Angelfish Facts

The emperor's body is disc-shaped and flattened side to side, built to slip through tight coral crevices; adults run 10 to 15 inches and 2 to 3 pounds. Like all marine angelfish (family Pomacanthidae) it carries a strong backward-facing spine on the gill cover for defence. The striking part is the two-stage look. A juvenile is dark blue-to-black ringed in white and electric blue — disruptive camouflage that breaks up its outline in shadowy caves. The adult switches to horizontal yellow-and-blue bands, with a dark mask across the eyes that hides the pupil. The change takes months of physiological reorganization, and it serves a social purpose: because the young fish looks nothing like an adult, territorial males don't treat it as a rival and let it live unharassed on their reef. Its overlapping scales are the opposite solution to a dragonet's bare, mucus-covered skin, examined in why mandarinfish lack scales.

The species ranges across the tropical Indo-Pacific — from the Red Sea and East Africa east across the Indian Ocean to Indonesia and the Great Barrier Reef, and out into the Pacific as far as Hawaii and French Polynesia. It is a bottom-associated reef fish, usually between 3 and 300 feet deep, that needs structurally complex reef with plenty of caves and overhangs. Juveniles are secretive and solitary, tucked into dark surge channels and under ledges; adults are bolder, patrolling outer slopes, drop-offs, and lagoon channels rich in the invertebrates they eat.

Despite the "omnivore" label of the aquarium trade, wild emperors are specialist grazers on sponges and tunicates (sea squirts), with some other invertebrates and a little algae. Sponges are a hard food to exploit — stiffened with glass-like silica spicules and laced with chemical toxins — and the angelfish is built for it. Rows of overlapping bristle-teeth rasp away the tough tissue like shears, and the gut secretes thick mucus that sheathes the sharp spicules so they pass through without cutting. By keeping fast-growing sponges in check, emperors do real ecological work, stopping sponges from overgrowing and smothering slow-building corals.

Emperor angelfish are territorial rather than schooling. A dominant male holds a large territory — often thousands of square meters of prime reef — overlapping the smaller ranges of two or more females in a loose harem. He drives off rival males with broadside displays, flaring his fins to look bigger, and grinding his pharyngeal teeth to produce a loud, low "thump" that divers can hear underwater. The gulf between juvenile and adult patterns feeds directly into this system: the male doesn't read the ringed youngster as competition, so the juvenile gains shelter inside his territory.

Reproduction tracks the lunar cycle and rising tides. A bonded pair performs a rising courtship dance, climbing together off the reef into open water, and at the top of the ascent releases clouds of eggs and sperm at once before darting back to cover. The buoyant fertilized eggs drift into open water and hatch within 24 to 48 hours. The larvae are planktonic, adrift on ocean currents for weeks — the mechanism behind the species' vast range — before settling onto a reef and taking on the ringed juvenile form. Growth is slow; it can take up to two years to begin the shift to adult colours, and a wild emperor may live up to 20 years.

The IUCN rates the emperor angelfish Least Concern; its huge range and widespread populations buffer it against immediate risk. But as an obligate reef animal, its fate is tied to reef health. Ocean warming and acidification are driving mass coral bleaching, and a collapsing reef takes with it both the angelfish's shelter and its sponge food. Its beauty is a second pressure: it is prized in the marine aquarium trade, and in poorly regulated areas local populations are thinned by destructive collecting, including cyanide stunning. Sustainable trade management, and action on climate change and reef protection, are what keep this fish common.

• IUCN Red List - Pomacanthus imperator
• National Aquarium - Emperor Angelfish