Reef fish cultivates algal crop

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Scientists have discovered a fish species that cultivates a crop of a specific algal species by weeding-out algae of other species.

The discovery is believed to be the first example of obligate plant-herbivore cultivation mutualism in the marine environment.

The damselfish, Stegastes nigricans, manages algal farms by defending its patch against invading grazers and by weeding-out unpalatable algae of other species so that the target crop - a species of Polysiphonia - grows more readily.

Hata and Kato of Kyoto University, who reported their findings in the journal Biology Letters, studied algal assemblages inside and outside the territories of five damselfish species found off Japan's Ryukyu Islands using both morphological and molecular techniques.

One algal species, named Polysiphonia sp. "1", grew only inside the territories of Stegastes nigricans and was never found in other areas.

The authors wrote: "Since only Polysiphonia sp. 1 is harvested and consumed by the damselfish as a staple food, this interdependent relationship is an obligate cultivation mutualism.

"This is the first record of an obligate plant-herbivore cultivation mutualism in a marine ecosystem. Our data also suggest that three other Polysiphonia species are facultatively mutual with, commensal with, or parasitic on other damselfish species."

The 14cm/5" Stegastes nigricans, which is known as the Dusky farmerfish, is normally found living in the dead branches of Acropora corals, where it cultivates its algal crop.

It defends its territory so vigorously against other species that it can be heard producing audible clicks and has even been known to bite divers.

The species occurs across much of the Indo-Pacific, excluding the Hawaiian Islands, and has also been reported from the Red Sea. You can view its distribution on Fish Mapper.

For more details see the paper: Hata H, Kato M (2006) - A novel obligate cultivation mutualism between damselfish and Polysiphonia algae. Biol Lett. 2006 Dec 22;2(4):593-6.