"We demonstrated that the coral is emitting some signal or cue that attracts the fish to remove the encroaching seaweed," Hay said. "The fish are not responding to the seaweed itself."
Similar waters collected from a different species of coral placed in contact with the seaweed did not attract the fish, suggesting they were only interested in removing seaweed from their host coral.
Finally, the researchers obtained the chemical extract of the toxic seaweed and placed it onto nylon filaments designed to stimulate the mechanical effects of seaweed. They also created simulated seaweed samples without the toxic extract. When placed in contact with the coral, the fish were attracted to areas in which the chemical-containing mimic contacted the coral, but not to the area contacting the mimic without the chemical.
By studying the contents of the fish digestive systems, the researchers learned that one species Gobidon 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.
The two species of fish also eat mucus from the coral, as well as algae from the coral base and zooplankton from the water column. By defending the corals, the gobies are thus defending the home in which they shelter and feed.
"The fish are getting protection in a safe place to live and food from the coral," Hay noted. "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."
As a next step, Hay and Dixson would like to determine if other species of coral and fish have similar symbiotic relationships. And they'd like to understand more about how the chemical signaling and symbiotic relationship came into being.
|Contact: John Toon|
Georgia Institute of Technology Research News