ButterflyfishesChaetodontidae

Much of this family eats coral, and eats it one polyp at a time. The snout ends in a brush of fine teeth, which is what the family name means, and the fish picks individual polyps out of the surface of a living colony or clips the crowns off tube worms, taking a fraction of a meal from a hundred colonies in a morning and killing none of them. It is the opposite of how anything else on the reef feeds.

Some species can switch to other food. Several cannot, and that turns them into an instrument. An obligate coral feeder has no fallback, so when the coral dies the fish goes even though the reef still looks like a reef: the skeleton stands, the shape is unchanged, and the butterflyfish are simply not there anymore. Survey teams count them for exactly that reason. The number drops before the structure does, and after a bleaching event the coral eaters thin out first while the generalists stay put.

They go in twos. A butterflyfish pair is not a courtship that ends; it can hold for years, and the two feed within a body length of each other, work the same ground day after day and search when they are separated. For an animal whose food is scattered in small fixed amounts across a territory, two fish that know every colony on it and can drive off an intruder together are worth more than either alone.

The family they get confused with is the angelfishes, and one mark settles it: an angelfish carries a spine at the lower corner of the cheek and a butterflyfish does not. Fifteen species are listed here and several of them, including the two you will see most, live here and nowhere else. Their young are worth a footnote: the larva swims in a case of bony plates over the head and shoulders, armor that no adult has and that it sheds when it settles onto a reef.