Fish that tend patches of stringy algae seem to shield branching corals from the worst effects of marine heat waves and help them recover after bleaching.
In 2019, the reefs near the French Polynesian island of Moorea in the South Pacific Ocean endured their worst heat stress event in 14 years. Because of some six weeks of unusually warm waters, branching corals there bleached en masse, in which they lose the symbiotic algae living in them that supply most of their food.
In some areas, 鈥渁ll of the colonies that used to be alive and had colour to them were now completely bleached white and on their way to dying鈥�, says , who studied the coral while at the University of California, Santa Barbara.
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Some of Moorea鈥檚 coral colonies were in the 鈥済ardens鈥� defended by territorial farmerfish (Stegastes nigricans), which cultivate their own algae for food and chase off fish that eat plants and corals. So, Honeycutt and her colleagues decided to investigate whether the presence of these fish affected coral bleaching and recovery.
By snorkelling or scuba-diving to more than 1100 coral colonies, the researchers observed that the presence of farmerfish gardens didn鈥檛 influence how severely a colony bleached.
But by continuing to watch nearly 400 colonies following the bleaching event, the researchers discovered that, after one year, just 44 per cent of colonies inside gardens died compared with 67 per cent of those outside gardens. What鈥檚 more, colonies on the turf of the territorial fish were twice as likely to recover living tissue to the levels they had been before bleaching.
鈥淩ecovery is a big deal,鈥� says at the Georgia Institute of Technology in Atlanta. Those recovered corals serve as a resource to reproduce and repopulate the reef, and the farmerfish, which might have been thought to cause problems for coral by tending to competing algae, seem to actually be having a positive effect, he says.
It isn鈥檛 clear how the fish help corals bounce back from bleaching, but the researchers say it could be because fish urinating in the water provide a nitrogen source that boosts the growth of the corals鈥� algal partners. Or, they suggest, that when farmerfish chase off coral-munching fish, the corals have a better chance of relying on an alternative feeding method to using internal algae: extending tentacles into the water to feed on zooplankton to fuel their rebound.
鈥溈烀ǘ淌悠祍 are really trying to understand how climate change is going to drive coral reefs now and going into the future,鈥� says team member at the University of California, Santa Barbara. 鈥淭here鈥檚 a lot of these species interactions that may play really important roles.鈥�
Because farmerfish are found in tropical waters around the world, they could provide a protective effect for corals in many regions, she says.
PLoS One
