OSLO, March 24 (Reuters) - Climate change is likely to make reef-building stony corals lose out to softer cousins in a damaging shift for many types of fish that use reefs as hideaways and nurseries for their young, a study showed.
Soft corals such as mushroom-shaped yellow leather coral, which lack a hard outer skeleton, were far more abundant than hard corals off Iwotorishima, an island off south Japan where volcanic vents make the waters slightly acidic, it said.
A build-up of carbon dioxide in the atmosphere is turning the oceans more acidic in a process likely to hamper the ability of creatures such as lobsters, crabs, mussels or stony corals to build protective outer layers.
"Soft coral has the potential to be a winner in coral reefs," lead author Shihori Inoue of the University of Tokyo told Reuters by e-mail of the findings, the first study of likely winners and losers among soft and hard corals.
"Reef communities may shift from reef-building hard corals to non-reef-building soft corals under (carbon dioxide levels) predicted by the end of this century," the authors wrote in Monday's edition of the journal Nature Climate Change.
When it reacts with water, carbon dioxide in the atmosphere can form carbonic acid. That damages hard corals, tiny animals that secrete calcium carbonate to form their stony protective layer.
"When combined with their ability for rapid colonisation, soft corals may out compete hard corals in coral reef environments subject to ocean acidification," the scientists wrote.
"Hard corals are important reef builders and provide complex three-dimensional habitats for many reef organisms," they wrote, adding that a shift could have damaging effects for many fish and other marine creatures that live around reefs.
A community of soft coral "hardly works like hard coral as a nest for small living organism such as ... shrimp, and little fish," Inoue said. The 2003 animated movie "Saving Nemo" shows how clownfish live and grow around reefs.
Covering less than one percent of the ocean floor, reefs support about 25 percent of all marine life, with over 4,000 species of fish alone, according to the International Coral Reef Initiative which seeks to protect reefs.
The scientists said that the levels of carbon dioxide in the water off Iwotorishima, an island near Okinawa in the Pacific, corresponded varied from equivalent to 550 parts per million (ppm) of carbon dioxide in the atmosphere to as high as 970. Both levels are within the range seen as possible by 2100 according to the U.N. panel of climate scientists.
Current atmospheric levels are about 395 ppm, a sharp rise from 280 ppm before the Industrial Revolution. And emissions of carbon dioxide are continuing to rise - China, the United States and the European Union are the top emitters.
Other studies have also shown that stony coral reefs are vulnerable to rises in temperature that can cause a whitening that can lead to death.
Gas vents in the Mediterranean and corals around a carbon dioxide seep off Papua New Guinea have also been used to study the likely future conditions of the oceans.
"Soft coral may not always be the winner in acidified waters. In temperate oceans, it has been shown that algae or sea anemones can be winners at high-carbon dioxide vents," they wrote. (Reporting by Alister Doyle; Editing by Jason webb)