The fish that farms alga

Damselfish are one of the most abundant fish on coral reefs across the world, from the Caribbean to Indonesia. Some species forage on tiny crustaceans but many stake out permanent gardens among the coral to cultivate algae. “They weed out the stuff they don’t want by taking a bite of it and swimming to the edge of the territory to spit it out,” says Jordan Casey, a PhD candidate at James Cook University in Queensland, Australia, who studies damselfish on the Great Barrier Reef. They also protect their algal gardens from would-be thieves. “They’re tiny fish but very aggressive,” says Casey. “They’ll attack humans.”

Exactly which type of algae damselfish prefer varies with species and location. In Mauritius, for example, the damselfish Stegastes nigricans grows a mixed bed of alga. But in the Okinawa islands off Japan, the Red Sea, and the Great Barrier Reef, S. nigricans maintains a monoculture of red algae from a family called Polysiphonia.

It seems the damselfish turned to farming red algae because they lack the enzymes needed to digest many common algal species. Polysiphonia is easy to digest. It’s also less robust than the inedible algal species it usually competes with, meaning it needs protection. So the relationship works for both parties: the damselfish gets a steady supply of food and the alga avoids competition.

What works for the damselfish might not be so good for the reef. Last year, Casey and her colleagues found that bacteria associated with black band disease are more abundant inside damselfish territories on the Great Barrier Reef than outside of their territories.

This suggests damselfish behaviour may be contributing to an increase in the disease, though Casey points out that rising temperatures have a much bigger impact.


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