Blackback Butterflyfish

Find this fish asleep at night and it is wearing different clothes. The back that was white by day has gone black, broken only by two white patches, and the change also comes on in daylight if the fish is badly frightened. Divers who have photographed the same individual an hour apart have come back with what looks like two species.

By day it is white with fine dark diagonal lines, a yellow snout and yellow dorsal and anal fins, a black bar through the eye and a black saddle across the base of the tail. The saddle is the mark to look for, and it holds up in a poor photograph.

What it eats was worked out on the reef flat at Sharm El-Sheikh, and the answer is unusual. Nearly all of its bites, over ninety five percent, went into soft corals rather than stony ones, mostly Sinularia and Litophyton, with a clear preference for the pulsing Heteroxenia and a clear refusal of Sarcophyton and Lobophytum despite both being right there. It fed at about forty bites every five minutes and held a territory averaging roughly five hundred square meters.

Soft corals are chemically defended, which is why most fish leave them alone, and this one has evidently solved that problem. It also means the fish is not chained to the same food as the strict stony-coral feeders on the same reef. When a bleaching event kills the Acropora and those species disappear, this one has something left to eat. Soft corals are not immune to heat either, so this is an advantage rather than a rescue.

Adults are usually alone or in a pair, in the top 20 m, over patches of soft coral and among staghorn thickets. Juveniles stay inshore.