One of the most striking features of bioluminescence is the enormous diversity of organisms that have developed the ability to emit light. Bioluminescent organisms range in size from bacteria less than one micrometre in length to the five metre-long 'Megamouth' shark. Despite their tremendous diversity, all bioluminescent organisms produce light via the same process. As anyone who has ever touched a burning electric lightbulb knows, most lightsources radiate a lot of heat, reducing the amount of free energy that can be converted into light. Virtually all of the energy generated by the luciferin-luciferase reaction is converted into light with almost none lost in heat or sound production.
Bioluminescence is literally a 'cold fire'.
Bioluminescent creatures can be divided into two basic categories, based on the type of light organ they possess. Light is produced by specialized photogenic cells and reflected through a 'lens' and clear outer covering, producing a gentle, even glow whose intensity can be precisely modulated. The second category 'borrows' the luminosity of bacterial symbionts. Subcutaneous bags of bioluminescent bacteria are maintained and nurtured by the host organism in exchange for a more brilliant level of light. The host is unable to control the intensity of light produced by the bacteria, and so organisms with bacterial light organs have evolved some fascinating mechanisms to 'turn out their lights' when they would be disadvantageous.