The Black-axil chromis is a damselfish

Damselfish

Damselfish comprise the family Pomacentridae except those of the genera Amphiprion and Premnas. They can grow up to long. While most are marine, a few species inhabit the lower stretches of rivers in freshwater. Damselfish usually have bright colors. in tropical coral reefs, and many of those are...

 from the tropical Indo-Pacific

Indo-Pacific

The Indo-Pacific is a biogeographic region of the Earth's seas, comprising the tropical waters of the Indian Ocean, the western and central Pacific Ocean, and the seas connecting the two in the general area of Indonesia...

. This fish can reach almost 5 in (12 cm) in length. This fish inhabits lagoon

Lagoon

A lagoon is a body of shallow sea water or brackish water separated from the sea by some form of barrier. The EU's habitat directive defines lagoons as "expanses of shallow coastal salt water, of varying salinity or water volume, wholly or partially separated from the sea by sand banks or shingle,...

s and reef

Reef

In nautical terminology, a reef is a rock, sandbar, or other feature lying beneath the surface of the water ....

s. It often occurs in large numbers, feeding above Acropora

Acropora

Acropora is a genus of scleractinian coral in the Phylum Cnidaria. Some of its species are known as table coral, elkhorn coral and staghorn coral. There are currently 149 described species...

corals. This fish mostly feeds upon copepod

Copepod

Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Some species are planktonic , some are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...

s, amphipods, and zoea.

This fish is often confused with Chromis viridis

Chromis viridis

The Green Chromis, Chromis viridis, is a species of damselfish. Individuals tend to be iridescent apple-green and light blue, and reach a length of about 3 in .It is sometimes called "Blue-green Chromis", but that may also refer to C...

. It differs from this species by having a black blotch at the base of the pectoral fin.