Millions of years of evolution have given the ability to some kind of plankton to glow in the dark. It is a defense mechanism called bioluminescence or chemiluminescence, that some creature on land also have.
Moreover, zoo plankton and single-celled animal plankton sometimes create amazing spots for divers and snorkelers when thousands of them glow next to the seashore.
Why do they glow ? How does it work to estrange predators ?