Bronze, with a complete dark ring round the tail base rather than a spot.
To about 15 cm, bronze to coppery gold with two blue-edged bands running through the eye from the snout to the gill cover, and a dark band encircling the tail base. The band is the identification: many cardinalfishes carry a dark mark near the tail, but in this species it is a complete ring around the peduncle rather than a spot on the side, and it stops short of the base of the tail fin instead of running onto it. Cardinalfishes in this genus are numerous and similar, and the combination of a complete ring, a plain bronze body with no lengthwise stripes, and the paired blue-edged head bands is what places it. The yellowstriped cardinalfish Ostorhinchus cyanosoma is smaller, yellow with blue body stripes, and carries no tail ring at all.
It spends the day in aggregations under ledges and overhangs, around jetty pilings, wreck structure and rubble piles, hanging more or less motionless in the shade, and disperses at night to feed on zooplankton and small bottom-living crustaceans. Groups of a few dozen are usual and they reform in the same places from one day to the next.
Like other cardinalfishes it is a paternal mouthbrooder: the male carries the egg mass in his mouth for a week or more, and a brooding male with visibly distended jaws will periodically open the mouth and roll the eggs to aerate them. That is the moment the species is usually photographed, and it takes patience at a fixed distance rather than a close approach, since a disturbed male may spit or swallow the brood. Recorded from about 3 to 40 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.
