Seaweed Attacks, Corals Calls For Help
November 9, 2012

Attacked By Toxic Seaweed, Coral Uses Chemicals To Call For Help

April Flowers for — Your Universe Online

When toxic seaweed threatens the health of coral, what does the coral do? It calls for help, of course. A new study, published in Science, reveals that corals under attack send signals to fish "bodyguards." The fish respond quickly, moving in to trim back the toxic alga that can kill the coral if not removed in a timely manner.

A team from the Georgia Institute of Technology has found evidence that these mutualistic fish respond to chemical 911 signals from the coral in a matter of minutes. The gobies, which are an inch long fish, spend their entire lives in the nooks and crannies of a specific coral. Here they find protection from their own predators while removing threats to the coral.

There are over 2,000 species in the family Gobiidae, ranging in size from three-eighths of an inch to one foot in length. Goby fish are found in shallow marine environments, such as seagrass meadows and coral reefs. A few species have also been found in mangrove swamps and freshwater rivers. Gobies are a popular fish for aquarists.

This is the first known example of one species chemically signaling a consumer species to remove the competitors. The relationship between Acacia trees and the mutualistic ants that live in them to receive food and shelter while protecting the tree is similar.

"This species of coral is recruiting inch-long bodyguards," said Mark Hay, a professor in the School of Biology at Georgia Tech. "There is a careful and nuanced dance of the 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."

Supported by the National Science Foundation, the National Institutes of Health and the Teasley Endowment at Georgia Tech, the findings of this study were published in the journal Science as part of a long-term study of chemical signaling on Fiji Island coral reefs. The team hopes to understand these threatened ecosystems and discover useful chemicals for pharmaceuticals.

The importance of large herbivorous fish in controlling seaweed growth on coral has been known from some time; however Georgia Tech postdoctoral fellow Danielle Dixson suspected that the role of the smaller gobies might be more complicated. She and Hay set up a series of experiments to see how the fish would respond to threats.

Using Acropora nasuta, which is a species of coral important to reef ecosystems because it grows quickly and provides a great deal of the structure of the reef, the team threatened the coral with filaments of Chlorodesmis fastigiata. C. fastigiata is a species of seaweed that is particularly toxic to corals. Dixon and Hay brought C. fastigiata into contact with A. nasuta and within minutes two species of gobies - Gobidon histrio and Paragobidon enchinocephalus — had moved in and started trimming the filaments away from contact with the coral.

"These little fish would come out and mow the seaweed off so it didn't touch the coral," said Hay, who holds the Harry and Linda Teasley Chair in Environmental Biology at Georgia Tech. "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."

Over a three-day period, the offending seaweed declined over 30 percent in corals occupied by gobies. The damage to the coral declined between 70 and 80 percent. Corals used as controls — which had no gobies living within — had no change in toxic seaweed and were badly damaged.

Dixson and Hay collected water samples from three locations to determine what was attracting the fish: near the seaweed itself, where the seaweed was in contact with the coral, and from coral that had been in contact with seaweed 20 minutes after the seaweed had been removed. They released the samples near other coral. The gobies were attracted to the samples taken from seaweed-coral contact areas and the damaged coral, but not from the seaweed by itself.

"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."

The team collected similar samples from a different species of coral, but those did not attract the fish. This suggests that the gobies are only interested in protecting their host coral.

As a final step, the team created pseudo-seaweed with nylon filaments designed to simulate the mechanical effects of the seaweed. Some of them were treated with a chemical extract of the toxic seaweed and some not — the gobies were attracted to the treated filaments when they came into contact with the coral, but not the untreated ones.

One species of goby — Gobidon histrio — actually eats the seaweed, the researchers found, while the other species bites it off without ingesting it. For G. histrio, consuming the toxic seaweed provides further protection from predators.

Both species also eat mucus from the coral, algea from the coral base, and zooplankton from the water column.

"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."

Dixson and Hays would like to expand their research to look at other fish/coral relationships. This will allow them to understand more about how the chemical signaling and symbiotic relationship came into being.

"These kinds of positive interactions needs to be better understood because they tell us something about the pressures that have gone on through time on these corals," said Hay. "If they have evolved to signal these gobies when a competitor shows up, then competition has been important throughout evolutionary time."

"These findings illustrate the complexity of coral reef systems," said David Garrison, program director in the National Science Foundation's Division of Ocean Sciences, which funded the research. "This newly discovered relationship may be a key factor in the resilience of some coral species."