What do a Pacific reef-forming coral and Justin Bieber have in common? They both have bodyguards. But, in the case of the coral, the bodyguards are adorable little fish called gobies. Branching corals such as Acropora nasuta face many threats, including warming waters and ocean acidification; but with the help of the gobies, there is at least one threat they can do something about: seaweed. Seaweed can grow on coral and block out light; some types of seaweed may even wage a chemical war on the coral. Researchers suspected that coral-dwelling fish may be helping to protect their hosts from this nearby competitor—so to test this, the scientists put the coral-harassing seaweed Chlorodesmis fastigiata next to heads of A. nasuta—and they found that corals occupied by the gobies Gobiodon histrio or Paragobiodon echinocephalus stayed healthier. In fact, the corals seem to be signaling to the gobies for help: The gobies didn't respond to injections of a chemical extract from C. fastigiata in water, but when the researchers injected the seaweed extract near the coral, the guard gobies promptly rushed over, apparently looking for the offending seaweed. It's not just guard duty for the gobies; they get something out of it, too. The fish produce a frothy mucus when threatened—exposure to the mucus makes predators lose their equilibrium, tipping forward or sideways—and consuming the seaweed seems to amp up the power of that mucus, the researchers found. When the mucus came from gobies living on a coral adjacent to the nasty seaweed, the predators' dizziness came on much faster.
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