In the waters surrounding the Fiji Islands, the coral reef has vigilant defenders. Researchers have discovered that when alerted by chemical signals transmitted by corals, two resident species of the goby fish will swing into action and limit a growth of seaweed that contributes to the bleaching of precious reefs.
The gobies, inch-long gemlike creatures, report to the affected areas of the corals and nibble the aberrant seaweed back into place, making it look ?like somebody went out there with little hedge trimmers,? said Mark Hay, a marine ecologist at Georgia Tech.
For their study, published in the latest issue of the journal Science, Dr. Hay and his co-author, Danielle Dixson, a postdoctoral fellow in the biology department at Georgia Tech, set out to investigate interactions between seaweed and reefs as well as the chemical output of corals.
While a coral?s chemical production is relatively well documented, as is the role that herbivorous grazers play in the upkeep of a reef, the use of chemicals to recruit fish to deal with a threat proved a revelation.
This ?really specialized chemical communication? is a unique finding, said Nancy Knowlton, a coral reef biologist and the Sant Chair of Marine Sciences at the Smithsonian Museum of Natural History.
In effect, the two goby fish species, Gobiodon histrio and Paragobiodon enchinocephalus, are acting to protect their home base.
Known as the broad-barred and redhead gobies, they are also popular aquarium pets because of their vivid coloring and stripes. While these species are not necessarily directly threatened by being collected for sale as pets, the service they perform for corals may raise questions about the wisdom of the aquarium trade, the researchers suggest. ?The primary concern would be that overharvesting of the fish would lead to decline in coral abundance,? Dr. Knowlton said.
Coral bleaching is also closely associated with global warming and with the proliferation of seaweed as a result of overfishing in general.
To demonstrate the goby?s role, Dr. Hay and Dr. Dixson studied the small staghorn coral, a host for the fish. Although the coral is a common fast-growing species, it is nonetheless threatened by a seaweed called Chlorodesmis fastigiata, or turtleweed. This lurid green hairlike plant is damaging to many corals because of an oily substance on its surface that reacts chemically when it brushes against coral, causing intensive bleaching within two to three days.
Dr. Hay and Dr. Dixson set out to test the response of the fish. ?We took the worst-case scenario and saw if these fish could defend the coral against it,? said Hay.
Donning scuba gear, the researchers collected seawater samples next to corals that usually hosted gobies and had been brushed with the toxic weed. They then released samples that were laden with the coral?s chemical signals near corals that were occupied by the fish.
In the same experiment with resident damselfish, the damsels abandoned the reef. ?The neighborhood?s going downhill, we?re moving,? Dr. Hay said, describing their response. But when it came to gobies, they ?came out and trimmed the seaweed back just enough? that it no longer slathered the coral with its chemical mix, he said.
The redhead goby seems just to trim the plant, Dr. Hay said, while the broad-barred fish actually devours the seaweed, in that way increasing its own toxicity and warding off predators.
The researchers still do not know what chemicals the corals release to activate the fish. ?We know they are responding to it via smell,? Dr. Hay said, ?but we don?t know what the compound is at this stage.?
Either way, on corals without the protective gobies, there is a reduction of photosynthetic ability of roughly 80 percent. With the fish on duty, that suppression drops to around 20 percent.
The fish play ?a very critical structural role in the maintenance of the reef? ? fulfilling a purpose, Dr. Knowlton suggested, that may make them better suited to living with the ocean?s corals than dwelling inside a home aquarium.
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