The nature of transmission of the most recent deadly coral disease in the Caribbean has been a mystery since it spread beyond its origin on Florida reefs. Newly-published evidence suggests container ships may be to blame.

Stony coral tissue loss disease (SCTLD) first appeared in Florida in 2014. By 2019, the disease had spread across the Florida Keys, destroying reefs along the way. It had also reached other areas in the Caribbean, such as the US Virgin Islands, without touching the islands of the Bahamas, which are physically closer to Florida. It eventually did reach the Bahamas, but not until the end of 2019. Because of this leapfrogging and backtracking nature of transmission, scientists determined that the disease was being transported via ballast water in container ships, according to a studypublished in Frontiers in Marine Science in July.

Additional evidence from the pattern of disease spread shows that reefs closer to ports had higher mortality rates than those farther away, also supporting the ship transmission hypothesis. Once present on an island, the disease spreads rapidly from coral to coral through close contact and water currents.

SCTLD is a severe threat to Caribbean coral reefs, given that it is waterborne, highly transmissible, and affects over 20 species of coral. Its dramatic presentation makes it seem even more alarming, as the live tissue of a coral appears to melt off its skeleton. The more information scientists have about how it spreads increases the chances that managers and activists can stop its devastation.