Bioluminescence

Light produced by living organisms; on night dives, plankton flash blue-green sparks around every movement when divers cover their lights.

Bioluminescence is light produced chemically by living organisms, in the ocean most visibly by dinoflagellate plankton that flash when disturbed. On a night dive, cover your dive light, wave a hand, and the water erupts in blue-green sparks trailing every movement, one of the most magical and memorable sights in diving, and completely free at many sites on the right night.

The chemistry is a reaction between luciferin and the enzyme luciferase, and it is astonishingly widespread: an estimated three quarters of deep-sea species produce light for defense, hunting, or signaling. Divers meet it as sparkling plankton, glowing comb jellies, flashlight fish with luminous cheek organs, and in special spots like the Puerto Rico bioluminescent bays or Toyama Bay's firefly squid.

To see it best, choose a dark night around the new moon, let your eyes adapt, and shield your torch, some guides run a lights-off minute at the end of a night dive for exactly this. A related spectacle, the ostracod string-of-pearls display in the Caribbean, appears shortly after sunset a few days after the full moon. Worth a line in the dive log when you catch it.