The cloud of small blue-green fish over a staghorn head, and the first thing to vanish into it.
To about 9 cm, pale blue-green and slightly translucent, appearing green from above and blue from the side. Breeding males turn yellow-green with a black-tipped tail. The similar Chromis atripectoralis is larger and carries a black spot at the base of the pectoral fin, and the two frequently share a coral head.
Aggregations of tens to several hundred hover a metre above a branching coral colony taking zooplankton, and drop into the branches as one when threatened. That behaviour makes the species a reliable indicator: a school that has gone into the coral has seen something. Males clear a patch of sand or dead coral at the base of the colony for eggs and guard them. Recorded from the shallows to about 20 m.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.