Just below the ocean's surface, coral reefs are under constant assault by seaweeds that seek to take control by stealing the corals' prime sunlit location for themselves. Many of these plant invaders come equipped with deadly chemical weapons that knock down the corals' metabolism, which might come off as an unfair fight against a seemingly unarmed foe. But corals are not defenseless: as a recent paper in Science shows, they have fish bodyguards at the ready to mount a defense.

Study co-author Mark Hay, a biology professor at the Georgia Institute of Technology, and his postdoctoral student Danielle Dixson were studying coral-seaweed interactions in Fiji. The scientists noted that when they introduced the toxic seaweed Chlorodesmis fastigiata to the reef-building coral Acropora nasuta, small gobies would emerge within seconds from their hiding places to pick at and eat the seaweed.

To really understand what was going on, the scientists took a variety of water samples and exposed the fish to them in the lab. They found that gobies were drawn only to water from corals that had been damaged by seaweed but not to the chemical signature of an alga by itself. “We found that the gobies were being ‘called to’ the area damaged by the algae and that the ‘call’ was coming from the damaged coral, not from the seaweed,” Hay says. The gobies are not being entirely selfless. Gobies don't just eat seaweed—they also eat mucus from the coral itself. “The fish are getting a safe place to live and food from the coral,” Hay notes. “The coral gets a bodyguard in exchange for a small amount of food. It's kind of like paying taxes in exchange for police protection.”

Adapted from Science Sushi at blogs.ScientificAmerican.com/science-sushi

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