Eyeline Surgeonfish

Overview

The Eyeline Surgeonfish (Acanthurus nigricauda) is a tropical marine fish widely distributed across the Indo-Pacific region. Often found in small groups or alone, this species is typically associated with reef environments where it navigates sandy and rocky substrates.

Distribution & habitat

This species is found throughout the tropical and subtropical Indo-Pacific, ranging from East Africa and the Mascarene Islands eastward to the Tuamotu Islands. Its range extends north to the Ryukyu Islands and south to the southern Great Barrier Reef. They primarily inhabit coastal areas, including lagoons, bays, and outer reef slopes, often frequenting sandy bottoms and occasionally brackish waters near river mouths.

Appearance

Adults reach a maximum length of approximately 40–45 cm and possess a laterally compressed, deep-bodied, oval shape. Their coloration is generally dark brown or purplish-grey, though it can shift in intensity. Key identifying features include a prominent black streak behind the eye, a horizontal black band behind the upper gill opening (absent in juveniles), and a crescent-shaped, lunate caudal fin. A thin yellow band is often visible on the pectoral fins.

Biology & behavior

Like other surgeonfish, this species features a sharp, retractable spine on each side of the caudal peduncle used for defense. They are known to be active during the day and are generally observed foraging near the bottom. Spawning is believed to occur in pairs.

Feeding

This fish is an omnivore with a strong tendency toward detritivory. Its diet consists of organic matter, benthic algae, sand, and small invertebrates, which it processes using a specialized, thin-walled stomach.