It is said the moorish idol got its name from the Moors of Africa, who purportedly believe the fish to be a bringer of happiness. Moorish idols are also popular aquarium fish, but despite their popularity, they are notorious for their short aquarium lifespans and difficulty.
With distinctively compressed and disk-like bodies, moorish idols stand out in contrasting bands of black, white and yellow which make them look very attractive to aquarium keepers. The fish have relatively small fins, except for the dorsal fin whose 6 or 7 spines are dramatically elongated to form a trailing, sickle-shaped crest called the philomantis extension. Moorish idols have small terminal mouths at the end of long, tubular snouts; many long bristle-like teeth line the mouth.
The eyes are set high on the fish's deeply-keeled bodies; in adults, perceptible bumps are located above each. The anal fin may have 2 or 3 spines. Moorish idols reach a maximum length of 23 cm. The sickle-like dorsal spines actually shorten with age.
Sponges, tunicates and other benthic invertebrates constitute the bulk of the moorish idol's diet. Captive kept moorish idols typically are very picky eaters. They will either eat nothing (common) and perish or eat everything (very uncommon). Eating a variety of items is healthy. Even small portions of avocado and banana are sometimes fed in captivity.
Moorish idols are pelagic spawners; that is, eggs and sperm are released in midwater and the fertilized eggs are left to drift away with the currents. The impressive range of these fish may be explained by the unusually long larval stage; the fish reach a length of 7.5 cm before becoming free-swimming juveniles. Before this time, the developing larvae will have drifted considerable distances.
Moorish idols are notorious for being difficult to maintain in captivity. They require enormous tanks, often exceeding 200 U.S. gal, are voracious eaters, and are infamous for becoming incredibly destructive. Their captive survival rate is very low: most do not survive for a full year. Most that live past this mark typically die shortly thereafter. It is not recommended that any aquarist attempt to keep this species, because it is considered cruel by many and is nearly impossible (see diet). To avoid these shortfalls, some aquarists prefer to keep substitute species that look very similar to the Moorish Idol. These substitutes are all butterflyfishes of the genus Heniochus, and include the pennant coralfish, Heniochus acuminatus; threeband pennantfish, H. chrysostomus; and the false moorish idol, H. diphreutes.