Butterflyfish are fairly small, mostly from 12 to 22 cm (7-9 in) in length. The largest species, the lined butterflyfish and the saddle butterflyfish, C. ephippium, grow to 30 cm. The common name references the brightly coloured and strikingly patterned bodies of many, bearing shades of black, white, blue, red, orange and yellow. Some species are dull in colour however. Many have eyespots on their flanks and dark bands across their eyes, not unlike the patterns seen on butterfly wings. A great location to find a large variety are national parks like the Similan islands or Surin islands. 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.

Generally diurnal and frequenting shallow waters of less than 18 m (though some species descend to 180 m), butterflyfish stick to particular home ranges. Here diving in Phuket so many of the dive sites are 15 meters and above anyway it means that the huge variety of butterfly fish found here are with in easy reach of anyone doing a dive course Phuket will amaze you. 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. Here in the waters around Phuket these fish are around in huge numbers. This coupled with the great 40 meter plus vizability of the Similan islands is what makes the Similan islands diving such a draw for so many each year.

Their coloration also makes butterflyfish popular aquarium fish. However, most species feed on coral polyps and sea anemones. This poses a problem in most reef tanks where a delicate balance is to be maintained. By far the best place to see them is a place like the Similan island national park just off the north west coast of Phuket. Species kept in the hobby are therefore the few generalists and specialist zooplankton feeders. Any dive shop Phuket has quite a few but a good one will have all the books and fish ID slates to be able to help you identify them or at least the more common ones. Then if that fails then we still have the internet too.

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. They lose their bony plates as they mature. uch a curious armoured stage is seen in only one other family of fish, the scats (Scatophagidae). The condition in the Similan Islands and the water all around Thailand make this such a perfect place for fish like the butterfly fish to live which is what makes Similan islands diving as popular as it is.

Taxonomy, systematics and evolution