An epizootic - the wildlife equivalent of a human epidemic - of black band disease has appeared in the Great Barrier Reef, say Australian researchers.
Scientists, who have been monitoring the progress of the disease, say this the first time an epizootic of this type has been documented in Australian waters.
Black band disease has decimated coral populations in the Caribbean and researchers are concerned it could spread here.
Black band disease infecting on a Montipora coral colony. The disease forms a thin black band that kills coral as it eats through tissue, exposing the fragile skeleton (Source: Yui Sato)
Marine biologist Yui Sato of James Cook University in Townsville and colleagues report their findings in the latest issue of Proceedings of the National Academy of Science.
Sato, who is a research student with the ARC Centre of Excellence for Coral Reef, says the black band disease flourishes in warm seawater, killing coral as it eats through tissue, exposing the fragile skeleton.
He is concerned that predicted warmer ocean conditions caused by global warming will lead to longer outbreaks and faster tissue loss.
Increased prevalence
The researchers compared photos of 485 colonies of Montipora species - a hard, plate-like coral - with photos from two previous surveys to track coral death and infection.
Sato says infectious diseases in reef building corals have emerged at an increasing rate throughout the last few decades.
He says sporadic occurrence or low prevalence had been documented in the Reef since 1993, but this is the first time big outbreaks had been found.
"It has not previously been documented like this with such a big impact on the coral assemblage," says Sato. "It shows impacts of the disease are greater than previously reported on the Great Barrier Reef and likely to escalate with ocean warming."
The researchers found that at least 10% of Montipora corals in the Great Barrier Reef were infected during summer with about 5% of those dying.
They also found that surviving coral were three times more likely to be reinfected than uninfected coral. This may be due to bacterial pathogens remaining in the colony or due to a compromised immune system.
Sato says infection occurs after a bacterial mat forms a black band, which migrates across healthy coral colonies. The mat is comprised of multiple species of bacteria, mainly cyanobacteria (blue-green algae) and bacteria associated with the sulfur-cycle.
"We don't really know where they come from or how they make such a complicated bacterial community, but once formed it can kill coral quickly," eating through 4 millimetres a day, he says.
The study area had been disease-free during the past 15 years; therefore the sudden appearance of multiple infections since 2006 is being considered an epizootic.
Factors
The researchers also examined whether new infections were influenced by water temperature and light intensity. They found high temperatures stressed the coral hosts and increased susceptibility to infections.
But they believe light might be the more important facilitator of disease, with most infections occurring two months before water temperature peaked when light levels were greatest.
The researchers believe the bacteria responsible for the disease are transported by water movement. Fish may also be responsible, eating and then spitting out bacteria as they move from colony to colony.
Humans could also be contributing to the spread of the disease.
"Black band disease is possibly impacted by sewage in the sea. The more nutrients around the disease mat, the faster it grows," says Sato.
"Past studies show that diseased areas are common around where people live but it's not always the case on the Great Barrier Reef as it has been found in remote areas too."