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Coral reefs are critical habitats for many species lower down on the marine food chain, and thus play a vital role in the health of all marine species. With the loss of over 50% of the Great Barrier coral since 1980, and the loss of 80% of living coral in the Caribbean, scientists have been studying reef interactions in hopes of finding ways to slow down and hopefully reverse this decline.

Recent research in the relatively healthy coral colonies near Fiji have led scientists to discover an interesting example of a symbiotic relationship which illustrates a communication between certain corals and “bodyguard” fish which protect it in return for shelter and food.

In studying the chemical messaging between species in the coral reef environment, scientists hope to develop a pharmaceutical application which may help restore damaged reefs.

The research team led by Georgia Institute of Technology’s Mark Hay discovered that coral polyps known as Acropora nasuta emit chemical signals when a certain toxic species of seaweed encroaches on their territory. Reef-dwelling gobi fish respond quickly to the signal by eating or mowing the seaweed clear of the coral and its algal sheath. The study is published in the journal Science.

Coral reefs are in competition with seaweed for sunlight, and some seaweeds have an advantage in emitting toxins which break down the coral. Researchers placed 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 contacting the coral, two species of gobies — Gobidon histrio and Paragobidon enchinocephalus — moved to the site of contact and began neatly trimming away the offending seaweed.

The small gobi fish have a vested interest in protecting the coral. They live their entire lives on the delicate reefs, using the complex coral structures to hide from predators. The gobi also feed on reef algae which sheath the coral formations.

To determine whether the gobi fish were reacting to the presence of the seaweed or to a chemical signal from the coral, researchers took water samples from next to undamaged corals, corals damaged by algae while the algae was still present, corals damaged by algae after the algae was removed, and the algae alone away from coral. They exposed gobies to these water samples and watched how they responded. Within 15 minutes, gobies were drawn to the water from damaged corals, but didn’t react to the chemical signature of algae (seaweed) by itself. “We found that the gobies were being “called to” the area damaged by the algae, and that the “call” was coming from the damaged coral, not from the seaweed,” said Hay.

The speed of response to the chemical signal by the gobi fish indicates to researchers the importance of this symbiotic relationship. The fish respond within minutes of smelling the chemical cue. Researchers determined that the gobi fish reduced the volume of seaweed by 30 percent over three days, effectively containing the threat and reducing coral damage by about 80 percent.

“We found that the gobies were being “called to” the area damaged by the algae, and that the “call” was coming from the damaged coral, not from the seaweed,” said Hay.

To illustrate the message-based behavior of the gobi fish , researchers obtained the chemical extract of the toxic seaweed and placed it onto nylon “seaweed-like” filaments. 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.”

“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. 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,” says Hay.

Symbiosis occurs in many species in the natural environment. The Yew tree, for example, emits toxic growth inhibitors which ward off competitor species. But, as Hay states, “the symbiotic relationship between the 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 research finding is interesting and may lead to methods which help preserve diminishing coral reefs. But it also serves as a reminder of the undiscovered wonders of the natural world which offer solutions to problems which plague mankind, and why it is critical to preserve species diversity and critical habitats.