The breathtaking reds, yellows and purples of Mesoamerican Reef are getting sick, leading researchers on a desperate quest to understand and combat the mysterious disease that kills Caribbean corals.
In just over a year, the Mexican Caribbean lost more than 30 percent of its corals due to a little known disease called SCTLD, or stony coral tissue loss disease that causes them to calcify and die.
Experts warn that the disease could kill much of Mesoamerican Reef, a magnificent arch of more than 1,000 kilometers of coral shared by Mexico, Belize, Guatemala and Honduras.
The SCTLD has put the region in difficulty: it could potentially devastate the vital reef tourism sector, the second largest in the world after Australia's Great Barrier Reef.
And yet it is highly possible that a lot of tourism is actually fueling the problem, scientists say.
SCTLD, which has also been infecting coral off Florida since 2014, was first detected in the Mexican Caribbean in July last year, near the northern tip of the reef.
Since then it has spread 400 kilometers south, reaching Belize and causing more coral losses than in the last 40 years in the region, according to the Healthy Reefs for Healthy People environmental group.
The disease only takes a few weeks to kill coral tissue that took decades to grow, said Melina Soto, the organization's Mexico coordinator.
"If we continue at this rate, this ecosystem will collapse in the next five to 10 years," she told AFP.
On the verge of extinction
Scientists say the SCTLD is even more dangerous than coral bleaching, another damaging condition that has affected reefs around the world, including the Great Barrier Reef.
Bleaching happens when environmental changes, such as warming ocean temperatures, cause corals – which are colonies of small animals – to expel the microscopic algae known as zooxanthellae that live inside them and give them vibrant colors.
A reef can recover from coral bleaching if its environment recovers over time.
But the SCTLD is fatal.
"It's a complete detachment of coral tissue that dies and leaves behind a white skeleton," said Claudia Padilla, a scientist at CRIP Puerto Morelos, a marine biology research center on Mexico's Yucatan Peninsula.
For untrained eyes, the impact of the disease is not yet very visible.
"They looked wonderful to me. I would never think they were dying, as the experts say," said Emanuel Fernandez, 34, an Argentine chemical engineer, after a snorkel trip off the coast of Cancun, the region's most famous resort.
But the impact is very visible to the experts.
"You used to dive in and see these thriving (coral) colonies. Now they are all dead," Padilla said.
Twenty-five of the region's 40 coral species were affected, she said. Of these, three are on the verge of becoming extinct in the region.
Researchers are currently building a DNA bank of endangered corals, hoping they can one day recover them from extinction in the wild if needed.
Scientists are rushing to understand what causes SCTLD.
One of the main suspects is poor water quality, caused by sewage and a recent wave of decaying sargassum algae – another environmental emergency in the region.
Another likely factor is tourists' sunscreen chemicals, which authorities have already banned.
"Many studies indicate that a particle found in sunscreen, oxybenzone, prevents coral reproduction," said Christopher Gonzalez, regional director of the national parks commission.
This month, authorities temporarily closed three sections of the reef, Palancar, Colombia and El Cielo, which receive thousands of visitors each year.
Now government officials, the tourism industry and residents are forced to strike a delicate balance: a level of tourism that will not kill the reefs or the economy.
About 725,000 tourists have visited the Mexican Caribbean reefs so far this year, similar to previous years, officials said.
"If we lose the reefs, we will lose our main economic activity: tourism," warned Maria del Carmen Garcia, chief of the Coral Reef National Park in Puerto Morelos.