VCG Photos
Written by Ava Steger ‘27
Edited by Helen Chow ‘26
In the shallow waters of the Pacific Ocean, one marine organism defies the laws of the universe. It swims around in the ocean, hunting for food and hiding from predators as most organisms do, but there is one thing different - this animal has no shadow. As it swims under the light of the moon, light seems to pass straight through, as if it is a ghost. This unique, mystical ability belongs to the Hawaiian bobtail squid and its secret is not magic; it’s symbiosis.
The Hawaiian bobtail squid’s ability to hide its shadow has nothing to do with fantasy, and everything to do with a little organism called Vibrio fischeri. These squids have evolved a symbiotic, or beneficial, relationship with Vibrio fischeri, which are bacteria known to produce bioluminescence, or light made by living organisms [1]. These microorganisms live in something called a light organ, which was not originally meant to hold them, but has evolved over time to become the perfect environment for Vibrio colonies. The squid must take careful action to provide a productive space for the bacteria, as when the squid is born, this light organ does not even exist - it is actually a vestigial ink sack [1]. The organ gains its purpose and title of a light organ when Vibrio fischeri floating in the ocean sneaks into the ink sack, originally making up less than 0.1% of the squid’s microbiome [1]. This process is assisted by the squid, as the squid begins to secrete mucus into the ink sack that holds chemicals that kill other bacteria, but encourage growth of Vibrio fischeri.
Over the period of four weeks, Vibrio fischeri slowly begins to colonize the ink sack, outcompeting other microorganisms [1]. Not only does the ink sack change, but the bobtail squid also becomes nocturnal and its diet shifts to include foods that benefit the bioluminescence of the bacteria. Specifically, the enzyme that Vibrio fischeri uses for luminescence, called bacterial luciferase, requires air to release photons of light and the bobtail squid responds to this need for air by switching from digesting food through respiration to fermentation, which increases the amount of oxygen available [1]. The squid will then pump this oxygen into the light organ, which stimulates the luciferase to produce light.
This intricate relationship between bacteria and squid is further complicated as the need for light produced from the light organ fluctuates throughout the night and day. As the squid hunts during the night, it needs a lot of light to be produced by the Vibrio to hide the squid’s shadow. To do this, the bobtail squid releases high levels of oxygen into the light organ around dusk, right as it begins to hunt. This stimulates bright light and, as the night goes on, the oxygen is used up and the luminescence lowers with it [1,2]. Not only does the light fluctuate, but the level of Vibrio fischerii in the light organ also fluctuates throughout the 24 hour cycle. During the night, the Vibrio fischerii stays constant to produce the light, but immediately after sunrise, almost all of the bacteria is pushed out of the light organ and back into the ocean. As the next night begins, the remaining couple cells repopulate the light organ, using the influx of oxygen and acidic nutrients [2]. This process is repeated each day for the squid’s entire life.
This fascinating relationship between the Hawaiian bobtail squid and Vibrio fischerii have provided a prime example of the intricacy of symbiotic relationships and microbiomes. In nature, there is always so much more happening than what meets the eye. It is exciting to think about how many other complex symbiotic relationships are waiting to be discovered.
References
Nyholm, S.V., McFall-Ngai, M.J. A lasting symbiosis: how the Hawaiian bobtail squid finds and keeps its bioluminescent bacterial partner. Nat Rev Microbiol 19, 666–679 (2021). https://doi.org/10.1038/s41579-021-00567-y
Boettcher, K.J., Ruby, E.G. & McFall-Ngai, M.J. Bioluminescence in the symbiotic squid Euprymna scolopes is controlled by a daily biological rhythm. J Comp Physiol A 179, 65–73 (1996). https://doi.org/10.1007/BF00193435
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