Fierce warriors, algal farmers

With scales coloured sparkling neon blue to eye-catching bright yellow, and patterns of polka dots to dazzling stripes, the spectacular damselfish are fish world fashionistas. Unsurprisingly, these coral reef inhabitants are prime attractions in many saltwater aquaria around the world. But there’s more to these fish than their beauty: they are the only known farmers in the aquatic world and they farm algae. These fish also fiercely protect their farms—they aggressively chase away other herbivorous fish and sea urchins that intrude on their territories to feed.

Many species of damselfish, like the dusky farmer fish (Stegastes nigricans) found in the reef flats and lagoon reefs of the tropics, make tiny patches at the bottom of the reef to grow their algal farm. They grow specific species of algae—the red algae Polysiphonia in the case of dusky farmer fish—in these yards by killing the coral. Since these fish do not have teeth to chew or enzymes to digest fibrous species of algae, they are choosy about what they grow. They maintain their monoculture crop of specific algae by regularly weeding out other species of algae using their tiny mouths—they pick out the algal bits and dump them away from the farms.

Longfin damselfish, found in the reefs of Central America, go the extra mile in their farming skills: they not only farm algae like other species but also provide refuge to tiny crustaceans called mysid shrimps above their algal farms. Although the fish do not eat the shrimp, scientists recently found that shrimp poop acts as a fertiliser to the growing algae, making them more nutritious and perhaps tastier for the fish. Damselfish surely have a green thumb, don’t they?

- Spoorthy Raman