How does Caribbean fire coral thrive as others vanish?
Unusual ability to grow in two forms gives these species an edge
Fire corals can be the bane of a scuba diver’s existence. An accidental brush against one can cause agonizing pain. But they also may help save Caribbean reefs, which have been plagued by hurricanes, global warming, disease, and an overabundance of algae. A long-term study has revealed that fire corals ( Millepora ) are thriving there even as other corals disappear and could help preserve some of the 3D environment that helps make reefs such great homes to fish and other organisms.
Fire coral “are going to be very important habitat providers because they are able to survive under these stresses,” says Colleen Bove, a marine ecologist at Boston University who was not involved with the work.
Thirty years ago, Peter Edmunds began doing annual surveys of underwater life off St. John, one of the U.S. Virgin Islands. The marine biologist from California State University, Northridge, marked out a 20-meter transect along an underwater reef. Each summer he has photographed what grew there, including an expanded transect of 40 meters.
By analyzing the abundance of each organism in these “photoquadrats,” Edmunds has traced how algae and various corals have fared through hurricanes, warming sea temperatures, and other environmental stresses. “What he’s done is really remarkable,” says Caroline Dubé, a marine biologist at Laval University who studies plasticity in Pacific fire coral. “There are so many disturbances occurring in coral reefs that this is something that needs to be done more.”
Fire corals resemble typical stony corals but are actually close relatives of jellyfish; hence their wicked sting. They have the ability to grow either as sheets—expanding as a flat coating across rocks and other surfaces—or as “trees,” sprouting upward with a stem and branches. More than 40 years ago, Jeremy Jackson, an ocean biologist with the American Museum of Natural History and the Smithsonian Tropical Research Institute, proposed that this plasticity would give fire corals an edge as Caribbean reefs experienced global warming and hurricanes. Edmunds now concludes Jackson was right.
Overall, Edmunds’s long-term data document that numerous kinds of multicellular algae called macroalgae have overrun Caribbean reefs. But if hurricanes or other factors destroy the macroalgae, fire corals quickly move in and encrust surfaces, Edmunds reports today in the Proceedings of the Royal Society B . When the reef gets crowded, fire coral sprouts into its branching tree form, so it can continue to thrive in tight spaces and provide an upright structure that other organisms nibble on, live in, or otherwise use.
Periodically, unusually warm water causes the corals to lose their green algal partners and die, opening the way for macroalgae to move in again. Hurricanes also blow off the tree form’s branches. But the fire coral quickly comes back in one form or another, Edmunds found. Thus, this coral has been able to hold steady and even increase a little in abundance.
“Their stony coral buddies don’t do a great job with producing sheets and trees,” Edmunds says. “So, in a world with frequent storms and stiff competition for space on the bottom, fire corals are poised to inherit the shallow reefs.”
Jackson is pleased: “Edmunds’s remarkable persistence allowed him to witness the ups and downs of fire coral dynamics.” Unfortunately, Edmunds’s data also show other corals are getting rarer and rarer. “ Millepora might replace them as they decline due to marine heat waves and coral bleaching,” Jackson says.
That may still be bad news for reefs overall. Nikolaos Schizas, a marine scientist at the University of Puerto Rico, Mayagüez, cautions that fire corals may not save reefs because they don’t typically form reefs that are meters high and wide. “We need to be realistic on the magnitude of that potential,” Schizas says.
Edmunds’s data also reveal the fire corals were repeatedly knocked down by hurricanes and other disturbances, points out Terry Hughes, a marine scientist at the Australian Research Council Centre of Excellence for Coral Reef Studies. “But the study strongly suggests that they will fare better than the majority of corals.”