By Mera McGrew
Similar to people in danger, corals under attack do what anyone would do—they call for help. A new study reported in the journal Science, shows that corals under attack by toxic seaweed send chemical distress signals to “emergency responder” fish, which respond within a matter of minutes to what is effectively a chemical 9-1-1 call.
Large herbivorous reef fish have long been known to control the growth of seaweeds that damage coral and play a major role in maintaining a healthy coral reef ecosystem. However, the role that smaller herbivorous fish play in maintaining thriving reef ecosystems are much less known and generally less understood.
Scientists at the Georgia Institute of Technology suspected that the role of inch-long fish, known as gobies, might be more complicated yet equally, if not more, critical to the health of coral reefs.
Gobies have a mutually beneficial or symbiotic relationship with the coral on which they live. Gobies spend their entire lives in the crevices of specific corals receiving protection from predators while removing threats to their host corals.
To study the complexities of the relationship between gobies and their host coral, Georgia Tech Professor Mark Hay and postdoctoral fellow Danielle Dixon set up a series of experiments to observe how the small fish would respond when the corals that shelter them were threatened.
They studied a coral species, Acropora nasuta, which is quick growing and makes up large portions of the structure on many reefs. To induce a threatened response in the coral, the researches moved filaments of a noxious seaweed species, Chlorodesmis fastigiata, into contact with the coral. Within a few minutes of the seaweed contacting the coral, they found that two species of gobies, Gobidon histrio and Paragobiodon enchinocephalus, would move toward the site of contact and begin neatly trimming away the offending seaweed.
“These little fish would come out and mow the seaweed off so it didn’t touch the coral,” said Hay. “The coral releases a chemical and the fish respond right away.”
To determine what was actually attracting the fish, Dixon and Hay collected water samples near the toxic seaweed, next to the contact site between the seaweed and coral and from beside coral that had been in contact with seaweed previously.
“We demonstrated that the coral is emitting some signal or cue that attracts the fish to remove the encroaching seaweed,” Hay stated. “The fish are not responding to the seaweed itself.”
To complete the study, Dixon and Hay also tested the mechanical effects of the seaweed to ensure that the fish were responding to the chemical compounds. In addition, they looked at the contents of the gobies’ digestive tract to see if the fish were actually consuming the toxic seaweed.
In the end, Hay explains, “the fish are getting protection in a safe place to live and food from the coral. 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.”
Research Source: Danielle L. Dixson and Mark E. Hay, Corals chemically signal mutualistic fishes to remove competing seaweeds, Science (2012).