Description and ecology

Description and ecology

A school of false moorish idols, Heniochus diphreutes

Butterflyfish mostly range from 12 to 22 centimetres (4.7 to 8.7 in) in length. The largest species, the lined butterflyfish and the saddle butterflyfish, C. ephippium, grow to 30 centimetres (12 in). The common name references the brightly coloured and strikingly patterned bodies of many species, bearing shades of black, white, blue, red, orange and yellow. Other 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.^[2] Their deep, laterally narrow bodies are easily noticed through the profusion of reef life. The conspicuous coloration of butterflyfish may be 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 waters of less than 18 metres (59 ft) (though some species descend to 180 metres (590 ft)), butterflyfish stick to particular home ranges. The corallivores are especially territorial, forming mated pairs and staking claim to a specific coral head. Contrastingly, the zooplankton feeders form large conspecific groups. By night butterflyfish hide in reef crevices exhibit markedly different coloration.

Their coloration also makes butterflyfish popular aquarium fish. However, most species feed on coral polyps and sea anemones. Balancing the relative populations of prey and predator is complex, leading Hobby aquarists to focus on the few generalists and specialist zooplankton feeders.

Butterflyfish are pelagic spawners; that is, they release many buoyant eggs into the water which 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 post-larval fish is covered in large bony plates extending from the head. They lose their bony plates as they mature.^[2] Only one other family of fish, the scats (Scatophagidae) express such an armored stage.

Etymology

The family name derives from the Ancient Greek words chaite ("hair") and odontos ("tooth"). This is an allusion to the rows of brush-like teeth found in their small, protrusile mouths.

Taxonomy, systematics and evolution

The Chaetodontidae can be, but is not usually, divided into two lineages that arguably are subfamilies. The subfamily name Chaetodontinae is a little-used leftover from the period when the Pomacanthidae and Chaetodontidae were united under the latter name as a single family. Hence, Chaetodontinae is today considered a junior synonym of Chaetodontidae. In any case, one lineage of Chaetodontidae (in the modern sense) contains the "typical" butterflyfishes around Chaetodon, while the other unites the bannerfish and coralfish genera. As the "Perciformes" are highly paraphyletic, the precise relationships of the Chaetodontidae as a whole are badly resolved.^[3]

Before DNA sequencing, the taxonomy was confused about whether to treat these as species or subspecies. Also, numerous subgenera have been proposed for splitting out of Chaetodon, and it is becoming clear how to subdivide the genus if that is desired.^[4]

The fossil record of this group is marginal. Their restriction to coral reefs means that their carcasses are liable to be dispersed by scavengers, overgrown by corals, and any that do fossilize will not long survive erosion. However, there is Pygaeus, a very basal fossil from the mid-late Eocene of Europe, dating approximately from the Bartonian 40-37 million years ago (mya). Thus, the Chaetodontidae emerged probably in the early-mid Eocene. A crude molecular clock in combination with the evidence given by Pygaeus allows to place the initial split between the two main lineages to the mid-late Eocene, and together with the few other fossils it allows to deduce that most living genera were probably distinct by the end of the Paleogene 23 mya.^[5]