Within the photophores (luminous organs) of some fishes and squid, symbiotic luminous bacteria produce the light. This does not happen in the crustaceans, however, there have been reports of infections in amphipod species, and two isopods rendering them temporarily luminous (Herring 1985).
The bacteria are the simplest bioluminescing organisms, they do not produce light alone but instead do so in large numbers (Herring 2002). The necessary compounds needed for the reaction exist externally of the prokaryotes and when the bacteria congregate in large aggregations light is emitted. This may happen once the bacteria have reproduced sufficiently on a food source, the bioluminescent glow invites higher order organisms to eat the bacteria; where they enter the gut and colonize (Herring 2002).
Vibrio fischeri is a well studied marine bacteria that exhibits similar behaviour. Large populations produce a substance known as autoinducer allowing them to bioluminesce as a colony (Herring 2002). Vibrio fischeri live symbiotically within marine species such as the Hawaiian bobtailed squid (Figure 4.1), in exchange for shelter and a stable environment they provide the necessary luciferins for chemiluminescent reactions within the squid’s photophores.