The symbiotic relationship between the Hawaiian bobtail squid Euprymna scolopes and the marine gram-negative bacterium Aliivibrio fischeri has been well studied. The two organisms exhibit a mutualistic relationship in which bioluminescence produced by A. fischeri helps to attract pray to the squid host, which provides nutrient-rich tissues and a protected environment forA. fischeri. Bioluminescence provided by A. fischeri also aids in the defense of the squid E. scolopes by providing  camouflage during its nighttime foraging activity. Following bacterial colonization, the specialized organs of the squid undergo developmental changes and a relationship becomes established. The squid expels 90% of the bacterial population each morning, because it no longer needs to produce bioluminescence in the daylight. This expulsion benefits the bacteria by aiding in their dissemination. A single expulsion by one bobtail squid produces enough bacterial symbionts to fill 10,000m of seawater at a concentration that is comparable to what is found in coastal waters. Thus, in at least some habitats, the symbiotic relationship between A. fischeri and E. scolopes plays a key role in determining the abundance and distribution of E. scolopes. There is a higher abundance of A. fischeri in the vicinity of a population of E. scolopes and this abundance markedly decreases with increasing distance from the host's habitat.
Bioluminescent Photobacterium species also engage in mutually beneficial associations with fish and squid. Dense populations of P. kishitanii, P. leiogathi, and P. mandapamensis can live in the light organs of marine fish and squid, and are provided with nutrients and oxygen for reproduction  in return for  providing bioluminescence to their hosts, which can aid in sex-specific signaling, predator avoidance, locating or attracting prey, and schooling.<!-- Empty reference