In the oceans, secret messages are being exchanged all the time. Some are obvious if you look closely enough, or at the right angle, or with sharp enough eyes. But mantis shrimps—a group of belligerent crustaceans—exchange signals that are so secretive that only they can see them.

Mantis shrimps have arguably the weirdest eyes in the oceans. Each eye can focus on objects with three separate regions, giving the animal ‘trinocular’ depth perception with a single eye. Some species have 12 types of light-sensitive cells, each tuned to a different colour; we have just three. They can even use filters to tune these cells to local light conditions.

Their eyes can also detect polarised light, which vibrates in a