Ostorhinchus monospilus
A small, easily-missed reef fish logged across the Visayas in the Philippines, sitting in 25.2 to 28.9C water year-round.
A compact, big-eyed fish only about 6 cm long, pale to translucent brownish body typical of cardinalfish, with a single dark spot on the rear half of its flank being the main feature to check. It lacks the bold stripes or bars of many reef-associated cardinalfish, so the plain body plus that one spot is what separates it from busier-looking relatives. Usually seen hovering close to shelter rather than out in the open.
Typically found hovering low over rubble or tucked near reef structure rather than roaming in open water, in the loose, sheltering way most small cardinalfish behave. Divers log it as an uncommon find, meaning you'll need to slow down and check crevices rather than expect it to be obvious on a reef scan. Like other cardinalfish it's more active around dusk, but daytime sightings near cover are typical too.
Cardinalfish in this family are mouthbrooders, with males typically carrying the egg mass in their mouth until the young hatch, a reproductive strategy shared across the group even though it hasn't been specifically documented for this species.
No clear seasonal pattern shows up in the sightings, and water temperatures at recorded sites stay a comfortable 25.2 to 28.9C, so it's a year-round sighting rather than a seasonal one.
Reef neighbours a diver could mistake it for. Use the identification notes above to tell them apart.
Most consistently logged in the Visayas region of the Philippines, which accounts for the vast majority of recorded sightings, with additional records from Sulawesi and Bali in Indonesia. If you're diving Philippine reefs in the Visayas, this is the region to keep an eye out in rubble and reef crevices.
Mapping where Moluccan Cardinalfish is logged…
The Visayas region of the Philippines is by far the most reliable area, with scattered records also from Sulawesi and Bali in Indonesia. Look in rubble patches and along reef structure rather than open water.
There's no strong seasonal pattern in the sightings, and it's found in warm water year-round (around 25 to 29C), so any time of year on Visayan reefs is a fair shot.
Look for a small, plain-bodied cardinalfish around 6 cm with a single dark spot on the rear flank. It lacks bold stripes, which helps separate it from more heavily marked cardinalfish species sharing the same reefs.
They stay small, topping out around 6 cm, which is typical for cardinalfish and part of why they're easy to overlook on a reef.
Cardinalfish as a family are known for mouthbrooding, usually by the male, and this species belongs to that same group, though its specific breeding behaviour hasn't been well documented by divers.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
Iridescent Cardinalfish
1,784 sites
