Contributed by Kelly Costopoulos, Taylor Werkema, and Nina Zook.
One of the most spectacular biological phenomena is an organism’s ability to luminesce. The light can be produced through chemical reactions in specific cells and even, in more rare cases, through a symbiotic relationship of an organism and unique bacteria. An extraordinary example of this is the cooperative relationship between the Bobtail squid and the bacteria Vibrio fisheri.
The Bobtail squid lives in the coastal waters off the Pacific Ocean and some parts of the Indian Ocean. During the day the squid buries itself in the sand and at night it comes out to hunt. The squid mimics the moonlight by using luminescent V. fischeri and eliminates its shadow on the ocean floor below to avoid predators. It’s nature’s own invisibility cloak!
This incredible act starts every morning when free living bacteria in the water are taken up by special light organs in the squid’s mantle. There, they are nourished by special cells that promote the growth of only V. fischeri, all other competitors are actively selected against. Once the bacteria population reaches a certain density, the bacteria produce light. The squid has the ability to control the intensity of light through specialized filters to match the moon’s light exactly. When the sun comes up in the morning, the squid expels the bacteria in a process known as venting, and the cycle starts over.
The video above gives a graphical representation of how the squid uses its invisibility cloak. As the moon comes out, the squid becomes luminescent and its shadow on the ocean floor disappears. This hides the squid from predators below.
The bobtail squid is only one of many squid species in the sepiolid family with V. fischeri symbionts. How did such an intricate symbiosis evolve in these squid species? Because both organisms can be easily cultured and raised in the laboratory setting, researchers can do evolutionary studies to shed light on how and why the sepiolid squid-V. fischeri system has evolved. Researchers have determined the relatedness between similar squid species and the relatedness of their symbiotic bacteria. Interestingly, the bacterial tree lines up with the squid tree, indicating that the squid and bacteria of each symbiotic pair have coevolved. This means that as squid species diverged from one another, so did their native bacteria such that the bacteria became more host-specific.
Evolutionary studies have also helped us understand how bacteria compete to establish symbiosis with the their squid hosts. Experimental competition, or raising a squid in the presence of two different bacteria, has shown that a squid’s native bacterial symbiont will outcompete nonnative strains. Further, the more closely related the nonnative strain is to the native bacteria, the more competitive it will be in inhabiting the squid’s light organ. Thus, the evolutionary history of sepiolid squid species and their unique symbionts is one of fidelity and preference for the native strain with whom the squid has evolved intimately over time.
For more on the squid-vibrio system, check out:
McFall-Ngai, M. 2008. Hawaiian bobtail squid. Current Biology : CB, 18(22), R1043–4.
Nishiguchi, M.K., Ruby, E.G., & McFall-Ngai, M.J. 1998. Competitive dominance among strains of luminous bacteria provides an unusual form of evidence for parallel evolution in sepiolid squid-vibrio symbioses. Applied and Environmental Microbiology 64(9): 3209-3213.
O’Brien, Miles, and Marsha Walton. “Glowing Squid.” National Science Foundation. The National Science Foundation, 22 Nov. 2010. Web. 28 Apr. 2014.