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Bacterial Symbionts
This website was created as a project for and undergraduate Biology Program at Davidson College.
Last Updated 10.25.2005

Symbiotic bacteria

Sepiolid squids are an example of the use of extrinsic luminescence from bacteria that live symbiotically with the organism. Vibrio and Photobacterium are the two most widely utilized genera. The bilobed light organ is complex and resides in the mantle portion of the squid. Invaginated portions of ectoderm house the bacteria in what are known as ‘crypts’. Light produced is reflected and focused ventrally by reflective tissue and lenses. An ink sac functions like a diaphragm to control the intensity of the light emitted. This unique morphology allows the squid to perform counterillumination, broadcasting varying light patterns to potential predators viewing them from below.

Due to the likely high metabolic costs of maintaining a complete bacterial population in the light organ, 95% of the bacteria or 10^12, are expelled with the arrival of sunlight each day. The squid then bury themselves in the sand during the day, building up the numbers of bacteria slowly over a number of hours until the bioluminescence is needed in the nighttime (Graf 2005).
Euprymna scolopes exhibits this mutualism and also displays the fascinating phenomenon of counterilluminescence.

Figure (above): Sepioteuthis lessoniana Photographed by Roger Hanlon

Figure (below): Sepioteuthis sepioidea Photographed by Roger Hanlon.