Sea ghosts.

Optic Topic (Luminescence) : Light lakes

In the Australian summer of 2008/2009, a hauntingly beautiful phenomenon illuminated the Gippsland Lakes and created scenes that those present will never forget. An unusually high concentration of Noctiluca scintillans, (a species of dinoflagellate that feed on algae, plankton and bacteria) a bioluminescent microorganism, turned the water a bright, glowing, ethereal blue.

N. scintillans uses its bioluminescence as a defense mechanism, lighting up when it senses a predator coming near. The ghostly glow attracts even larger predators to eat the first predator, keeping the N. scintillans safe to glow another day.

Here is a video that shows how it works,