Butterfly fish (family Chaetodontidae) are some of the most colorful and varied fish of the oceans, the majority of which live on or close to coral reefs. Most species measure from 5-9.5 in (13-24 cm) in length and have deep, flattened bodies that are frequently adorned by extended fins. In some species these may form a large arc over the body. In addition to refinements in the body shape, the colors and patterns of most butterfly fish are quite enthralling. The head and body is usually a dark background color which is broken up by a series of stripes and other patterns. Coloration varies considerably but often includes patches of yellow, orange, blue, and white. A "false eye" is commonly seen on some butterfly fish; this is usually located towards the back of the fish or even on a fin, the objective being to distract a striking predator from the butterfly fish's own head.
Like the parrotfish and wrasses, butterfly fish swim by synchronous rowing strokes of the pectoral fins, while the tail fin is used as a rudder for direction and balance. They are capable of very rapid movement and rely largely on their agility to avoid capture from other larger species. Butterfly fish are strictly diurnal, exploiting the diversity of feeding and living spaces in and around coral reefs and atolls. As dusk approaches, however, most begin to seek out a safe hiding place where they will rest for the night.
Butterfly fish are specialist feeders: many species have the mouth placed at the end of a short, tubular snout that facilitates the animals poking into tiny crevices in coralline reefs and extracting prey from seemingly inaccessible places. Their diet may either be restricted to living polyps plucked from just a few coral species, while other members of the family spare the polyps themselves, preferring to graze on algae growing on the corals. Some even actively pursue small shrimps and copepods that lurk within the many crevices of the reef face.
The social behavior of butterfly fish varies according to the particular species. Many species are solitary, but some do form stable monogamous relationships with a member of the opposite sex. In the latter case, the two fish commonly patrol and defend a particular patch of coral against other members of the same species. A series of threat and aggressive gestures including color changes have evolved in these species which help prevent aggressive encounters from developing. Spawning frequently occurs at dusk—a strategy that may have evolved to increase the survival rates of young butterfly fish, as many of the tiny plankton feeders are less active in the evening.