Ostorhinchus cyanosoma
A tiny reef regular with a blue racing stripe, most reliably spotted around Koh Lipe, the Visayas and Fiji's Viti Levu year-round.
A small, oval-bodied cardinalfish around 5 cm long, pale yellow to translucent with a bold electric-blue stripe running from snout to tail. A black spot often sits at the base of the tail, and the large dark eyes are typical of the cardinalfish family. It tends to hover nose-into-current in small loose groups rather than swim in tight formation.
Look for it hovering in loose clusters over coral heads, rubble patches and under ledges by day, often mixed in with other small reef fish rather than schooling tightly. It is generally unbothered by divers, holding position or drifting a short distance off rather than bolting for cover. Most sightings are on the reef itself during daylight hours, though like many cardinalfish it becomes more active after dark.
Cardinalfish are mouthbrooders: males carry the fertilised eggs in their mouths until they hatch, a trait shared across the whole Apogonidae family.
No strong seasonal pattern shows in the data, and its 25 to 29.3 C comfort range matches tropical reefs year-round, so any month on these reefs is a fair bet.
Reef neighbours a diver could mistake it for. Use the identification notes above to tell them apart.
Most reliably logged in the Philippines, especially across the Visayas, and around Koh Lipe in Thailand, with strong numbers also in Fiji's Viti Levu and Egypt's Marsa Alam and Deep South sites. Good secondary populations turn up in Flic-en-Flac in Mauritius, Sabah in Malaysia, and reefs across Indonesia and Oman.
Mapping where Blue-striped Cardinalfish is logged…
It shows up widely across the Philippines (especially the Visayas), Koh Lipe in Thailand, Fiji's Viti Levu, and Egypt's Marsa Alam area. It is also a fairly dependable find in Mauritius, Malaysia's Sabah region, Indonesia and Oman.
There is no clear seasonal pattern in sightings, and it tolerates a broad 25 to 29.3 C range, so it is present on tropical reefs across all months.
Look for a small yellowish fish around 5 cm with a sharp blue stripe running the length of its body and a dark spot near the tail base. It hovers in loose groups near coral rather than schooling tightly.
They stay small, reaching only around 5 cm in length, making them one of the smaller reef fish divers regularly log.
No. They are harmless, small reef fish that pose no risk to divers and generally hold their position as you pass by.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
Iridescent Cardinalfish
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