The damselfish, Stegastes nigricans, cultivates a preferred and specific algae species by defending its algae patch against other herbivorous fish, and by actively weeding out undesired species.
The preferred algae from the genus Polysiphonia – a type of red algae – can only survive in algal patches that are cultivated by damselfish.
Polysiphonia cannot compete with other un-palatable algal species and would also be subject to over-grazing without the protection of the damselfish, this suggests that both species are interdependent upon one another and thus need each other in order to survive - a phenomenon known as obligate mutualism.
S. Nigricans digests the filamentous algae through the use of a very acidic stomach, linked to a long intestine and slow gut turnover rate.
S. nigricans farms its algae intensively, not only does it defend the patch against invaders, but it actively weeds out undesired algae.
This is in contrast to some other damselfish species that defend larger patches of mixed algae species only by excluding invaders, with no weeding.
Hiroki Hata from Ehime University, Japan, explored this 'gardening' behaviour with a team of researchers; he says: "We surveyed 320 territories of 18 damselfish species and thoroughly examined algae from each fish territory from coral reefs in Egypt, Kenya, Mauritius, the Maldives, Thailand, Borneo, the Okinawa Islands, and the Great Barrier Reef. We found that although the crop alga species shifted in the West Indian Ocean, the intensive farming by damselfish was seen throughout this geographic range".
The findings are thought to be one of the first examples of mutualism in a non-terrestrial environment; many of us are already familiar with the cultivation of fungi and aphids by termites and ants, for example.
The author believes that the findings can be used ".....as a model system through which we can approach the origin, establishment, and coevolutionary processes of the cultivation system.
For more information see the open-access paper: Geographic variation in the damselfish-red alga cultivation mutualism in the Indo-West Pacific. Hiroki Hata, Katsutoshi Watanabe and Makoto Kato
BMC Evolutionary Biology (in press)