05 June 2018

Symbiosis special: What makes the Hawaiian bobtail squid glow?

Photo by Todd Bretl, Monterey Bay Aquarium
As adorable as the endemic Hawaiian bobtail squid Euprymna scolopes can be, its famed relationship with a microbe is even more special: among thousands of marine bacteria, only one microbe, Vibrio fischeri, is known to successfully colonize the squid's light organ within its mantle, turning the squid into an enchanting and 'disappearing' luminescent creature [1-4,8]. 

Few hours after the hatchlings are out from their egg case, V. fischeri from the surrounding water begin to enter the pores on either side of the light organ, settling on the epithelium-lined inner crypt spaces (tiny spaces within the organ)  [1,2,5].

Specificity is achieved early on through a mutual dialogue between the young host and the symbiont (i.e., agreement on entry and attachment of the microbe). Once the microbes have established, this transformation triggers a series of biological changes in both organisms, strengthening their relationship [7].

Now, the squid matures, and as if charging a weapon for the night, V. fishceri reaches its highest concentration in the light organ. And the squid shines its brightest [2,6].

While the squid hunts for prey, the microbes perfectly match the intensity of the moonlight welling down from above, reducing the squid's silhouette, ultimately giving it an 'invisibility cloak' (counter-illumination) against predators seeing from below [5,8]. What's more interesting is that the host is equipped not only to detect but to also control the amount of light emitted by the bacteria through its specialized light organ features [5]. 

Here's a video from Ed Yong (The Atlantic), illustrating this fascinating partnership.

As the dawn breaks, it secretes from its light organ a thick mucous containing 95% of its symbionts back into the sea, while the rest of the microbes replenish themselves to start another cycle [2,6]. Over the rest of the day, the squid becomes dormant and retreats into the sand [6].

For over 20 years, this squid-vibrio relationship has been key in studying many biological phenomena, like cephalopod development and the structure of tissue interacting with light [6]; this one-on-one connection has also been crucial in understanding host-microbe interactions in a natural microenvironment [1,2]. 

Beyond this, the squid-vibrio partnership is important, because, it turns out, the microbe 'remakes' and protects its host: reaching the adult state is only possible when the squid harbors the right microbial ally [8]. 

And, charmingly, the squid need not look further.

To know more about bobtail squids, visit SeaLifeBase.

[1] Rader, B. A., & Nyholm, S. V. (2012). Host/microbe interactions revealed through “omics” in the symbiosis between the Hawaiian bobtail squid Euprymna scolopes and the bioluminescent bacterium Vibrio fischeriThe Biological Bulletin, 223(1), 103-111.

[2] Schleicher, T. R., & Nyholm, S. V. (2011). Characterizing the host and symbiont proteomes in the association between the Bobtail squid, Euprymna scolopes, and the bacterium, Vibrio fischeriPLoS One, 6(10), e25649.

[3] Boettcher, K. J., & Ruby, E. G. (1990). Depressed light emission by symbiotic Vibrio fischeri of the sepiolid squid Euprymna scolopes. Journal of Bacteriology, 172(7), 3701-3706.

[4] Yazzie, N., Salazar, K. A., & Castillo, M. G. (2015). Identification, molecular characterization, and gene expression analysis of a CD109 molecule in the Hawaiian bobtail squid Euprymna scolopesFish & Shellfish Immunology, 44(1), 342-355.

[5] Peyer, S. M., Pankey, M. S., Oakley, T. H., & McFall-Ngai, M. J. (2014). Eye-specification genes in the bacterial light organ of the bobtail squid Euprymna scolopes, and their expression in response to symbiont cues. Mechanisms of Development, 131, 111-126.

[6] McFall-Ngai, M. (2014). Divining the essence of symbiosis: insights from the squid-vibrio model. PLoS Biology, 12(2), e1001783.

[7] Visick, K. L., & McFall-Ngai, M. J. (2000). An exclusive contract: specificity in the Vibrio fischeri-Euprymna scolopes partnership. Journal of Bacteriology, 182(7), 1779-1787.

[8] Yong, E. (26, Jan 2018). The lovely tale of an adorable squid and its glowing partner. The Atlantic. Retrieved from https://www.theatlantic.com/science/archive/2018/01/the-lovely-tale-of-an-adorable-squid-and-its-glowing-partner/551549/