Unwanted sea urchins and starfish are ejected from the farms, and unpalatable algae are meticulously weeded out to promote lush turfs of the preferred species.
(See related photos of colorful coral reefs.)
In a study that appeared last October in the journal Biology Letters, Hata and colleagues described what happens when the damselfish are removed from their plots. The team built cages to set around the actual gardens and thus control what could get in.
If all herbivores are kept out—including the damselfish—within a week the Polysiphonia gardens become completely overrun by other species of algae.
"When only the damselfish are removed," Hata said, "it takes just a couple of days for other grazing fish to move in and obliterate all the algae growing inside the gardens."
Spreading the Seeds
Nancy Knowlton is a marine biologist at the Scripps Institution of Oceanography in La Jolla, California.
Knowlton, Hata, and colleagues plan to study similar farming damselfish on the reefs of Panama later this year.
One of the questions the team wants to examine, Knowlton said, is how the fish developed such a specialized relationship with the algae.
Many kinds of farming animals practice vertical inheritance, in which they transfer some of their crops from established gardens to set up new colonies.
But dusky farmerfish instead seem to establish new territories when waterborne spores or fragments of algae drift to new parts of the reef from nearby gardens. The fledgling plots can then flourish under the care of new damselfish.
"There is no evidence that tiny larval damselfish would be able to transfer algae from their parental gardens to their own new territories," Knowlton said.
"Without vertical inheritance it's unclear how the specific fish-algae relationships could come about."
Meanwhile, Hata said, existing damselfish gardens face an uncertain future as global warming continues to negatively impact coral reefs.
Although it is difficult to predict, he noted, increased sea temperatures could eventually lead to a breakdown of the relationship between fish and algae.
(Related: "Soft Corals 'Melting' Due to Warming Seas, Expert Says" [July 13, 2007].)
"The damselfish depend on new coral skeletons to establish their gardens on," he said.
"Without new coral growth, older skeletons will erode away and eventually could leave the damselfish with nowhere to cultivate algae."
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