Glowing squid use bacterial flashlights that double as an extra pair of "eyes"

In the darkness of the deep ocean, some animals create their own light. Among these is the Hawaiian bobtail squid Euprymna scolopes, which forms a partnership with the luminous bacterium Vibrio fischeri. The squid houses colonies of these bacteria in special light organs, and it can control the brightness and direction of their illuminations. But these organs do much more than produce light - they detect it too.

Deyan Tong from the University of Wisconsin has discovered that the organs generate nervous signals when they sense light and they're loaded with proteins responsible for detecting it. The light organs are effectively an extra set of primitive eyes, each equipped with its own "iris" and "lens".  The squid comes equipped with a pair of living, 'seeing' flashlights.

Scientists have studied the light organ of E.scolopes for over 20 years and its similarity to an actual eye hasn't gone unnoticed. The core of the organ where the bacteria live is surrounded by a reflective layer of tissue and part of the squid's ink sac. These can expand and contract like an iris to control how much light escapes the core. The entire package is covered by a thick, transparent tissue - a "lens" - which diffuses the light produced by the bacteria.

The similarities aren't just superficial ones; these structures contain arrays of proteins that are remarkably similar to their counterparts in actual eyes. Tong analysed the genes that are switched on in the light organs of young E.scolopes. He found 11 that are very similar to those used by proper eyes to convert light into electrical signals. The proteins they produce, such as the visual pigment opsin, arrestin and rhodopsin kinase, strongly resemble their eye counterparts too and are found in the same parts of the light organ.

And as a final piece of evidence, when Tong found that the light organs produced electrical signals when he shone pulses of white light onto them, even when they were devoid of bacteria.

Tong suggests several different uses that the squid could have for its extra pair of light-detectors. For a start, the ability to sense light could be part of a feedback loop that controls the development of the light organ itself. The glow given off by the bacteria is a trigger that tells the organ to switch from a juvenile phase designed to recruit the luminous microbes, to an adult version designed to control their light. The ability to sense light allows the squid to know when to make that switch.

The organ could also be used to detect light from the environment, to help the squid with an act of deception called "counterillumination" where it actually produces light in order to make itself less visible. A small amount of natural light filters down to the depths where it lives, and against these rays, the squid's silhouette would be obvious to any predator watching from below. To avoid sticking out, the squid gives off light from its underside to match the natural light welling down around it. It's possible that by functioning as sensors as well as flashlights, the light organs are better equipped to match the natural light around them. 

Most intriguingly, the ability to detect light could help the squid to manage its bacterial partners. Producing light costs energy and groups of luminous bacteria are vulnerable to infiltration by "dark mutants" that gain the benefits of colony life without actually contributing any light of their own. The rise of dark mutants spells trouble for the squid and previous studies have found that juveniles don't allow these cheaters to stick around. Perhaps the light-detecting ability of their light organs allows them to weed out these slackers.

This is the first study of its kind so it's impossible to say if the light organs of other squids or indeed, of other deep-sea animals, have similar abilities. For the moment, the most interesting mystery is how the light organs evolved. Given that they share many of the same active genes and proteins, the most enticing possibility is that they arose by co-opting the developmental programmes that produce the squid's actual eyes.

Reference: PNAS 10.1073/pnas.0904571106

Images: bobtail squid by William Ormerod; eye image from PNAS

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