. The Banggai cardinalfish is naturally found in a small Indonesian island group just two-thirds the size of Connecticut.45 It features a stunning combination of black stripes on a white base color, with white-speckled black pelvic, anal, and caudal fins. Unlike the pajama cardinal, the Banggai has a unique approach to brooding. The male holds just forty to fifty large eggs in his mouth, significantly fewer than other cardinals that can brood thousands of much smaller eggs. He broods these for nineteen to twenty days before they hatch, when he retains them for an additional ten days in his mouth.46 During this extended period of paternal care, the young grow into well-developed, miniature versions of their parents.

Banggai cardinalfish. Lembeh Strait, Sulawesi, Indonesia.

I once spent a dive observing and photographing a brooding male, but was most intrigued by watching the inquisitive fry jostling for the best vantage point to watch me from inside their father’s mouth. Because of their advanced development, the Banggai skip the pelagic oceangoing stage of development, and, after gaining their freedom, immediately form a small school around a protective home such as a sea urchin, where they shelter among the long spines. The babies are then immediately committed to the reef; as a result the species hasn’t naturally spread beyond the confines of the Banggai Islands, which are surrounded by deep, unpassable water.

I have been privileged to see the striking Banggai cardinalfish on several occasions, but I have never been to the Banggai Islands. In the mid-1990s there was huge demand for these fish in the aquarium trade. They were heavily collected, and in 2000 a small group was discovered in Lembeh Strait, a popular diving location separating the northeast Indonesian islands of Lembeh and Sulawesi. These Banggai cardinalfish are believed to have escaped and naturalized from an aquarium trade consignment. In Lembeh Strait their numbers have since increased exponentially and their population is now abundant. Several years later, most likely released by enterprising scuba professionals, they appeared in northwest Bali, and, in 2017, the first individuals were taken and released near Ambon Island, a diving area in central Indonesia. Without their natural predators, the non-native Banggai’s populations have exploded, with potential impacts on native species. It is hard to know the long-term implications of this influx, but you don’t have to look hard to find examples of widespread devastation by other invasive organisms. Ironically, the natural populations of these fish in the Banggai Islands have continued to suffer; they have reportedly been reduced by 90 percent through removal for the aquarium trade, and have been listed as endangered and continue to decline in numbers.

Recently released juvenile Banggai cardinalfish shelter among the spines of an urchin. Lembeh Strait, Sulawesi, Indonesia.

The final major explanation for high biodiversity in the Coral Triangle is that the geographic ranges of many species from the Indian and Pacific Oceans overlap within the Asian archipelago, causing higher diversity where they coexist. While the true source of the Coral Triangle’s high diversity can possibly be attributed to a convergence of several factors, it mostly boils down to the huge variety of habitat types the islands offer. Varied habitats beget varied organisms.

In Our Hands

We know that the world’s highest marine biodiversity is in the Coral Triangle, but this wasn’t always the case. Major tectonic events over the past fifty million years have shifted the location of the hotspot at least three times over that period.47 Southwest Europe was once the location of the world’s richest reefs. While natural processes like tectonic events can impact the survival of coral reefs and their inhabitants, human activity currently threatens to enact unprecedented changes on the ecosystems.

Male Banggai cardinalfish mouth brooding his fry. Lembeh Strait, Sulawesi, Indonesia.

Given its amazingly high biodiversity, the Coral Triangle’s conservation is a global priority. Even today dozens of new fishes, crustaceans, corals, and echinoderms are being discovered on the region’s coral reefs—each vital to the mechanics and well-being of their environment. New species can emerge in the most unexpected places. Some have been hiding in plain sight; others have offered only the briefest glimpses into their lives. We have so much more to learn. If we want to meet undiscovered organisms before we lose the chance forever, we must conserve reefs around the world.

26 Fredrik Moberg and Carl Folke, “Ecological Goods and Services of Coral Reef Ecosystems,” Ecological Economics 29, 2 (1999): 215–233.

27 Rebecca Fisher et al., “Species Richness on Coral Reefs and the Pursuit of Convergent Global Estimates,” Current Biology 25, 4 (2015): 500–505.

28 Alasdair D. McIntyre, ed., Life in the World’s Oceans: Diversity, Distribution and Abundance (Blackwell Publishing, 2010): 65–74.

29 Renema, Willem, ed., Biogeography, Time and Place: Distributions, Barriers and Islands 29 (Springer Science and Business Media, 2007): 117–178.

30 Gerald Allen, “Conservation Hotspots of Biodiversity and Endemism for Indo‐Pacific Coral Reef Fishes,” Aquatic Conservation: Marine and Freshwater Ecosystems 18, 5 (2008): 541–556.

31 J. Veron et al., “Delineating the Coral Triangle,” Galaxea, Journal of Coral Reef Studies 11, 2 (2009): 91–100.

32 A. Green and P. Mous, “Delineating the Coral Triangle, its Ecoregions and Functional Seascapes,” Version 5.0. TNC Coral Triangle Program Report 1, 08 (2008).

33 J. Veron et al., “Delineating the Coral Triangle,” Galaxea, Journal of Coral Reef Studies 11, 2 (2009): 91–100.

34 J. Veron et al., “Delineating the Coral Triangle,” Galaxea, Journal of Coral Reef Studies 11, 2 (2009): 91–100.

35 Hedley Grantham et al., “A Comparison of Zoning Analyses to Inform the Planning of a Marine Protected Area Network in Raja Ampat, Indonesia,” Marine Policy 38 (2013): 184–194.

36 Gerald Allen and Mark Erdmann, “Reef Fishes of the Bird’s Head Peninsula, West Papua, Indonesia,” Check List 5, 3 (2009): 587–628.