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Squid’s Night Out With Vibro fischeri
The Hawaiian bobtail squid lives just off the coast of Hawaii in shallow, knee deep water. It’s nocturnal. During the day the squid buries itself in the sand and sleeps, coming out at night to hunt. Vibrio fischeri is a bioluminescent marine bacterium that lives as the squid’s symbiont; glowing in the dark like a firefly, but with blue light rather than yellow because the color blue travels far in water.

The little bobtail squid has two lobes under its mantle that function as light organs. The vibrios live in these lobes and glow. In exchange the quid feeds them. Its light organs are loaded with proteins and sugars, a candy store for bacteria. 

The V. fischeri have a much richer, happier, fatter life in the squid than fending for themselves in the open ocean. From the bacterium’s point of view, bioluminescence means free food. And the squid wants to keep these bacteria happy because it uses their light as an anti-predation device.

On very bright nights when there’s a lot of starlight or moonlight, the light penetrates the pools of water. The squid has detectors on its back that tells it exactly how much light is hitting the water. Using its ink sack as a shutter, the squid lets out exactly the same amount of V. fischeri-generated light below as that coming from above so the squid doesn’t make a shadow. In other words, the squid counter illuminates itself against predators that could find and kill it if they saw a shadow. 

When the sun comes up, the squid pumps out 95% of the bacteria. The number of V. fischeri plummets and the light goes out. During the day the bacteria replicate and by night the light comes on exactly when the squid needs it. The whole system, a squid “chemostat,” is controlled by circadian rhythm.

V. fischeri only light up when there are a lot of them. Scientists figured out why about forty years ago. It’s a social thing. Bacteria tend to be gregarious; preferring to crowd together in multicellular communities rather than lead solitary, reclusive lives. Vibros are no exception, they chatter continuously and their words are chemical. When there are enough V. fischeri in the squid’s light organs, a communication system called “quorum sensing” kicks in and all the bacteria light up at once. When their numbers drop in the morning, the light goes off. 

Bacteria have been the main form of life on our planet for nearly 4 billion years. During that time they’ve evolved an extraordinary variety of biochemical capabilities ¾language and bioluminance are only two of them. About a billion years ago, a great experiment occurred:

Bacteria and another great domain of life, the Archaea, came together in a fusion event that resulted in beings called Eukarya; which include squid and we humans. Squid, like us, developed all sorts of complicated and mutually beneficial relationships with bacteria. The friendship between the bobtail squid and V. fischeri is only one example of all the wonderful things going on out there.

Further reading:

Bassler, B.L., Losick,R. 2006. Bacterially Speaking. Cell, 125:237-246

Bassler, B. L. 2002. Small talk: cell-to-cell communication in bacteria. Cell 109: 421-424.

Engebrecht, J. & M. Silverman. 1984. Identification of genes and gene products necessary for bacterial bioluminescence. Proc. Natl. Acad. Sci. USA 81: 4154-4158.

Henke, J. M. & B. L. Bassler. 2004. Bacterial social engagements. Trends Cell Biol. 14: 648-656. 

Nee,S. 2005. The great chain of being. Nature. 435:429. 

Taga, M. E. & B. L. Bassler. 2003. Chemical communication among bacteria. Proc. Natl. Acad. Sci. USA 100(suppl.2): 14549-14554.

Marcia Stone is a science writer based in New York City. More of her work can be seen on http://www.mstoneworks.net

 
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