Aeoliscus strigatus
Spot razorfish in the Visayas, Bali or Sipadan, they swim head-down in tight schools above sea urchins. Best Feb, Aug in 25-29 °C Indo-Pacific waters.
Razorfish are unmistakable once you know the trick: they swim permanently head-down, bodies nearly vertical, in tight synchronised schools. The body is laterally compressed (flattened side-to-side) and encased in bony plates, giving it a rigid, shrimp-cracker silhouette. Colouration is pale silver to cream with a bold dark brown or black stripe running the length of the body from snout to tail. The tail end carries a flexible spine where you might expect a tail fin, giving the fish its razor-like profile.
Schools hover motionlessly above long-spined sea urchins (Diadema spp.) or beds of branching coral, all individuals pointing straight down, they use the urchin spines as refuge, darting between them when threatened. When you approach, the school tends to shift position rather than scatter, keeping the spines between you and them. They feed by sucking up tiny crustaceans from the water column, so you often find them in areas of gentle current where plankton accumulates.
The head-down posture is not resting behaviour, it is how razorfish swim all the time. The tail carries the propulsive fin while the head hangs below, meaning the fish is essentially swimming upside-down relative to a conventional fish's orientation. It is one of the most reliably bizarre sights in Indo-Pacific diving.
Across the northern-hemisphere sites where seasonality is best recorded, sightings concentrate between February and August. The species appears tied to warm tropical shallows in the 25-29 °C range, so destinations closer to the equator, Palau, Borneo, Indonesian Sulawesi, can produce sightings outside those months as well.
The Philippines, especially the Visayas, is far and away the most consistent place to encounter razorfish, followed closely by West Sabah and Labuan in Malaysia, including Sipadan. Indonesia rounds out the top three, with strong records from Bali, Lombok and Sulawesi. Palau's Rock Islands, India's Andaman Islands, and sites across Vietnam and New Caledonia also hold populations worth keeping an eye out for.
Mapping where Razorfish is logged…
Razorfish are most commonly logged in the Philippines (particularly the Visayas), Malaysian Borneo including Sipadan, and Indonesia across Bali and Sulawesi. Palau's Rock Islands and India's Andaman Islands are also reliable. All confirmed sightings come from warm Indo-Pacific waters.
At northern-hemisphere destinations, February through August sees the highest concentration of sightings. In equatorial locations like Palau or Sipadan, the warm water year-round means there is no hard off-season, but Feb, Aug still appears to be the peak window overall.
Look for a school of small silver fish hovering perfectly vertical, heads pointing straight down, above a sea urchin or coral head. The rigid, blade-like body and the continuous dark stripe along the side are distinctive. No other common reef fish schools in that vertical, head-down posture.
They are not upside down, that vertical, head-down orientation is simply how the species swims. The flexible tail at the upper end propels them, while the head hangs below. This posture lets them mimic the spines of sea urchins and makes it far harder for predators to pick individuals out of the group.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
Soldierfish
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