III.

Light-Producing Organs

Creatures that use bioluminescence occur in two main categories: organisms that produce their own light and organisms that borrow light from symbiotic bioluminescent bacteria.

Many bioluminescent animals have special cells (called photocytes) that produce light using the species’ own enzymes and luciferins, or using luciferins obtained by eating other bioluminescent organisms. The luminescent tissue is often organized into complex light-producing structures called photophores. Photophores may include mirrors, lenses, shutters, and colored filters that modify and control the output of light. Most bioluminescent organisms produce flashes of light. Flashes may be a few milliseconds long or may be sustained for many seconds, with most lasting for a tenth of a second to a full second. In large animals, such as some jellyfish medusas, the flashes may involve spectacular patterns and whorls of light.

Fish such as flashlight fish found in the Philippines and squid such as the Hawaiian bobtail squid do not have light-producing organs of their own. Instead these animals have a symbiotic relationship with bioluminescent bacteria. The host animals keep the bacteria in special baglike organs where the microbes thrive on nutrients and oxygen from the animal's blood. In return, the light produced by the bacteria can serve as a way for the host to attract prey or mates, or to confuse predators. Bacterial luminescence is usually a continuous glow so these squid and fish have evolved special mechanisms to turn off or block the borrowed light.