Moorish idol

Zanclus cornutus is the only living species of the family Zanclidae, a reef fish with a very deep, flattened body, a protruding snout and a dorsal spine drawn out into a filament that usually exceeds the length of the body ^[1],[2]. It lives across much of the Indo-Pacific and also reaches the tropical eastern Pacific ^[1]. The catalogue of Peru's marine fishes gives it as far as the north of the country ^[3], and the global Red List classes it as Least Concern ^[4].

Linnaeus gave it two names in 1758: Chaetodon cornutus for the adult and Chaetodon canescens for its late postlarva ^[5],[1]. The genus Zanclus was created by Cuvier and Valenciennes in 1831 ^[2]. Although canescens appears on an earlier page of Linnaeus's work, the valid name is cornutus, because Günther, the first to realise in 1876 that both were the same fish, chose that one ^[1]. The Peruvian catalogue of 2001 still lists it as Zanclus canescens ^[3].

The body is extremely deep, its depth going 1 to 1.4 times into the standard length, and strongly compressed; the snout is narrow and juts far forward, and adults carry a pointed bony knob in front of each eye, larger in males ^[1]. The dorsal fin has 6 to 7 spines, nearly always 7, and 39 to 43 soft rays; its third spine is a very long filament that usually exceeds the standard length, and the anal fin has 3 spines and 32 to 36 rays ^[1]. The scales are tiny and each bears a row of backward-curving points, so the skin feels like fine sandpaper ^[1].

The colour is striking: white at the front and yellow behind, with 2 broad black bands, the first around the eye and spreading over the chest, pelvic fins and half the belly, the second on the rear half of the body, edged with white and black lines and running into the dorsal and anal fins; there is an orange patch ringed with black on the snout, the chin is black and the tail mostly black ^[1]. It reaches at most 22 cm in total length and commonly grows to 16 cm ^[1]; the Peruvian catalogue gives 20 cm ^[3].

It can be confused with butterflyfishes of the genus Heniochus, but these lack the long snout and the bony knob in front of the eyes; surgeonfishes, family Acanthuridae, differ in having one or more spines or keeled plates on each side of the tail stalk, which Zanclus cornutus lacks ^[1]. What defines its family is a broadening of the shoulder girdle into a pectoral disc and the large number of rays in the dorsal and anal fins; the family also includes 2 extinct genera from the Eocene of Bolca, in Italy ^[2].

It ranges over almost the whole Indo-Pacific, except the Red Sea and the Persian Gulf, and also reaches the tropical eastern Pacific ^[1]. In the western Pacific it has been seen north to Kominato, in Japan's Chiba Prefecture, and south to southern New South Wales, in Australia, records owed to postlarvae carried by warm currents beyond the area where it breeds; Randall, the guide's author, also saw a single adult at Easter Island ^[1]. In the Sydney region it is among the tropical fishes most often seen as stray juveniles, arriving from the Great Barrier Reef ^[6].

On the American Pacific coast, the Peruvian catalogue of 2001 gives it from Baja California, in Mexico, to northern Peru, including the Galápagos Islands ^[3]. A record published in 2018, from the northern Gulf of California, was the northernmost then documented in the eastern Pacific ^[7]. On Colombia's Pacific coast it turned up, among the less abundant species, in fish counts on the rocky shores of Utría National Park ^[8].

It lives on coral reefs or over rocky bottoms, from very shallow water down to 180 m ^[1]. The Peruvian catalogue describes it as a fish of coastal waters over rocky areas ^[3].

It is omnivorous but eats more animals than algae: sponges dominate the animal part of its diet, and bryozoans, small molluscs and crustaceans, hydroids and tunicates have also been found in its stomach ^[1]. In Wakatobi Marine National Park, Indonesia, it showed a clear preference for sponges only at a site regarded as degraded, with heavy sediment and little coral, where it fed mostly on the barrel sponge Xestospongia testudinaria; at a site with more coral cover, by contrast, 60 % of its bites went to coralline algae and the rest to hard coral, ascidians and other algae ^[9].

It is sometimes seen in pairs and occasionally in groups ^[1]. On the reefs of west Hawaii Island it was recorded making pulsed sounds during aggressive encounters and during what looked like courtship ^[10].

Its larva spends a long time in the open sea ^[2]. The late postlarva, which can reach 8 cm in total length, carries on each side of the head a thick curved spine in front of the eye, which it loses when it turns into a juvenile ^[1].

Weighing how many individuals they counted and how often they saw them bite sponges, the authors of the Wakatobi study placed it among the 4 fishes that could most influence the sponge communities there ^[9]. The same study recalls that in Pacific Panama, sponges hidden in crevices vanished quickly once exposed to their predators, this species among them ^[9].

Its populations on either side of the Eastern Pacific Barrier, some 5000 km of deep water that separate the central from the eastern Pacific, show no significant genetic differentiation, and by the study's estimates gene flow has been much greater from east to west than the other way ^[11].

Parasites have been described from it: monogenean worms of the genus Haliotrema in the South China Sea ^[12], and in the Lakshadweep Islands, in the Arabian Sea, a gallbladder myxosporean, Sphaeromyxa cornuti ^[13], and 2 species of the genus Ceratomyxa ^[14].

Juvenile tropical fishes reach the Sydney region every summer and today nearly all die in winter; an analysis of the external shape of those species placed this one among those with the best odds of filling a free niche in the temperate communities if they managed to stay ^[6].

Its global category on the IUCN Red List is Least Concern ^[4].

It matters in the aquarium fish trade, although it thrives only in large tanks; collectors look for small individuals, because adults adapt poorly to captivity, and individuals have been reported to live up to 10 years in an aquarium ^[1]. A 2024 study of its parasites in the Lakshadweep Islands presents it as a marine ornamental fish ^[14].

The catalogue of Peru's marine fishes, published in 2001, places the southern end of its range on the American Pacific coast in the north of the country, without naming a particular locality, and describes it as a coastal fish of rocky areas ^[3].