by Edward Chen
figures by Jovana Andrejevic
On a sunny, nondescript Hawaiian day, a Vibrio fischeri bacterium arises the same as on most other nondescript days: homeless. It hurries along on a ride to work. No, not by car. Not onboard a trolley either. Yes! The great, dynamic Pacific Ocean current. Currency-free and open to all, it’s the road to opportunity and fortune for aquatic hard workers like the V. fischeri we are observing. Maybe even the acting job our protagonist hopes to get. If it had eyes, it would see that all around it were other applicants – after all, no job is guaranteed – other small, microscopic organisms that, like it, are homeless in an underwater, majority-homeless universe. The world really is teeming with life. For our silent, stubby V. fischeri, it’s not only a career on the line, but its livelihood. And not only its livelihood, but the livelihood of another animal we call the Hawaiian bobtail squid. Squid, meet V. fischeri. Bacterium, meet our reader. Hi!
What is the Hawaiian bobtail squid?
Hawaiian bobtail squids are small, primarily nocturnal creatures that hide by day and hunt by night. From dawn to dusk, they bury themselves under sand or mud to evade potential predators. Even so, their life is not as solitary as this habit might suggest. Instead, they associate with a single species of bacteria that literally lives inside them. Hence the casting call. (Trillions of bacteria live within you too.) The bacteria, Vibrio fischeri, have enzymes that produce blue-green light. In exchange for their natural bioluminescence, the squid provides nutrients such as sugars and amino acids for the bacteria, as well as a home within its light organ.
But why trade food for light?
Imagine looking towards the sky. (Better yet, actually do it!) You will see sunlight or, if it is nighttime, moonlight and starlight. If there is a bird flying nearby, why can you see it? Our visual system uses the contrast of objects against their background to distinguish them, whether it be a dark object against a light background, or a light object on a dark background.
In the ocean, predators looking upwards can see sunlight or moonlight streaming down the water. Just as birds cast shadows, sunlight hitting prey lights them up as conspicuous, tasty dark spots. Scientists hypothesize that the Hawaiian bobtail squid benefits from generating light along their underside to help mask them from seals and other predators they may encounter. But won’t bright lights stand out to predators looking downwards toward the squid? That’s when the squid’s ink sac comes into play. Located above the light organ housing the bioluminescent bacteria, the ink absorbs any light being emitted upwards. The squid also has tissue that functions as a lens that, together with reflective proteins termed reflectins, work to direct light from the bacteria downwards. This method of camouflage is the principle of counter-illumination: Absorb on the top, shine on the bottom.
Unrelated to the bacteria, the squid can also camouflage by using its chromatophores, pigmented cells that can be contracted or expanded to modify its color, and by carrying a covering of sand on top of itself, which adheres to its mucus secretions.
Is there more to counter-illumination than screwing in a lightbulb?
In the natural world, counter-illumination does not work with a constant light intensity. After all, moonlight does not shine consistently throughout the night. To address this reality, the squid can sense and adjust the bacteria’s bioluminescence. Additionally, the bacteria produce light in a cyclical pattern that follows the onset of darkness. Researchers have hypothesized that this is because the squid limits the availability of oxygen, which is necessary for generating light, during the day. A similar phenomenon has been observed in ponyfishes.
Light production by the bacteria also depends on its population density within the light organ, which is sensed through a mechanism termed quorum sensing. Coincidentally, quorum sensing was first discovered in V. fischeri before its mutually beneficial relationship with the Hawaiian bobtail squid, a now iconic example of symbiosis, first became an area of study. Another fun Vibrio fact: A different Vibrio strain produces the pufferfish toxin!
Research has also shown that the bacteria’s light affects the squid’s internal daily rhythm, or circadian rhythm, and what proteins the squid makes, including proteins involved in vision and gas exchange. The daily cycle is important: Both squid and bacteria change the quantity of proteins they make over the course of a day. This continuous adjusting is expected because the squid supplies different nutrients during the day and the night, and because the bacteria has a greater need for energy at night when it generates light.
Think of all this crosstalk as a squid director choreographing a whole cast of thalassic, thespian bacteria in a sunset-to-sunrise symphony starring sparkling shimmers. A light show where more than rave reviews are at stake. A timeless, fiery feature flick that truly posits – yet simultaneously questions – the inherently limited nature of nature; the inescapable transience of existence. Tuned just right, this is an opportunity for the literal—and metaphorical—small guy to beat the metaphorical—and literal—big guy. This is a repeat performance that somehow tricks those bigger-brained seals into missing out on a meal. Beautifully, it works! These two small life forms, tiny think-meat and all, come together to outwit much larger predators. Who ever said bigger brains are smarter?
You said something about homelessness?
Even after finding a Hawaiian bobtail squid and making its way into the light organ, which already is difficult, the bacteria do not have it easy. For starters, bacteria that produce less light are outcompeted by those that produce more light. This may be because oxygen consumption from the light-generating reaction prevents the squid from using that oxygen to generate pesky, harmful reactive oxygen species. Also, low oxygen levels may cause the squid’s surface-lining cells to secrete more nutrients.
But even when the bacteria do work hard, shelter is still not guaranteed. Each dawn, as the sun rises, the squid expels 95% of the bacteria within it by thinning the tissue in its light organ. The circadian rhythm is important here too as the squid cells start making more structural, cytoskeleton proteins just before dawn, in alignment with the daily shedding. This increases the local oceanic bacteria level by up to 30 times, which partially helps newborn squid collect bacteria of their own, and regulates bacteria levels within the host squid, which rebounds by mid-afternoon. And so, without fail, nineteen out of every twenty bacteria will find themselves wandering the seas again, waking up in the morning as strangers to the harsh world – in need of a job; in search of a home.
How can studying the squid and Vibrio fischeri inform humankind?
Reportedly, even the Air Force has turned to this tiny seafloor-dwelling squid for inspiration. Howard Hughes Medical Institute’s BioInteractive writes that military researchers have studied the squid and its exclusive symbiotic relationship with V. fischeri to design better camouflage for airplanes. What is more certain is that counter-illumination experiments in the early 1940s were not followed through because mankind was simultaneously developing something that does not need to “see” in the traditional sense: radar, which cannot be fooled by bright lights. Luckily for the squid, it seems this is not a feature it needs to outsmart.
And so concludes our dive into a world-reaching performance most of the world will never see. The next time you head to a theater, consider the one that is unscripted and outdoors, with chirps and clicks and whooshes and warbles. The big screen is big, but the small sights you will see, hear, and smell once you open your mind to adventure are even bigger—and larger than life.
The next morning, our protagonist might find itself starting out a nondescript day like it does most other nondescript days: homeless. Or, it might be lucky and find it kept a spot within its favorite bed and breakfast. Either way, both actor and director will continue a cycle that ticks the ecosystem along, one night at a time.
Edward Chen is a first-year Immunology MMSc student at Harvard Medical School. Though he used to think of himself as a seal person, writing this article has leaned him towards squid people.
Jovana Andrejevic is a fifth-year Applied Physics Ph.D. student in the School of Engineering and Applied Sciences at Harvard University. She’s open-minded and is neither a squid nor a seal person.
For More Information:
- Want to read about the science behind squid-Vibrio symbiosis? Check out this SITN article that focuses on each partner. How do they find each other? How does the squid select for only Vibrio fischeri? How do both partners physically change the other?
- Interested in classroom activities about counter-illumination? Check out this 2018 activity guide and these high school-oriented videos from HHMI BioInteractive.
- Curious about what real Hawaiian bobtail squids look like? Check out some of these websites.