Lyn Bently of Dunedin, asks :-

What causes the flashes of light seen in the sea on some dark nights?

Barrie Peake, a marine chemist at Otago University, responded.

These flashes of light that you have observed in the sea are in fact coming from chemical reactions in a wide variety of marine species living in the sea (particularly certain types of microscopic phytoplankton) that give out light. This process is called bioluminescence and because the intensity of the light is very weak, it is most easily seen on a dark night. Such reactions are also referred to as being chemiluminescent because the energy to form the light comes solely from the chemical reaction itself.

Bioluminescence is also the mechanism responsible for light emitted by glow-worms and insects such as fireflies. Bioluminescence should be carefully distinguished from fluorescence or phosphorescence where the energy comes from light first shining on the organism and then being remitted typically at a different wavelength and hence colour.

At least two chemicals are involved in the light-emitting reactions involved in bioluminescence. These are given the common names of "luciferin" which is the compound actually producing the light and "luciferase" which drives or catalyzes the reaction typically in the presence of oxygen. These compounds are often associated with other biological chemicals such as proteins and their exact chemical nature varies with the type of biological organism.

Although the behavioral or ecological role of the emitted light remains unknown for many marine species, it is often seen as a means of communication. For example, in some types of plankton, bioluminescence occurs when the organisms are disturbed such as by wave action and is therefore considered to be a type of predator defence. It can also be observed in some fish living at great depth (1000 - 4000 m) where it is believed it might help in species recognition in the darkness.

An excellent web site with further details on bioluminescence including photos of light emitting organisms is 'The Bioluminescence Web Page' with URL of http://lifesci.ucsb.edu/~biolum/ .