Consider the fact that darkness continuously shrouds more than two thirds of the earth, since light does not penetrate much below watery depths of 1000 metres, and only does so in the cleanest and clearest of tropical seas. Consider further that the sun’s rays illuminate any earthly surface during only half the day, and one can understand the evolutionary selection for animals to make their own light. Many do just that.

At depths of a few hundred metres or more, sunlight is restricted to a narrow spectral range of between 460 nm and 490 nm. Not surprisingly, bioluminescence emitted by almost all animals at this depth and deeper, even to abyssal depths, has a similar spectral range, with a peak at approximately 475 nm. Evolutionarily, it is the exception that proves most interesting.

The stomiid fish as represented by Malacosteus niger, the deep sea dragon fish, on this month’s cover, have two different forms of bioluminescence. The usual bluish range of emission is produced by a retro-orbital light organ, but in addition to this organ there is an infraorbital organ located beneath the eye, as seen …