threadfin butterflyfish

The threadfin butterflyfish is a small, laterally compressed reef fish reaching about 23 cm, with a distinctive dark diagonal band across the head and a long filament extending from the dorsal fin—the threadlike structure that gives it its name. It inhabits coral and rocky reefs across the Indo-Pacific, typically at depths between 5 and 40 metres, and feeds primarily on coral polyps and small invertebrates. Threadfins are generally seen in pairs or small groups moving methodically through branching corals, making them a common and approachable encounter for divers.