One of the prime suspects in the destruction of Caribbean coral has been found guilty, but researchers were wrong about its motive. For years scientists thought that the threespot damselfish was nibbling star coral to death because overfishing had reduced its natural predators. But a new survey shows that the damage is being done because the damselfish was forced from its natural habitat—and in fact the fish is also suffering. The good news is that it might be possible to restore the reefs to health, though that effort could take at least a decade.
The case begins not with the star coral but with another species, the branching staghorn coral (Acropora cervicornis). Once, the dominant coral in the Caribbean, staghorns—and their less-common relative, the elkhorn—were decimated in the 1970s and '80s, the victims of a bacterial infection called white-band disease. More than 90% of the coral died, and populations hit their lowest levels in over 3000 years.
That was bad news for the damselfish (Stegastes planifrons), which survived on the staghorn coral. The ill-tempered fish, which has been known to bite the fingers of divers who approach its territory, nibbles incessantly on the coral—not to eat it but to kill the living tissue so that algae, the fish's favorite food, can grow on the dead coral skeletons. The damselfish never destroyed the staghorn coral, because the coral grew fast enough to recover from the constant nibbling.
But when the damselfish was forced to relocate to star coral (Montastraea), this coral began dying. Many scientists concluded that overfishing had thinned the ranks of damselfish predators, such as snappers and small groupers, and that the population of damselfish had exploded, wreaking havoc on star coral reefs.
But no one had tested this hypothesis. So a team of researchers set up surveys at 10 coral sites in the Florida Keys, Bahamas, and Cayman Islands and off the coasts of Belize and Jamaica. "We wanted to find out if the more heavily fished reefs had more threespot damselfish, and if not, what was controlling the abundance of threespots," says paleontologist Richard Aronson of the Florida Institute of Technology in Melbourne. He explains that the team chose the sites to compare the heavily fished areas, such as off Jamaica, with the moderately fished reefs of the Keys and Bahamas and the protected reefs of the Caymans and Belize.
Damselfish are indeed responsible for the star coral die-off, the team found, but not because their numbers are booming out of control. In fact, the researchers report this week in PLoS ONE, the fish's numbers have declined overall. Aronson says that's because star corals offer fewer places to hide than the staghorn, leaving the damselfish more vulnerable to predation.
So why is star coral dying? Aronson and colleagues conclude that it grows back much slower than the staghorn, so it doesn't recover from the damselfish algae farming. The change has placed all Caribbean reefs—already in jeopardy from pollution, silting, warming waters, and oil spills—in even greater danger. "The threespots have now killed substantial amounts of star coral, and [they] are doing damage that will take decades or centuries to fix," Aronson says.
But the reefs may not be lost. One practical solution, Aronson explains, is to restore the staghorn and elkhorn to the reefs. This can be done via a painstaking aquatic form of tree farming. Biologists first grow nubbins, or small baby coral, on cinder blocks that have been submerged, and then they transplant the growing corals to the reefs. "Once the staghorn is restored, the threespots will move back into their preferred neighborhoods," Aronson says.
The paper "drives home the fundamental importance of disease outbreaks in the changes we've seen on Caribbean coral reefs over the last several decades," says marine biologist John Bruno of the University of North Carolina, Chapel Hill. It's "an important contribution from a group of scientists who know more about these reefs than anyone else in the world."