Ostorhinchus rueppellii
Small, shy reef fish logged across Western Australia in 22-29C water, most often spotted uncommon to rare under ledges year-round.
A small, deep-bodied fish around 8cm with a big dark eye and a subtle stripe running through it, typical of the cardinalfish shape. Body colour is a muted silvery to pale brown, with two dorsal fins held separately, the front one spiny and often raised. Look for it hovering rather than swimming hard, tail-first into a crevice, rather than out cruising open water.
Expect to find it tucked under ledges or in shaded crevices, often hanging almost motionless with a handful of others rather than schooling in the open. Divers who spot it usually do so on slower, close-look passes along a reef wall or overhang rather than mid-water. It is not flighty but it is easy to miss, blending into shadow and holding still rather than darting off.
Like other cardinalfish, this species is a mouth-brooder, with the male most likely holding fertilised eggs in his mouth until they hatch, a trait that makes the whole family notable among reef fish for parental care.
Sightings show no clear seasonal pattern, and with a comfortable range of 22.3 to 28.6C it appears to be present on Western Australian reefs across the year rather than tied to a particular month.
Reef neighbours a diver could mistake it for. Use the identification notes above to tell them apart.
Every logged sighting of this fish comes from Australia, with Western Australia standing out as the region where divers consistently find it. It is not a headline reef fish but a steady, if quiet, presence on WA reef and ledge sites rather than something reported elsewhere in the world.
Mapping where Rüppell's Cardinalfish is logged…
All recorded sightings come from Australia, with Western Australia the clear stronghold. It turns up on reef ledges and crevice habitat there rather than being reported elsewhere.
There is no detected seasonal peak, and the water temperature range it tolerates, roughly 22 to 29C, spans most of the year in Western Australia, so sightings are possible year-round.
Look for a small, deep-bodied fish around 8cm with a large dark eye and a faint stripe through it, holding still under ledges rather than swimming in open water. Pale silvery-brown colouring and a separated two-part dorsal fin help distinguish it from similar cardinalfish.
They stay small, typically around 8cm in length, which is standard for cardinalfish and part of why they are easy to overlook on a reef.
Like most cardinalfish, it is a mouth-brooder, meaning eggs are carried and protected in the mouth of one parent until the young hatch, rather than being left on the reef.
No. They are small, non-aggressive reef fish with no venom or bite risk, and simply retreat further into cover if approached.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
Iridescent Cardinalfish
1,784 sites
