Mutualism of the month: A farming fish

Every month, excusing the long hiatus, we showcase a relationship between two or more species considered a mutualism: a relationship where both members benefit. This month’s mutualism is between a group of damselfish and the algae that they garden.

Predatory grouper prowling the reef. Photo source.

Predatory grouper prowling the reef. Photo source.

In the coral reef, there isn’t much algae (or at least there shouldn’t be). It’s tough being an herbivore in a coral- and rock-dominated community. You have to swim long distances to find your food, risking your life as you pass each coral head, which could be hiding the next predatory grouper. One group of fishes has gotten around this issue by growing its own food. Damselfish aggressively defend territories on the reef from other fish (or divers for that matter). There, they cultivate a locally high concentration of edible algae by weeding out inedible algae.

To accomplish this feat, the damselfish protect the edible algae from other herbivorous fish all while fertilizing the area with their feces and liquid waste. These algae seem to truly benefit from this farming arrangement; one study found that algae tied up outside a damselfish territory will be half gone within an hour, but when placed within a territory, less than 5% will be consumed. In return, the damselfish receive a delicious meal whenever they want.

A damselfish protecting its garden (in the background) from a nosy diver. Photo source.

A damselfish protecting its garden (in the background) from a nosy diver. Photo source.

The trouble is that this behavior may hurt the coral reefs these fish live in. The damselfish physically remove corals as they settle while the cultivated algae compete with corals by overgrowing them and by releasing chemicals into the water. The algae within these communities also have higher incidence of coral pathogens such as black band disease.

Although damselfish may be inadvertently hurting coral reefs and replacing them with algae through their farming, this may not be good news for the farming damselfish as it may become more energetically expensive to farm and defend than to roam and graze. Combined with overfishing, pollution, and climate change, coral reefs are in trouble and it’ll be interesting to see if the relationship between damselfish and their algae change as a consequence as time passes. Maybe some of the gardening damselfish will throw down their pitchforks in search for a nomadic life once again.

To read some original scientific articles about gardening damselfish click here, here, and here. The article may be behind a pay-wall so if you really want to read it, send me an email and I’ll grab it for you.

Don’t forget to check out more mutualisms of the month and all the other great articles on FTDM! Stay hungry!

More Mutualisms of the Month