Yellow-side Damselfish

The pale fish hanging over the coral in half the photographs taken in 🇪🇬 Egypt lives in this sea and its doorway and nowhere else. The yellow-side damselfish is known from the 🌊 Red Sea and the 🌊 Gulf of Aden, from the head of the Gulf of Aqaba down to the Somali coast, which is a small world for a fish that common.

It is built like a coin: deep-bodied, laterally flat, pearly white with a gray wash over the front of the back and the rear half of the body washed yellow, which is what flavilatus means. At up to 10 cm it stays smaller than the whitebelly damselfish it shares reefs with, and that one has no yellow across the back half of the body.

The living is done in the water column. Groups hover a meter or two above coastal fringing reefs and outer slopes, usually between about 12 and 20 m, facing into the current and picking out individual plankton animals, then dropping into the coral together when something fast comes past. The reef below is shelter and dormitory rather than pasture.

Breeding puts them back on the rock. The male clears a patch of hard surface, courts a female over it, and takes over the clutch she glues down: he guards it against every egg eater on the reef and fans it with his pectoral fins for the days it takes to hatch. The larvae then spend a few weeks out in open water before settling.

Allen and Randall described the species in 1981. It is one of a short list of fish that a diver can only see between Aqaba and Aden, and it is worth knowing that the commonest thing on the reef is sometimes also the most local.