Hawaii's Reef Fishes

Surgeonfishes (Acanthuridae)

Ringtail Surgeonfish Acanthurus blochii (Valenciennes, 1835)

The Hawaiian fishes of this family are readily identified by having 1 or 2 spines on the side of the caudal peduncle. The species of the genera Acanthurus, Ctenochaetus, and Zebrasoma have a single lancet-like spine which folds into a groove; whereas those of the genus Naso have 2 fixed keel-like spines.

All surgeonfishes have a deep compressed body with the eye high on the head; the mouth is small with a single row of close-set teeth; the teeth may be spatulate with denticulate edges and fixed in the jaws (as in Acanthurus), or numerous, slender with incurved tips, and movable (as in Ctenochaetus).

There is a single unnotched dorsal fin with 4 to 9 spines; the anal fin has 3 spines (except Naso with 2); the pelvic fins have a spine and 3 or 5 soft rays (3 in Naso). Some of the species of Naso have a horn-like projection on the forehead, the basis for their common name unicornfishes (others of the genus without the rostral horn are still called unicornfishes).

The species of Ctenochaetus and several of Acanthurus, such as A. xanthopterus and related species, have a thick-walled gizzardlike stomach. All of the surgeonfishes have a very long intestine. Most feed on benthic algae. The species of Acanthurus graze principally on filamentous algae; those with gizzard-like stomachs often ingest sand with their algal food to assist in trituration of the algae.

Some species of Acanthurus, such as A. triostegus and A. leucopareius, form feeding aggregations to overwhelm territorial herbivorous damselfishes. The algal-feeding species of Naso, N. lituratus and N. unicornis, browse mainly on leafy algae such as Sargassum. The species of Ctenochaetus feed on detritus and algal fragments which they whisk with their comb-like teeth from the substratum (at the same time employing suction). One Hawaiian species of Acanthurus and five of Naso feed mainly on zooplankton well above the bottom.

Surgeonfishes are able to slash other fishes (or humans who do not handle them carefully) with their caudal spines by a rapid sidesweep of the tail; some species have bright hues around the caudal spines as warning coloration.

Spawning from aggregations has been observed in some species of Acanthurus, Ctenochaetus, and Zebrasoma at dusk (and for some at dawn, hence correlated with low light intensity). The eggs are small and pelagic. The late larval stage is orbicular, transparent with silvery over the abdomen, and small scales in narrow vertical ridges on the body. This stage has venomous second dorsal, second anal, and pelvic spines; these spines remain venomous in the adults of at least some of the species of Naso.

Twenty-four species of surgeonfishes are recorded from Hawai'i; however, the colorful Acanthurus lineatus, is known only from two individuals.

Eyestripe Surgeonfish