Acanthurus thompsoni
Uncommon in the Maldives' Central Atolls, this brown surgeonfish shows a white tail crescent in 25-29C water year-round.
A slim, oval-bodied surgeonfish about 19 cm long, uniformly dark brown to chocolate-coloured over the whole body. The giveaway is a pale, almost white crescent-shaped band across the base of the tail, standing out sharply against the dark flanks. It often swims in loose groups above the reef rather than hugging the bottom, showing a more streamlined, mid-water profile than most reef surgeonfish.
Usually spotted cruising in loose, mixed groups over reef slopes and along current-swept edges rather than picking at the substrate like many of its relatives. It tends to hold a bit off the bottom, feeding on plankton in the water column, and generally keeps a comfortable distance from divers without darting for cover. Most sightings come from open reef and channel habitats where a little current keeps the water moving.
Unlike most surgeonfish that graze algae off the reef, this species feeds mainly on plankton drifting in open water, which is why divers see it hanging in the current above the reef rather than nose-down on the substrate.
Sightings show no clear seasonal pattern, so it is considered a year-round species; it sits comfortably in the 24.6 to 28.8C range typical of these tropical reefs in every month logged.
Reef neighbours a diver could mistake it for. Use the identification notes above to tell them apart.
Most reliably logged in the Maldives, especially the Central and Southern Atolls, where it turns up on outer reef slopes and channel edges. Strong secondary populations sit in Sulawesi, Indonesia, and around Atauro Island in Timor-Leste, with smaller but consistent numbers in Palau's Rock Islands and Peleliu, and in the Philippines around Luzon.
Mapping where Chocolate Surgeonfish is logged…
The Maldives is the most reliable spot, particularly the Central and Southern Atolls, with further sightings in Sulawesi (Indonesia), Atauro Island (Timor-Leste), and Palau's Rock Islands and Peleliu.
There is no strong seasonal pattern in sightings, so they are considered present year-round on tropical reefs sitting in the 24.6 to 28.8C range.
Look for a slim, all-brown surgeonfish around 19 cm with a pale, near-white crescent band at the base of the tail. It is often seen loosely grouped in mid-water above the reef rather than grazing on the bottom.
They typically reach around 19 cm in length, a modest size compared to some other reef surgeonfish.
Most surgeonfish graze algae directly off rock and coral, but this species feeds on plankton in open water, which explains why it is so often seen hovering above the reef rather than browsing the substrate.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
Striped Surgeonfish
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