Hawaii’s coral reefs have got a problem on their hands.

A new aggressive species of algae has been spotted growing on the coral reefs of Papahānaumokuākea Marine National Monument, the US National Monument encompassing 1.51 million square kilometers (583,000 square miles) of ocean water that's home to many rare species of coral, fish, bird, reptile, and marine mammals. 

Scientists have never seen anything like it here before and they remain unsure where the algae came from. Nevertheless, already this red-colored seaweed has taken over whole coral reefs in the area since it was first discovered back in 2016. 

“This is a highly destructive seaweed with the potential to overgrow entire reefs,” Heather Spaldin, study author and assistant professor of biology and the College of Charleston, said in a statement. “We need to figure out where it’s currently found, and what we can do to manage it. This type of research needs trained divers in the water as quickly as possible. The sooner we can get back to Papahānaumokuākea Marine National Monument, the better.”

Reported in the journal PLOS One this week, marine biologists from the University of Hawaii at Manoa, College of Charleston, and the NOAA describe the newly-named Chondria tumulosa algae. A series of dives around Pearl and Hermes Atoll revealed the algae was most prevalent at a depth of around 10 to 15 meters (~33 to 50 feet) where it formed mats up to 18 centimeters (7 inches) thick that smothered the reef.

A multi-pronged analysis showed that the aggressive seaweed is not closely related to any known Hawaiian native species, which generally live in harmony with the coral reefs. However, it remains unclear whether it is an invasive species brought into the area from elsewhere.

If an invasive seaweed covers a coral, it blocks out the sunlight and means the single-celled zooxanthellae that live within their cells and provide them with food are not able to photosynthesis. As a result, the starved corals become smothered and die. It can also have a much wider knock-on effect throughout the marine ecosystem as reefs are home to a range of other marine creatures.

Once an invasive species such as algae takes hold of a marine ecosystem, there is no straight forward way to reverse the change. So, for now, the researchers are on tenterhooks to assure they do everything they can to prevent the species from spreading to other marine areas. 

“Until we understand whether it is native or introduced, and until we better understand what is driving this outbreak, it is critically important that research divers and research ships do not inadvertently transport this species to other islands,” Randall Kosaki, NOAA research coordinator at Papahānaumokuākea Marine National Monument

He added, “All of our dive gear was soaked in bleach, and all of our dive boats were sprayed down with bleach prior to returning to Honolulu.”