Butterfly Fish

The butterflyfish are a group of conspicuous tropical marine fish of the family Chaetodontidae. Found mostly on the reefs of the Atlantic, Indian and Pacific Oceans, butterflyfish are fairly small, most from 12 to 22 cm in length. The largest species, the lined butterflyfish, Chaetodon lineolatus, grows to 30 cm. There are approximately 127 species in eleven genera. They should not be confused with the freshwater butterflyfish of the family Pantodontidae.

Butterflyfish are named for their brightly coloured and strikingly patterned bodies in shades of black, white, blue, red, orange and yellow (though some species are dull in colour). Many have eyespots on their flanks and dark bands across their eyes, not unlike the patterns seen on butterfly wings. Their deep, laterally compressed bodies are easily noticed through the profusion of reef life, leading most to believe the conspicuous coloration of butterflyfish is intended for interspecies communication. Butterflyfish have uninterrupted dorsal fins with tail fins that may be rounded or truncated, but are never forked.

The family name Chaetodontidae derives from the Greek words chaite meaning "hair" and odontos meaning "tooth." This is an allusion to the rows of brush-like teeth found in their small, protrusile mouths. Butterflyfish closely resemble the angelfish of the family Pomacanthidae but are distinguished from the latter by their lack of preopercle spines (part of the gill covers).

Their coloration also makes butterflyfish popular in the aquaria hobby. However, most species feed on coral polyps (corallivores) and sea anemones; this poses a problem in most reef tanks where a delicate balance is to be maintained. Species kept in the hobby are therefore the few generalists and specialist zooplankton feeders.

Generally diurnal and frequenting shallow waters of less than 18 m (some species found to 180 m), butterflyfish stick to particular home ranges. The corallivores are especially territorial, forming mated pairs and staking claim to their own head of coral. Contrastingly, the zooplankton feeders will form large conspecific groups. By night butterflyfish hide amongst the crevices of the reef and exhibit markedly different coloration than they do by day.

Butterflyfish are pelagic spawners; that is, they release many buoyant eggs into the water which then become part of the plankton, floating with the currents until hatching. The fry go through what is known as a tholichthys stage, wherein the body of the postlarval fish is covered in large bony plates extending from the head. This curious armoured stage is seen in only one other family of fish; the Scatophagidae (scats). The fish lose their bony plates as they mature.