White-band disease is a widespread coral disease that has plagued elkhorn (Acropora palmata) and staghorn corals (Acropora cervicornis) for decades. These corals that dominated the waters of the Caribbean for many years, have now all but disappeared, with the disease being responsible for the devastation of approximately 95% of the elkhorn and staghorn corals in the Caribbean 1.

Because of their decline, both the elkhorn and staghorn coral species are now listed as threatened under the U.S. Endangered species act. According to a new study from Florida Institute of Technology, ocean warming has played a significant role in this dramatic decline. In an attempt to discover whether the effects of climate change have contributed to the spread of the disease throughout the Caribbean, researchers at the institute examined the relationship between white-band disease and rising oceanic temperatures, reporting there findings in the February issue of Nature Climate Change.

“Our data shows that climate change has helped drive down staghorn and elkhorn corals by boosting white-band disease,” says Florida Tech Ph.D. student Carly Randall. “We still don’t know if the disease is caused by a marine microbe, but now we do know that changes in the environment contributed to the problem.”

No known pathogen has been isolated for white band disease, although there has been shown to be a shift of bacterial composition in the surface layer where the band eats away as the coral tissue, with composition shifting from a dominant pseudomonad population to an increasingly dominant Vibrio carchariae population 2. The disease causes the affected coral tissue to decorticate off the skeleton in a white uniform band for which the disease was given its name. The band progresses up the coral branch at an approximate rate millimetres per day, causing tissue loss as it works its way to the branch tips.

The study was achieved through quantifying eight Sea Surface Temperature (SST) metrics, including rates of change in SST and contemporary thermal anomalies, and comparing them with records of white-band disease on elkhorn and staghorn corals from 473 sites across the Caribbean, surveyed between 1997 to 2004. The results suggested that decades-long climate-driven changes in SST, increases in thermal minima, and the breach of thermal maxima have all played significant roles in the spread of white-band disease.

The study also found that the disease is more common in places where the waters have been warming most rapidly and where the waters stay unusually warm in the winter season. The scientists think that without a cooling-off period, the effects of the hot summers linger, and the disease is more likely to worsen. The disease has been previously observed to be more active in summer and less prominent in winter, suggesting that warmer water temperatures contribute to the waterborne spread of the disease to affected corals 2.

The increased severity of such marine diseases in elevated temperatures may occur for several reasons. Elevated water temperature causes corals physiological stress, which undermines their immune system and makes them more susceptible to infection from white band disease or other coral diseases 3.

In response to the teams findings, Florida Tech’s Robert van Woesik says : “We are a step closer to predicting where these diseases are occurring because now we know why they are occurring.”

– JK

Article Source: J. Randall, R. van Woesik. Contemporary white-band disease in Caribbean corals driven by climate change. Nature Climate Change, 2015; DOI:10.1038/nclimate2530

Photo Header Credit: CARLY RANDALL, FLORIDA INSTITUTE OF TECHNOLOGY

Additional References:

1. Gladfelter, W. B. (1990) Population Structure of Acropora palmata on the Windward Fore Reef, Buck Island National Monument, St. Croix, U.S. Virgin Islands”. U.S. Department of the Interior, National Park Service.

2. Gignoux-Wolfsohn, Marks, and Vollmer (2014) White Band Disease transmission in the threatened coral, Acropora cervicornis

3. Bruno, J. (2013). Coral reefs and climate change. Retrieved from http://www.eoearth.org/view/article/151492