Visually Separating the Chromis: Blue-green Chromis and Black-axil Chromis

These two species are very similar, and sometimes school together. With some careful observation, it is not hard to tell them apart.

Black-axil Chromis, Chromis atripectoralis, is more distinctive. It has a black patch outlined in bright pale blue on the upper pectoral fin base. This axilspot is only fully visible when the fin is held in the down position, though some of the bright border can often be seen when the fin is up. This species often has pale areas in the center of each scale on the upper back, giving a string-of-pearls look. It also has a bright blue line from the tip of the snout to the eye. Note that the overall color is somewhat variable: while this one is darker in the pictures shown here, that is not always the case.

Blue-green Chromis, Chromis viridis, might have a tiny dingy axilspot, but never a well-defined round spot with border. Generally it is unmarked pale blue-green, with a slightly paler belly. It sometimes shows a line from snout tip to eye similar to the one on atripectoralis, though generally not as bright.

There are behavioral differences. Blue-green Chromis is rarely seen far from the safe cover of branching corals, while Black-axil is braver, willing to venture higher above the reef. When one of these fishes is more than a few inches from safety, it is usually Black-axil.

Here's a typical view which is a little tricky. No axilspot is visible. But the pectoral fin is in the "up" position, so it might just be hiding that axilspot. You can see a pale blue line curving above the pectoral fin base: that is the outline of the hidden axilspot. So this is a Black-axil. You can confirm that with the brighter blue spots on the back, and bold lines on the face.