Marine Disease Shapes Tropical Reefs, Study Finds

Palmyra Atoll, a secluded national marine monument over a thousand miles south of Hawaii, is a quintessential tropical paradise: forests of a rare flowering tree cover the islands, which provide refuge to migrating seabirds and rare coconut crabs. A colorful healthy coral reef filled with fish lies beneath the waves that lap the shores.

The only human habitation on Palmyra is a small research station. The reefs are not polluted by sewage, runoff, or fishing. Nevertheless, Bigelow Laboratory researchers found evidence there of the global changes being driven by human activities.

“Even though this is one of the most remote locations on the planet, we still see the signal of human impact,” said Nichole Price, a senior research scientist and the primary investigator on the project. “Nowhere is unaffected anymore.”

Palmyra’s remote location, however, has offered it some protection, and the coral reefs surrounding it are among the healthiest remaining on the planet. These factors make it an ideal place for Price to study a fungal disease that afflicts an important reef-building organism called crustose coralline algae.

Tropical ecosystems worldwide have suffered catastrophic losses of these Pepto-Bismol pink seaweed and corals, which create the underlying calcified structure of reefs, and provide a complex habitat for countless species. Price is working with The Nature Conservancy, U.S. Fish and Wildlife Service, and U.S. Geological Survey National Wildlife Health Center to assess the impact the fungus has on the Palmyra reef ecosystem.

In particular, the researchers are investigating the relationship of this marine disease to two major global change phenomena: ocean warming and acidification. Scuba diving allows Price’s team to monitor the pink seaweed and fungus, and assess how they are responding to recent, dramatic changes in ocean temperature.

The team is also using laboratory experiments to examine how the pink seaweeds are likely to fare in the future. Price expects the acidified seawater will make the pink algal crusts more permeable to the pathogenic fungus, and more vulnerable to disease. The scientists simulated future ocean conditions in seawater aquariums with carbon dioxide bubbling through them, and observed how diseased specimens responded. They will analyze the results of those experiments in 2018.

Their initial field results have already shed new light on reef dynamics. One significant finding is that herbivorous fish, like parrotfish and surgeonfish, selectively feed on the fungus and limit its spread.

“Across most of the Caribbean and parts of the Pacific, parrotfish populations have been destroyed by spearfishing and gill netting,” said Ben Neal, a postdoctoral scientist in Price’s lab. “This is potentially one of the major factors for why coral reefs are ecologically falling apart.”

Back in Maine, the team is now working to genetically analyze the fungus. This will tell the researchers more about the organism, which they suspect hasn’t yet been formally studied and named. Even without this understanding, they have already gained new insights from the fungus in regard to climate change and marine disease.

“Disease is not a factor that people often think about affecting tropical seaweeds, and this study has already revealed an unexpected and important component of global ocean changes,” Price said. “Evidence-based management of coral reef fishes may be key to reef resilience to disease outbreaks in the future.”