Sick corals might be stressed but not necessarily contagious â€" an important finding for predicting diseases outbreaks on threatened coral reefs, according to research by Mote Marine Laboratory and Florida Institute of Technology scientists who have completed the largest-ever study about the spatial distribution of coral diseases.
The study was published this week in the peer-reviewed journal Global Change Biology. Results show that three common diseases don't tend to spread from coral to coral, suggesting instead that disease outbreaks might owe more to environmental stressors such as climate change.
Climate change, pollution, more acidic oceans and diseases caused by harmful bacteria and other microscopic organisms are all contributing to global declines of coral reefs. Disease outbreaks have significantly reduced coral populations in the Caribbean, which is home to 8 percent of the world's corals and 66 percent of coral diseases. Due to multiple threats, live corals in the Caribbean cover an average area of just 8 percent of the reef, down from more than 50 percent in the 1970s, according to a report from the International Union for the Conservation of Nature in September 2012.
Cutting-edge research is just beginning to reveal how coral diseases attack, and how they interact with other threats such as warming water, which can cause corals to lose the vital symbiotic algae in their tissues and bleach (whiten), then die. This research can feed into mathematical models to predict how coral diseases will spread and change through time.
Mote scientists and collaborators have shown that disease-causing bacteria are more likely to harm corals when the corals are stressed by warming water, but most models of coral disease have not factored in environmental stress and instead suggested that coral diseases spread mainly because they are contagious and transmitted from coral colony to coral colony. This new research indicates that environmental conditions play a bigger role in coral disease than previously understood.
"Coral disease is like a triangle that includes the coral, the disease agent and the environment â€" all of these parts are important, but we wanted to find out if certain parts play a stronger role in disease outbreaks," said Dr. Erinn Muller, postdoctoral fellow at Mote. Muller co-led the study as a doctoral student at FIT and continues her research today at Mote. Her work in this study was supported by the by the National Oceanic and Atmospheric Administration's Dr. Nancy Foster Scholarship Program.
Muller and her co-author, Dr. Robert van Woesik of the Florida Institute of Technology, studied the distribution of diseased and healthy corals in the Caribbean. The researchers mapped the location of coral colonies at 253 sites of about 330 square feet each near St. Thomas and St. Croix, U.S. Virgin Islands, and near the Southwest end of Puerto Rico. They documented whether corals had common and often devastating diseases called yellow-band and white-plague, along with a less-studied condition called dark-spot syndrome.
"We didn't tend to see clusters of sick coral colonies â€" a sign that corals probably weren't catching diseases from each other," Muller said. "These corals seem to be getting infected through other means."
Diseased corals generally weren't clustered over a few feet to miles, but the data sometimes showed clustering patterns at a larger scale over multiple 330-square-foot study sites.
"These broader patterns might be showing the effects of environmental stress, which tends to happen over larger scales," Muller said. "For instance, water temperature changes in the past have affected Caribbean corals over several miles."
The researchers hope to continue their studies at other sites and combine these results with disease studies in the lab to develop new models of coral disease that will support management of reefs in a changing environment.
"This work not only gives us new knowledge to build stronger models â€" it also gives us more reasons to consider and address human impacts on corals and their natural environment," Muller said.