A drab brown fish that runs a farm and will fight a diver over it.
The patch of long, untidy algal turf on a reef flat is not neglect - it is cultivated. This damselfish weeds it: it bites out the algal species it does not want and spits them over the boundary, and what is left is a near-monoculture of one or two soft red algae that grow nowhere else on the reef because nothing else lets them. Remove the fish and the crop is grazed away within days by everything that has been waiting.
Defending it is a full-time job. The fish drives off parrotfish and surgeonfish many times its size, chases urchins out, and will charge and bite a diver who kneels in the wrong place. Fourteen centimeters (5.5 in) of very serious intent.
The farms also shelter other animals - small shrimps, gobies and worms live inside them under the fish's protection, which makes each one a little island of a different community on the reef. It is one of the clearest cases of an animal engineering its own habitat.
The species. Stegastes nigricans (Lacepède 1802) - blackish. ETYFish
- dusky brown-gray body, darker toward the tail, with a pale-edged dark spot at the rear of the dorsal fin
- always over a patch of long algal turf that stands out from the reef around it
- attacks anything entering the patch, including divers
- length up to 14 cm
Why it's threatened
There are no known major threats.
Threat classification from the IUCN Red List.
