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Gardening Fish "Domesticate" Crops of Algae

Helen Scales
for National Geographic News
July 23, 2007
 
Damselfish can appear quite contrary to species that wander into their gardens of algae by aggressively chasing off larger fish and even nipping at human divers.

But for some damselfish species, protecting their "crop" is a matter of survival for both the fish and the algae, according to recent research.

The dusky farmerfish has developed a co-dependent relationship with a species of the red algae Polysiphonia.

Both creatures are found on coral reefs in the Ryukyu archipelago, a scattering of islands that stretches between southern Japan and Taiwan (map of Asia).

"Not only do the fish rely on the algae as a source of food, but the algae only survive well if they are farmed," said Hiroki Hata, a marine biologist from Kyoto University in Japan.

"We saw dusky farmerfish feeding exclusively inside their farms, which are dominated by a single type of algae that we called 'Polysiphonia species one,'" Hata said.

Scouring the reefs also revealed that Polysiphonia species one grows only inside the gardens of dusky farmerfish.

In total, Hata and colleagues identified four new Polysiphonia species that have adapted to rely on particular types of damselfish.

"Life inside the damselfish gardens is so good for the algae that they seem to have come to depend on being farmed," Hata said.

Guarding and Weeding

Damselfish are among the handful of animals—including humans, ants, and salt-marsh snails—that are known to cultivate beneficial crops.

Although they are relatively tiny—on average about 6 inches (15 centimeters) long—damselfish seem to tend their gardens with zeal.

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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