Ostorhinchus doederleini
Small reddish cardinalfish clustering over reefs in Sai Kung, Hong Kong, in water from 20 to 29 C, year-round and easy to overlook.
Small, deep-bodied fish around 10 cm, coloured reddish-brown to coppery with large dark eyes typical of the cardinalfish family. Look for a faint dark line or blotch through the eye and a stocky, slightly compressed body held almost motionless in the water. It typically sits in loose clusters just off the reef rather than swimming out in open water.
Found hovering in small, loose groups close to reef structure, ledges and crevices, usually staying still or drifting gently rather than darting about. Like most cardinalfish it tends to be more active around dusk and at night, tucking closer into shelter during brighter daylight hours. It's generally unbothered by divers who keep a bit of distance, making it an easy subject for a slow, patient look rather than a chase.
Cardinalfish in this family are mouthbrooders, meaning the male typically carries the fertilised eggs in his mouth until they hatch, a detail worth watching for if you spot one with a noticeably swollen jaw.
No clear seasonal pattern shows up in the sightings, and its comfortable range of 20 to 29 C covers most of the diving calendar in these regions, so it's effectively a year-round find.
Reef neighbours a diver could mistake it for. Use the identification notes above to tell them apart.
Most reliably logged around Sai Kung in Hong Kong, which accounts for the bulk of recorded sightings, with additional records from Norfolk Island and New South Wales in Australia. Divers in Hong Kong's Sai Kung region have the best odds of finding it, usually described as uncommon rather than abundant.
Mapping where Doederlein's Cardinalfish is logged…
The strongest sightings come from Sai Kung in Hong Kong, with further records from Norfolk Island and New South Wales in Australia. It's logged as uncommon at most of these sites rather than guaranteed on every dive.
There's no strong season in the sightings data, and it tolerates water from about 20 to 29 C, so it can turn up on reefs in its range at any time of year.
Look for a small, reddish-brown fish around 10 cm long with large dark eyes and a faint eye-stripe, usually seen hovering in small loose groups near reef ledges rather than out in open water.
They're a small species, typically reaching around 10 cm in length, so they blend easily into reef nooks and crevices.
Like other cardinalfish, males are thought to be mouthbrooders, carrying fertilised eggs in their mouth until the young hatch, which protects the eggs from predators.
No, they pose no danger to divers and are a calm, easy species to observe up close without disturbing them.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
Iridescent Cardinalfish
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