MoonyfishesMonodactylidae

Most of this fish is fin, and the fins have scales on them. In nearly every other fish the scales stop where the fin starts and the fin itself is a membrane stretched on rays. In a moony they run out onto the tall dorsal and often onto the anal as well, so the two triangles that give the animal its outline are made of body rather than attached to it. Seen flat against the light it is a disc, sometimes taller than it is long, with a small head at one edge.

What is missing is underneath. The pelvic fins, which most fish use for braking and fine steering, are reduced in an adult to almost nothing. The family name reads as one finger, and the old English name is fingerfish, both of them pointing at those tall scaled fins.

This is not a reef animal. Moonies belong to the edge of the land: sheltered bays, harbor walls, tidal creeks and mangrove stands, in water shallow enough to stand in, usually as a loose school hanging against a vertical surface. They also move between salt, brackish and fresh water, which the species page takes apart properly.

That same flexibility is why this is a standard aquarium-shop fish worldwide, and one of the most reliably mishandled. A young mono is sold as a freshwater fish because a young mono tolerates fresh water. An adult is a marine fish, and the ones kept in a tropical community tank do not get to be adults.

One species is listed here, and it is the one fish in this stretch of the catalogue you are more likely to see from a jetty than from underneath it.