The Hawaiian Bobtail Squid uses bioluminescence in a unique way to avoid predators.
The squid harbours a population of Vivio Fischeri -a luminescent bacteria- in glands underneath its body. These bacteria, like many others, are able to communicate with one another through Quorum Sensing. Quorum Sensing involves the secretion of molecules called autoinducers into the extracellular environment. The bacteria then use specialised receptors to detect the concentration of autoinducers in their surroundings, which allows them to estimate the local population density of their own species or other bacteria-because other individuals in the area have also been secreting autoinducers.
What Quorom Sensing allows bacteria to do is to regulate gene activity on a community level. Once a certain population is reached, all the individuals in the colony know about it, and this information (the concentration of autoinducer) triggers the transcription of genes that were previously inactive. Viruses for example, only begin attacking the body when they know that there are enough of them around to actually have an effect on the host. Similarly, bioluminescence is only turned on when there is a threshold amount of bacteria in the glands of the squid.
Incredibly, the squid has managed to sync the bacterial bio-luminescence with its Circadian rhythm, expelling just the right amount of bacteria during the day (it would actually be harmful to keep them in for long periods), so that by night time, they’ve multiplied to exactly the right number to activate bio-luminescence via quorum sensing. Depending on the level of moonlight or starlight, the squid is then able to use this bio-luminescence to create the illusion that it does not create a shadow- allowing it to avoid predators.
And we thought humans were innovative.