Ostorhinchus fleurieu
Spot this small reef fish's dark tail-spot in Sabah, Sai Kung and Longdong, in water from 24 to 29C year-round.
A small, deep-bodied fish around 10cm with a reddish-brown to golden body and large dark eyes typical of cardinalfish. The name-giving mark is a bold dark blotch (the 'bullseye') sitting near the base of the tail, which is the quickest thing to key on underwater. Two thin dark stripes often run through the head and eye, fading toward a plain-ish body.
Look for it hovering in small, loose groups near overhangs, crevices and under ledges rather than out in the open water column. It tends to hold station close to shelter and eases back in rather than bolting when a diver approaches slowly, typical cardinalfish caution. Most sightings come from the reef structure itself, so scanning nooks and shaded gaps is more productive than watching open sand.
Like other cardinalfish, this species is a mouthbrooder, with the male carrying fertilised eggs in his mouth until they hatch, a detail worth remembering if you see one holding its jaw oddly full and shut.
Divers log it year-round, consistent with its comfortable 24.3 to 29.1C range and the tropical to subtropical reefs it favours. There is no clear seasonal peak in the sightings data, so any month works if you are diving Sabah, Sai Kung or Longdong.
Reef neighbours a diver could mistake it for. Use the identification notes above to tell them apart.
Most reliably logged in Sabah, Malaysia, with strong showings in Sai Kung, Hong Kong and Longdong, Taiwan. Across these three countries it turns up as an uncommon but steady find on reef and rocky-reef dives rather than a headline attraction.
Mapping where Bullseye Cardinalfish is logged…
Sabah in Malaysia is the strongest area, with regular sightings also logged around Sai Kung in Hong Kong and Longdong in Taiwan. It shows up as an uncommon reef resident rather than a guaranteed sighting at any one site.
There is no strong season in the sighting data, so it is essentially a year-round find. Its temperature niche of 24 to 29C covers most of the diving calendar in Sabah, Hong Kong and Taiwan.
Check for a small, reddish-brown fish around 10cm with big dark eyes and a distinct dark spot near the base of the tail. Thin dark head stripes and a habit of hovering near crevices help confirm it.
They reach roughly 10cm, a typical small-bodied size for cardinalfish, so they are best spotted at close range near reef structure rather than from a distance.
No, they pose no threat to divers. They are a small, non-aggressive reef fish that simply retreats toward shelter if approached.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
Iridescent Cardinalfish
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