Hot on the Trail

Peter Craig was curious. The recently retired chief biologist for the National Park of American Samoa for 20 years, Craig was on the island of Ofu, about 60 miles east of the main island (in the neighborhood of Fiji), when he came across a lagoon with a number of shallow pools, arrayed, he says, “like a string of pearls.” The pools were of varying depths, and supported almost 100 species of vibrant corals. And during the low afternoon tides in the summer, when the pools were cut off from the sea, they got really, really hot.

Corals, Craig knew, are not supposed to thrive in hot water. He wanted to see what, exactly, these corals were enduring, so he dunked thermometers in some of the pools. He found that the temperatures fluctuated by as much as 11° F during a single day. Sometimes, the water was well over 90° F. Craig was astounded. Corals elsewhere died when water temperatures rose by less than 2° F. Yet these corals seemed at home in swings more than five times that.

He suspected he had stumbled onto something big. To lure an academic scientist to investigate even more, he published his observations in the journal Coral Reefs in 2001 and more generally spread the word that Ofu was worth a look. “We certainly played up the romantic pictures of the South Pacific,” he says. “We played that card pretty shamelessly, actually.”

As hosts to some of the richest biodiversity in the ocean, corals are usually not a hard sell. More than half a billion people live near corals, relying on them for food, shelter from storm surges, and the income that tourism brings. And while a coral may look like a single entity, it’s actually a partnership between two microscopic organisms: a polyp, which is a tiny assemblage of mouths and tentacles; and a single-celled organism, usually an alga or dinoflagellate, which lives within that assemblage. The former builds a tiny calcium-carbonate structure that shelters the latter, and, through photosynthesis, the latter provides food to the former. Millions of these little partnerships accrete to form enormous, iconic reefs.

Globally, though, coral reefs are in trouble, and the reefs in American Samoa are no exception. Some suffer from heavy fishing pressures, while others are sullied by pollution. More broadly, there is the looming specter of climate change, which is predicted to increase sea temperatures. Rising temperatures can cause corals to expel their algal partners. The corals turn bone white and die—a phenomenon called “bleaching.” The national park, as Craig well knew, had endured major bleaching events in 1994, and would again in 2002.

Fortunately, Craig’s paper caught the attention of Stephen Palumbi, a biologist at Stanford University. Palumbi first visited Ofu in 2004 and has studied corals there ever since. Through a series of experiments, he and his students have shown that the Ofu corals are uniquely equipped to handle high water temperatures. In one recent study, published in Coral Reefs in 2011, Palumbi and Tom Oliver, a postdoctoral researcher, examined how corals from shallow pools on Ofu would fare in conditions predicted under climate change, compared with corals from nearby deeper pools that weren’t thought to be as robust. They filled two tubs, one with water at the same temperature as the ocean, the other with water 9°F warmer—a potentially fatal difference. They found that 50 percent of the corals from deeper pools died, but the corals from shallower pools were none the worse for wear. Not only that, but those shallow-pool corals grew faster than the same species from deeper pools. “It was almost like they hadn’t read the literature,” Palumbi says. “They didn’t know that they were supposed to be dead.”

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That such important work can come from a place as isolated as Ofu might be a surprise, but park staff have gone out of their way to make the island welcoming to research. In the past, there wasn’t much in the way of facilities. Scientists had to bring their own equipment, and often left it behind for others to use. The park saw an opportunity to provide a more permanent space and built an elaborate seawater filtration system, and now everyone has a sophisticated lab at their disposal—perhaps the most remote in the National Park System. Scientists and park staff depend on local Samoans to help maintain some experiments. “It’s a great way to tie research in with the community,” says Carlo Caruso, a park ranger at Ofu.

Now, Palumbi and his lab are trying to figure out why Ofu corals thrive. Is it their partner alga that helps them? (Some corals will exchange one alga for a stronger one when temperatures rise.) Or have they been conditioned to the high temperatures after being exposed to them for so long, leading to some kind of genetic advantage? “What we’re doing now is delving into the gene level,” says Dan Barshis, another of Palumbi’s postdoctoral researchers. “Our goal is to find a few genes that are diagnostic of strong corals and then go out and survey for those genes over a bunch of different areas. If we can identify corals with those genes, we can prioritize targets of protection.”

More encouraging still is that biologists now know that heat-resistant corals aren’t only at Ofu, having turned up in the Caribbean, in Indonesia, and off the coast of Hawai’i, among other places. Palumbi is conducting a series of transplant experiments to see whether it’s possible to move these so-called supercorals to areas with weaker corals and perhaps make them stronger, restoring shrinking reefs. If so, there’s a chance our coral reefs may not be in such hot water, after all.