around "Bacterial Illumination"(in http://www.this-magic-sea.com/COMBACTL.HTM)
"Squid Communicate with each other using a wide spectrum of colors and postures. At night they signal with luminescent organs.
The squid's luminescent communications result from communications between bacteria living in special light organs.
Squid, and a variety of fish, have luminescent bacteria housed in special sacs called light organs. The bacteria generate light with an enzyme called luciferase - but not all the time.
When the bacteria Vibrio fischeri swim free in sea water, their population densities are very low and they do not produce light. When they inhabit the light organs of squid, however, the squid cells nurture the bacteria and their populations soar to high levels. When the bacteria are packed tight, they begin to secrete messenger proteins called autoinducer. As this reaches a critical level, it enters the bacteria and activates a protein called LuxR.
Activated LuxR stimulates the bacterial genes to produce the light producing proteins luciferin, luciferase and associated light producing proteins.
The genes are stimulated to produce additional autoinducer messengers and these are released from the bacteria. The additional production of autoinducer messengers, triggered by the active LuxR, stimulate another round of protein synthesis. This positive feedback loop results in rapid production of light producing proteins. When these proteins reach a critical density, they react with each other to liberate that delightfully eerie blue phosphorescence of the sea."
As any squid knows, visual communication is a wonderful way to convey a message. It has a major downside, though; predators can tune in to the broadcast just as readily as the intended recipients (other squid) can. A recent study by Lydia M. Mäthger and Roger T. Hanlon, both biologists at the Marine Biological Laboratory in Woods Hole, Massachusetts, suggests that squid—and most likely their close relatives, cuttlefish and octopuses—have evolved a secret communication channel to which their predators are oblivious.
Squid, cuttlefish, and octopuses are known for their ability to change their skin color in a spectacular way. They can blend instantly into the background or produce a startling array of patterns and hues to express their physiological or motivational state. The secret to the show is the two distinct layers of cephalopod skin: The inner layer of iridophore cells is both iridescent and reflects polarized light. The outer layer is made up of pigmented organs, or chromatophores, which expand or contract to help change the color or pattern of the skin.
Cuttlefish, octopuses, and squid have a visual system to match the complexity of their skin. Unlike their vertebrate predators, they can detect differences in polarized light. Mäthger and Hanlon discovered that the two skin layers work independently, and that by taking advantage of the reflective properties of the iridophores, squid may be able to communicate with other squid via polarized light. At the same time, the squid can camouflage themselves from predators by altering the color pattern in the chromatophore layer, through which polarized light travels freely. What happens among squid stays among squid!"
http://www.tonmo.com/forums/showthread.php?8667-Secret-squid-communication
