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
