Fish
Moorish Idol
Zanclus cornutus · also called moorish idol
The Moorish idol is one of the most recognisable fish on a coral reef: a nearly circular body painted in broad bands of white, black, and yellow, with a long white streamer trailing from its back fin and a little tube of a snout for picking at sponges. It is also, for such a common and beautiful fish, almost impossible to keep alive in an aquarium.
Overview
The Moorish idol is found on reefs across the Indo-Pacific and eastern Pacific, one of the most widely distributed reef fish in the world. It is 16–22 cm long, strongly compressed and disc-shaped, banded in white, black, and yellow, with a trailing dorsal filament and, in adults, small bony bumps above the eyes. It eats mostly sponges.
A family of one
It is the only living species of the family Zanclidae, related to the surgeonfishes and tangs but on its own branch, with no close living relatives.
A very long larval life
The young drift in the open ocean for up to several months, in a spiny form once thought to be a separate animal, which is how the species spread across such a vast range.
Impossible to keep
Because of its sponge diet and its sensitivity, most Moorish idols taken into home aquariums refuse to feed and slowly starve. It is widely rated one of the hardest marine fish to maintain — an irony, given that the character Gill in Finding Nemo is a Moorish idol who dreams of the open sea.
Status
The Moorish idol is Least Concern, common and widespread, though collected for the aquarium trade despite its poor survival there and affected by the general decline of coral reefs.
Moorish Idol: common questions
Is the Moorish idol related to the butterflyfish it resembles?
No, and this is a classic case of two unrelated fish arriving at a similar look. The Moorish idol and several banded butterflyfish, particularly the bannerfish, share a disc-shaped body, bold vertical stripes, and a trailing dorsal filament, and they are easily confused. But butterflyfish belong to their own family, while the Moorish idol is the only species in a separate family related to the surgeonfishes. The resemblance is thought to be a shared solution to the same problems of moving among coral and confusing predators, rather than a sign of kinship. One quick difference is the Moorish idol's long tubular snout and, in adults, the little horns over the eyes.
Why is the Moorish idol so hard to keep in a home aquarium?
Two reasons combine. First, its natural diet is dominated by sponges and other encrusting reef organisms that are difficult or impossible to supply in a tank, and many individuals simply will not switch to prepared foods, so they gradually starve even while appearing healthy at first. Second, the fish is sensitive to handling, water quality, and the stress of capture and shipping, and it often arrives already weakened. Experienced marine aquarists sometimes succeed with wild-caught fish that were already feeding, in very large mature reef systems, but for most keepers the species is a near-certain loss and is discouraged.
What is the spiny young stage that was mistaken for another species?
Moorish idol larvae spend a long time drifting in open water before settling onto a reef, and during that phase they have a distinctive appearance, being more elongated and bearing a large bony spine projecting forward from the front of the head and other spines on the body. Early naturalists who caught these larvae, before the link to the adult was understood, described them under a separate name. It is now known to be the pre-settlement stage of the Moorish idol, and the head spine is lost when the fish takes up reef life.
Where does the Moorish idol live and how common is it?
It is one of the most widely distributed reef fish in the world, occurring right across the Indo-Pacific from the coast of East Africa to the eastern Pacific coast of the Americas, on coral and rocky reefs from shallow water down to well over a hundred metres. The long larval drift is what allows this enormous range. Across most of it the Moorish idol is a common sight, often in pairs or small groups picking over the reef, and it is listed as Least Concern. Its main long-term threat is the same one facing reef fish generally, the loss and degradation of coral reef habitat.