Fish as Farmers: Reef residents tend an algal crop

A damselfish cultivates underwater gardens of an algal species that researchers haven’t found growing on its own.

The special alga could be the fishy version of people’s domesticated crops, says Hiroki Hata of Kyoto University in Japan. Growth tests of the alga, surveys of its distribution, and genetic analyses support that idea, he and Makoto Kato say in an upcoming Biology Letters.

People have been slow to get the hang of farming. Starting millions of years before the rise of human agriculture, certain ants, termites, and ambrosia beetles grew fungi for food. Today, they sow, fertilize, and weed their crops. A few of these spineless cultivators even employ bacteria to make pesticides.

In simpler systems, sometimes referred to as protofarming, mollusks called limpets and certain damselfish graze in territories of edible algae. Hata and Kato have been analyzing the Stegastes nigricans damselfish’s patches of a Polysiphonia, which is categorized as a red alga. The fish defends what looks like a piece of “brown carpet on the reefs,” says Hata. The fish nips out bits of other algae and swims outside its territory to spit them out.

Hata and Kato began to suspect that the brown carpet might not persist untended. For example, when they kidnapped the resident damselfish, other fish and sea urchins ate up the alga within days. When the researchers caged farms to keep out both the farmer and interlopers, algae of other species quickly overwhelmed the brown carpet.

To test the farmer-alga bond, Hata and Kato recently collected various algae both inside and outside damselfish territories in the Ryukyu Islands of Japan. The scientists next distinguished four Polysiphonia species by analyzing a segment of each one’s DNA. One of the species, the brown carpet, turned up only in S. nigricans territories, Hata and Kato report. The other algal species were lacking in those territories but appeared both in territories of other damselfish and outside those boundaries.

A specialist in classifying red algae, Gary Saunders of the University of New Brunswick in Fredericton, comments that many animals consume these species, “but for the alga to be dependent on the animal—I just can’t think of another case.”

He finds it plausible that the alga depends on fish farmers on the reefs that Hata and Kato examined, but he cautions that in other regions, the brown-carpet species might survive independently. “It’s a big ocean,” he says.

Ulrich Mueller of the University of Texas at Austin, who compares farming species, notes that the algal crop restricts damselfish farms to sunny spots, a limitation that it shares with the crops raised by people. In contrast, fungus raised by insects can grow in dark, protected chambers that reduce exposure to pests.

“It is likely that many more protofarming systems will be discovered in other animal lineages,” Mueller predicts.