AngelfishesPomacanthidae

The name is about a spine, not about wings. At the lower back corner of the gill cover an angelfish carries a stout backward-pointing spike, and the family name means precisely that. It is the one reliable way to tell an angelfish from a butterflyfish, which is otherwise the same silhouette in the same place doing similar work, and it is not decoration: a fish that has backed into a crevice with that spine flared is a difficult thing to extract and an awkward thing to swallow.

Then there is what the big ones eat. Sponges make up much of the diet of the large angelfishes, and a sponge is designed to be inedible: the body is stiffened with needles of glass and loaded with chemicals whose whole purpose is to make an animal regret the mouthful. Almost nothing on a reef will touch one. Angelfish work through them with a small mouth full of brush teeth, along with sea squirts and the encrusting mat that covers everything else, which is about as narrow a living as a reef fish makes and the reason an angelfish in a home aquarium usually starves in slow motion.

They live in fixed arrangements rather than crowds. The large species go in pairs that hold the same stretch of reef for years and patrol it in a set circuit. The small ones live in harems, one male with a handful of females on a compact territory, and if that male disappears the largest female becomes him, which takes weeks and is permanent.

Seven species are listed here. Two of them, the Arabian angelfish and the yellowbar, belong to this corner of the world and are among the fish people come here to photograph, which is harder than it sounds: an angelfish that has decided you are watching it will keep a coral head between you and itself for as long as your air lasts.