CowriesCypraeidae

Cowries are the shells everyone knows by touch: glossy, egg-shaped, with the spire buried under later growth and a long toothed slit running the length of the underside. The polish is not wear. The living animal wraps two flaps of mantle up over the outside of the shell and lays down fresh material from the outer surface, so the shell is being repaired and repainted from the outside for as long as the snail lives.

That mantle is what divers miss. A cowrie out at night is covered in fleshy tissue studded with papillae, often nothing like the shell underneath in color, and it withdraws in seconds under a torch beam - which is why most people only ever see the bare shell and assume the animal is drab.

They were money for longer than coins have existed. The shells of two small species circulated as currency across Africa and Asia for centuries, which is where their names come from, and they remain the most collected marine shells in the world. The pressure that puts on the larger species is the reason a living cowrie is worth leaving on the reef.