Corals under attack by toxic seaweed do what anyone might when threatened: they call for help.

Results reported this week in the journal Science show that threatened corals send signals to fish “bodyguards” that quickly respond to chomp the destructive algae, which can kill the coral if not removed.

Scientists Danielle Dixson and Mark Hay at the Georgia Institute of Technology found evidence that these fish respond to chemical signals from the coral in a matter of minutes.

The fish are gobies and they spend their lives in the crevices of specific corals, receiving protection from their own predators while removing threats to the corals.

The symbiotic relationship between fish, and the coral on which they live, is the first known example of one species chemically signalling a consumer species to remove competitors.

“This species of coral is recruiting inch-long bodyguards,” said Mark Hay, a biologist at Georgia Tech and co-author of the paper. “There is a careful and nuanced dance of odors that makes all this happen.

“The fish have evolved to cue on the odor released into the water by the coral, and they very quickly take care of the problem.”

The importance of large herbivorous fish to maintaining the health of coral reefs has been known for some time. They control the growth of seaweeds that damage coral.

But Georgia Tech scientist Danielle Dixson, also a co-author of the paper, suspected that the role of the gobies might be more complicated.

To study that relationship, she and Hay set up a series of experiments to observe how the fish would respond when the coral that shelters them was threatened.

They studied Acropora nasuta, a species in a genus of coral important to reef ecosystems because it grows rapidly and provides much of the structure for reefs.

To threaten the coral, they moved filaments of Chlorodesmis fastigiata, a species of seaweed that is particularly chemically toxic to corals, into contact with the coral.

Within a few minutes of the seaweed touching the coral, two species of gobies–Gobidon histrio and Paragobidon enchinocephalus–moved toward the site of contact and began neatly trimming away the offending seaweed.

The research was part of a long-term study of chemical signaling on Fiji Island coral reefs. It was aimed at understanding these threatened ecosystems and discovering chemicals that may be useful as pharmaceuticals.

“These little fish would come out and mow the seaweed off so it didn’t touch the coral,” said Hay.

“This takes place very rapidly, which means it must be very important to both the coral and the fish. The coral releases a chemical and the fish respond right away.”

In corals occupied by the gobies, the amount of offending seaweed declined 30 percent over a three-day period, and the amount of damage to the coral declined by 70 to 80 percent.

Control corals that had no gobies living with them had no change in the amount of toxic seaweed and were badly damaged by it.

The researchers learned that one species – Gobion histrio – actually eats the noxious seaweed, while the other fish apparently bites it off without eating it. In the former, consuming the toxic seaweed makes the fish less attractive to predators.

Further Reading:
Corals Chemically Cue Mutualistic Fishes to Remove Competing Seaweeds, Danielle L. Dixson & Mark E. Hay, Science 9 November 2012: Vol. 338 no. 6108 pp. 804-807. DOI: 10.1126/science.1225748
National Science Foundation