The bobtailed squid is a cephalopod and a close cousin to (my favorite cephalopod) cuttlefish and can be found bathing in shallow coastal waters. They possess a very special and round mantle – within this stubby mantle there is a unique light organ that houses a mono-specific colony of bioluminescent bacteria, Vibrio fischeri. The bobtailed squid provides sugar and amino acid solutions to the bacteria and in return, V. fisheri hides the squid’s silhouette when viewed from below by matching the amount of light hitting the top of the mantle, called counter-illumination.
How does counter-illumination work?
The bobtailed squid have a complex light-emitting organ in the center of their mantle consisting of two lobes with 3 distinct crypts, containing extracellular bacterial symbionts. A series of accessory tissues in the crypts can control the intensity of light emission from the organ.
How you say?
Well, within these accessory tissues is a thick reflector, which can direct light ventrally (or away from the “belly”) by surrounding the crypts. Additionally, the ink sac covers the dorsal surface of the organ to absorb stray light – therefore saving the light for where it’s needed… in the ventral region. Even more amazing, diverticula, or fluid-filled pouches of the sac can rotate over the crypts allowing the host to modulate the intensity of the light emitted. And finally, the entire ventral surface of the organ is covered with a thick, muscle-like lens that refracts the bacterial light into the environment.
How do V. fisheri emit light?
The enzyme bacterial luciferase catalyzes light production in the presence of three compounds (aliphatic aldehyde substrate, reduced flavin monocleotide, and molecular oxygen) and emits light around 490 nm (I’m blue dabadiabbadi). Like any other reaction, the amount of light emitted is dependent on the amount of substrates. This mechanism of light emission is highly regulated by transcription of the lux genes (more on regulation can be found in Ruby’s review linked below). One very special type of up-regulation of the lux genes can occur by V. fisheri detecting the density the bacterial colony. This mechanism, through which bacteria can sense the density of self-specific cells, has been termed quorum sensing (QS) and this required for V. fisheri survival in the bobtailed squid.
How do the bobtailed squid acquire V. fisheri?
Were the bacteria passed from generation to generation through the egg or did each new generation of bobtailed squids acquire the bacteria from free-living bacterial population in the surrounding sea?
It is horizontal transmission, or from the hitchhikers of the sea, but how to the squid get the right bacteria to colonize them? Luckily for the bobtailed squid, V. fisheri is the only luminous bacterium that is symbiotically competent, so there wasn’t much choice. Additionally, when the bobtailed squid is hatched the light organ is built for catching V. fisheri floating by with complex, ciliated, microvillous epithelial structures (CMS) on the organ’s surface. V. fisheri aggregate in mucus that is secreted by the CMS (in response to the peptidoglycan, a membrane-associated sugar structure in Gram-negative bacteria) and then the bacteria migrate towards the pores of the light organ with a final destination at the crypts where they can colonize.
New research has seen, upon colonization, the crypts stop production of anti-microbial agents NOS (nitric oxide synthases) and NO (nitric oxide) – this suggests the squid has specific mechanisms to sense and respond to the presence of the correct symbiont (Wang et al., 2011). My questions are, what is the signal the squid receives to reduce NOS and NO production once V. fisheri has colonized? I guess I have my post-doc research project.
The relationship between symbiotic V. fisheri cells and the host epithelial cells lining the light organ crypts of an adult E. scolopes. In this transmission electron micrograph of a thin section through the light organ, the bacteria (B) fill the crypt space, and are closely invested within the field of microvilli that covers the surface of the epithelial cell (E) tissue layer.
An impressive side-fact:
If the bobtailed squid does not come into contact with V. fisheri, crypts will remain uncolonized by any of the million of other species of marine bacteria present in the sea.
An even more impressive side-fact:
Each morning the squid expels over 90% of the bacterial symbionts by venting most of the crypt contents into the sea water, allowing the remaining bacteria to re-populate the crypt within the next 12 hours – this makes it easy for the squid to burrow into the surface sand layer at the beginning of the day without being detected and reconstitute the light organ just in time for hunting and for hiding at night.
Since this field is still exploring the intricacies of this beautiful biological relationship, there are still many questions that remain to be answered. In the words of my science-Grandfather, Stanley Falkow, “one cannot neglect the complex role of the host (although microbiologists and molecular biologists often do”. Hopefully this field bobtailed squid are nice to work with in a laboratory environment.
Wang Y, Ruby EG. (2011). The roles of NO in microbial symbiose Cellular Microbiology, 13 (4), 518-526
Edward G. Ruby (1996).
LESSONS FROM A COOPERATIVE, BACTERIAL-ANIMAL ASSOCIATION:The Vibrio ﬁscheri–Euprymna scolopes Light Organ Symbiosis Annual Reviews in Microbiology, 50, 591-624