The Bobtail Squid

Dusk is settling in the Hawaiin archipelago, much like a 2nd year university student, the Bobtail squid is surfacing from its daylight period of inactivity, ready to go undetected in search for its next meal. These animals are arguably the cutest of the molluscs, averaging around 1-8cm in length, far from the colossus giant squids reported to ravage unsuspecting ships in the middle of the ocean. Aside from their endearing, diminutive figure, what attracts me most to these creatures is their intimate relationship with a specific bacteria, Vibrio fischeri.

As the Bobtail squid propels itself around the shallow waters in search of shrimps and worms amongst other delectable invertebrates, it inevitably has to avoid being prey itself to a range of seafloor dwelling predators. One of the problems with hunting at night is that the light reflecting off the moon hits the top of the squid and creates a silhouette when viewed from below which the predators can use to target it. How does the squid counteract this in order to avoid detection and what do bacteria have to do with it?

The squid has a specialised light organ. This organ emits light downwards, matching the light hitting the top of the squid, thereby eliminating the silhouette in an anti-predatory phenomenon called counter-illumination. Where this process really ‘sparks’ is that this light is not produced by the squid directly but by a special kind of bacteria that resides in the light organ of the squid.

In the first 8 hours of the Bobtail squid’s life, they secrete mucus in which many different populations of bacteria get trapped. From here the bacteria migrate into the small holes, or pores, on the surface of the light organ in order to take up residence there. What is most fascinating here is that in the mucus there are many different species of bacteria but only the light producing, Vibrio fischeri, make it into the light organ, forming a very specific relationship; like nachos and cheese, Jake Peralta and Charles Boyle, New Years Eve and disappointing plans. So after these first 8 hours, a strong partnership has been forged that will last a lifetime.

This, however, is not some sort of Montgomery Burns/Waylon Smithers one-sided relationship. The squid feeds the bacteria sugar and amino acids (the building blocks which make proteins) to enable the bacteria to survive, reproduce and in-turn the bacteria produce the light so valuable to the squid’s anti-predator defences. This is a very dynamic process as the properties of the light hitting the top of the squid will inevitably change, so how does the squid adapt to be able to match this in the light it emits?

Wavelength - the squid’s light organ contains a filter which can help match the wavelength of the moonlight. Amount - part of the light organ can control the size of the opening in which the light is emitted, like how the iris, the coloured part of the eye, controls the size of the pupil. Lastly, and for me, the most intriguing, the squid can control the intensity of light leaving the organ. To produce the light, the bacteria need oxygen. As a result if the squid wants to produce a higher light intensity, it supplies the bacteria with more oxygen and vice versa. Like any good factory manager, supplying the input to determine the output. These adaptations allows the properties of the light emitted below the squid to be the same as the light hitting the top of the squid, eliminating its silhouette.

How do you keep the bacteria honest? Often it is easy for individuals to not contribute to the common good; the millionaire who doesn’t pay his taxes or the friend who doesn’t chip in £10 for the fantasy football league. However, it has been shown that cheating bacteria which feed on the sugars and amino acids provided by the squid but do not produce light are ousted from the safety and comfort of the light organ within a couple days of colonisation. Efficient.

As scientific research progresses it is becoming more and more evident the importance of the interactions between large organisms and the bacteria which reside both inside and on their bodies. There has been a huge interest in studying the human microbiome and the positive impact that the bacteria we house in our guts have on our health. Future research will help to keep ‘illuminating’ the roles they play. Terrible pun intended.