Sometimes people decide to name animals the strangest things. I’m partial to oddly-named animals, so all you have to do is scroll through my past entries to find some weird names. Still, sometimes people name a thing and it kind of makes sense. Such is the case with today’s group of animals, the butterflyfish.

Butterflyfish are part of the family Chaetodontidae, along with bannerfishes and coralfishes. There are about 120 species of butterflyfish, and almost all of them are brightly coloured, in shades of blue, red, orange and yellow. They live mostly in tropical waters, and are particularly abundant in the Indo-west Pacific ocean. Most species live around coral reefs; I have a feeling that butterflyfish are one of the main reason I think of coral reefs as so bright and colourful.

A yellow longnose butterflyfish with predator fooling markings. Its long jaws are used to grab food from nooks and crannies.
Image By BEDO (Thailand) – Own work, CC BY-SA 4.0

Butterflyfish vary in size from nine to thirty centimetres, with the lined butterflyfish and the saddle butterflyfish being the largest species. All species of butterflyfish are very laterally compressed, meaning they are thin if you look at them head on. Some fish also have very strangely shaped jaws, which are adapted to the fish’s feeding habits. The jaws can make up over 25% of a fish’s length in some species.

There is some debate as to why these fish are so brightly coloured. One explanation is that the colours help in interspecies communication. In one species, the raccoon butterfly, when aggression occurs between two fish, the yellow colour on the fish is exaggerated, so this might be a plausible theory. Another interesting aspect of butterflyfish coloration is the strange patterns some fish have. In many species, this consists of dark colours around the head, and an eye spot by the tail. This coloration may confuse predators, who attack the false eye thinking it’s the fish’s head, and then collapse in shock when the butterflyfish swims the other way.

They are such pretty fish! Image By Leonardo Stabile – Own work, Public Domain

Reproduction in butterflyfish occurs by external fertilization, as is generally the case with fish. They are monogamous, and breeding pairs may be stable for at least three years. The fish release their eggs into the water, and the poor things just float in the ocean, going wherever the currents take them. Once hatched, the fry have a very strange feature: they are covered in bony plates. These disappear as they grow up. Butterflyfish are the only fish other than scats to go through such armouring.

As brightly coloured, beautiful fish, I can see why these guys are called butterflyfish. I can also see why they are so popular in aquariums, though this had made some species vulnerable. Still, at least right now they’re still around to brighten up our oceans!

Cover image By Bernard E. Picton BernardP – Own work, CC BY-SA 4.0