Noctiluca scintillans
It shows bioluminescence when disturbed: this produces a "milky seas effect". The bioluminescence is produced in the cytoplasm of this single-celled protist. It is done by a luciferin-luciferase reaction in thousands of spherical organelles, called scintillons.  Nonluminescent populations in the genus Noctiluca lack these scintillons.