Bacteria are the simplest luminescent organisms found in the ocean (Herring 2002). Bioluminescence is found in 4 genera of bacteria, 3 of which are marine: Vibrio, Photobacterium and Shewanella (Thouand et al. 2002).
Perhaps the most studied of these are Vibrio fischeri. Individual organisms do not show any signs of bioluminescence until large densities of the bacteria are present (Herring 2002). Each individual bacteria produces an extracellular chemical (an autoinducer) which stimulates the luciferase when a certain concentration is reached, initiating the light producing reaction. As individuals these organisms would not produce enough light to be visible to any other organism resulting in wasted energy. Instead they congregate on particulate matter and only biolumines when in high enough densities for their combined luminescence to attract predators to the particle. The predation upon the particle allows the bacteria to enter the host’s gut; an optimum environment for their growth (Herring 2002).
A substantial minority of organisms that utilise bioluminescence do not actually produce light themselves, but harness it from bioluminescent bacteria in symbiotic relationships (Herring 1977). The bacteria are concentrated in specialised light organs known as photophores (Haygood 1993), of which there are usually only one or two (Herring 2002). These light organs can be situated throughout the body, including being attached to the intestinal tract, or be external as seen in various species of anglerfish. There must always be some method of controlling the light, which usually involves some form of shutter or an alternative method of hiding the light. An opening to the external environment must also be present to allow dead or surplus bacteria to be disposed of (Haygood 1993).
As previously stated this is a symbiotic relationship which means that there must be some gain for both symbionts. In this case the host organism gains the ability to produce light, while the bacteria are provided with a protected and a nutrient rich environment (Nealson & Hasting 1979). The light gained by the host organism does not come without its costs. Herring (1977) explains that costs include: maintaining the bacteria at optimum conditions for light production, containing the bacteria in a localised area and either passing the correct bacteria onto the next generation or the next generation having a way of acquiring the bacteria anew.