LITMIR.BIZ

      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

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