The mutualistic fish Gobidon histrio is shown on the coral Acropora nausuta in the Fiji Islands. The coral is in contact with the green alga Chlorodesmis fastigiata, which is harmful to coral. (Photo : Georgia Tech/Danielle Dixson)

A kind of friendship under seawater helps in survival. When corals face threat from seaweed, they immediately call for help, reveals a new study.

Researchers from Georgia Institute of Technology have found that corals send chemical signals to "mutualistic" fish that act as bodyguards and mow the seaweeds that could kill the corals, if they are not removed.

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

"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," he said.

The "bodyguard fish" known as gobies live in the crevices of specific corals. They share a symbiotic relationship with the corals, where they protect themselves from predators by living with the same coral and also give protection to the coral from seaweed attack.

Researchers observed the corals, known as Acropora nasuta, growing in Fiji. The corals face threat from seaweed species known as Chlorodesmis fastigiata, as they are chemically toxic. In order to threaten the coral reefs, researchers moved the Chlorodesmis to make them come in contact with the coral.

In few minutes, two species of gobies - Gobidon histrio and Paragobidon enchinocephalus - moved towards the coral and trimmed away the seaweed approaching the coral.

Researchers noticed that there was 70 to 80 percent decline in the damage caused to the corals. However, they found that corals that did not have gobies living with them were damaged when exposed to seaweed.

To find out what attracts the fish towards the coral, experts collected water samples from three locations - near the seaweed, where the coral and seaweed came in contact and from the damaged coral - within 20 minutes after the seaweed was moved.

When they released the water near corals hosting the gobies, they found that the fish were attracted to the water samples collected from the damaged coral and the spot where the coral and seaweed came in contact. But the gobies were not attracted to water samples taken from the seaweed itself.

Similarly, water samples collected from other species of coral also did not attract the gobies.

Researchers obtained the toxic chemical from seaweed and placed on nylon filaments to have the effects of seaweed. They also created seaweed samples without the chemical extract. When the samples were placed near the fish, the gobies were attracted to nylon filaments with chemical extract and not to seaweed samples without toxic chemical.

Experts conducted further studies on the fish's digestive system, and found that one of the fish species (Gobidon histrio) actually consumes the seaweed, which makes it less attractive to predators. The other fish species just bites off the seaweed, but does not eat it. By doing so, the gobies are able to protect the corals that offer them food and shelter.

The findings of the study, "Corals Chemically Cue Mutualistic Fishes to Remove Competing Seaweeds," are published in the journal Science.