Squid typically aren’t my thing, but I can certainly be wooed by their microbes. Example: the very awesome symbiotic relationship between the Hawaiian bobtail squid, Euprymna scolopes, and its “luminous symbiont” bacteria, Vibrio fischeri.
This squid, like us, has its own body clock dictating it’s routine. But instead of waking up in the morning and heading to work, E. scolopes follows a vampire’s schedule. It emerges into the water column at night as a ruthless predator, and then burys itself in the sand once the sun rises, laying hidden during the day. All while looking super cute!!!
So obviously this squid is a Twilight vampire. Because in addition to drawing you in with its charming (but deadly!) looks, E. scolopes literally sparkles. This squid species can bioluminesce courtesy of extracellular V. fischeri bacterial cells, which live in “deep crypt spaces” in the squid’s light organ (I’m really not making up this vampire theme here).
And why would a squid sparkle?
Some behavioral evidence suggests that the night-active host animal uses the luminescence of the bacterial symbiont as an antipredatory camouflage in a process known as counter illumination. (Heath-Heckman et al. 2013)
What’s even cooler: these symbiotic bacteria function as a bacterial alarm clock, helping to regulate the squid’s daily circadian rhythms. A “circadian rhythm”, a.k.a. a body clock, is basically a giant cellular war between proteins — some molecules compete to accumulate, while others work furiously to break things down. This complex array of switches and feedback loops is ultimately regulated according light/dark cycles provided courtesy of the sun. Its the same process that makes us feel tired at night, and jet lagged when we switch time zones. In E. scolopes it regulates its urge to FEED!
A new study by Heath-Heckman et al. (2013) provides convincing evidence that bacterial light production (and metabolic products produced by V. fischeri during this process) are required for the squid to regulate its own body clock. This is crazy but totally cool biology – its akin to us humans being required to drink probiotic yogurt in order to sleep. In this hypothetical scenario, our own body wouldn’t know when it should sleep on its own, since sleep would only be cued after our body detected the correct signals from bacterial proteins.
Circadian rhythms in the Hawaiian bobtail squid are influenced by blue-light receptors called cryptochromes – way back in the day, these were DNA repair enzymes that have since mutated and evolved to respond to light. In E. scolopes, gene expression for one such receptor (escry1) *only* responds to light produced from bacterial bioluminescence, and not environmental light. Other bacterial products (lipid A, peptidoglycan monomer) are also important for regulating host gene expression. Experimental evidence is also supported by the fact that escry1 gene expression is crazy high in squid tissues surrounding bacterial symbionts.
We’re only just scratching the surface of this host-microbe interaction. The relationship between V. fischeri bacteria and E. scolopes is complex and interdependent – mutant squid that can’t support bacterial symbionts are defective in bioluminescence, and show stunted development of the light organ. V. fischeri appear to play a critical role from Day 1 of a squid’s existence.
The authors of this study consistently remind us that–in addition to being an awesome example of biology–bacteria in the gut may influence similar patterns on humans. But don’t start blaming bacteria for your late-night Twitter obsession just yet.
Heath-Heckman EAC, Peyer SM, Whistler CA, Apicella MA, Goldman WE, McFall-Ngai MJ. (2013) Bacterial Bioluminescence Regulates Expression of a Host Cryptochrome Gene in the Squid-Vibrio Symbiosis, mBio, 4(2):e00167–13–e00167–13.