The Hawaiian Bobtail Squid made headlines in the summer of 2011 for its symbiotic relationship with bioluminescent bacteria. These "Vibrio" bacteria glow when tucked into a specialized organ in the baby squid's belly. The bacteria have a nourishing environment. The squid becomes invisible from beneath because the glow appears to be moonlight through the water.
A Glowing Report for the Hawaiian Bobtail Squid
The Hawaiian Bobtail Squid, or Euprymna scolopes, buries itself into the sandy bottom of shallow Pacific reef flats to stay safe from daytime predators. It emerges at night to hunt; but how will it escape its own predators?
While it swims near the surface, it would be silhouetted by moonlight from above. The adaptation that protects the bobtail squid from this hazard is the glow from its "light organ". But the squid does not glow by itself.
Luminescent Vibrio fischeri Bacteria
The fish-dwelling bacteria, Vibrio fischeri, are luminescent: they glow due to a chemical in a family known as luciferins. Bacterial luciferin is a "reduced riboflavin phosphate". When this chemical is oxidized with the help of an aldehyde compound and a luciferase, it glows. (Different types of luciferins are found in other marine species, fireflies, and some fungi).
A relative of V. fisheri is the dangerous V. cholerae, known for causing cholera in humans.
The Symbiotic Life of the Hawaiian Bobtail Squid
The mother Hawaiian Bobtail Squid "lays" her eggs, which later hatch directly into the young squid. These juveniles do not hatch with V. fisheri bacteria in their light organs. Obtaining these bacteria is one of the highest priorities for the hatchlings.
The squid's light organ has tiny "hairs", or cilia, that wash sea water over the pores in the organ's skin. Each V. fisheri bacterium uses its whip-like flagellum to move, so it can enter the light organ.
Although other bacteria may try to colonize the squid's light organ, they are defeated by toxins released by the Hawaiian Bobtail. Obviously V. fisheri is adapted to thrive despite the squid's hydrogen peroxide disinfectant.
As well, the light organ excretes sugars and amino acids to feed the V. fisheri. After all, producing luciferin costs the bacteria a significant amount of energy.
Strangely, the squid's cilia die off soon after the light organ is colonized by the right bacteria. Also, the bacteria in a colonized light organ lack the flagella that allow them to "swim" in the open ocean.
Finally, the Hawaiian Bobtail Squid ejects at least nine-tenths of its V. fisheri bacteria every morning. The swirl of bacteria, mucous and some squid cells maintains the population of this bacterium in the ocean. This allows the next generation of squid to begin its own colony of symbiotic bacteria, to provide night-time camouflage for its hunting.
(Added July 17, 2011) Expelling most of the bacteria effectively "turns off" the bacteria's light. They only glow when their population is dense enough that it is worth spending the energy to emit photons. Like many other bacteria, V. fisheri detect a "quorum" by sensing chemicals released by each individual. In addition, the Hawaiian Bobtail Squid's "light organ" can adjust the amount of light it allows to escape, in order to match the moonlight from above. All in all, this is a finely tuned and highly versatile symbiotic relationship: the bacteria receive food and protection, while the Hawaiian Bobtail Squid gains camouflage.
References:
Christine Buckley, PhysOrg, "Glowing squid thrive in symbiotic relationship", published July 6, 2011, referenced July 12, 2011.
J.Graf, University of Connecticut, " The Light-Organ Symbiosis of Vibrio fischeri and the Hawaiian squid, Euprymna scolopes ", updated Aug. 2007, referenced July 13, 2011.
Haddock, McDougall & Case, The Bioluminescence Web Page of the University of California at Santa Barbara, "Luciferin Details", 2011, referenced July 13, 2011.
(Added) Marcia Stone, New York Academy of Sciences, " Microbial Chatter: How Bacteria Talk With One Another ", Spring 2006, referenced July 16, 2011.
Mike DeHaan - Copyright (c) Mike DeHaan, B. Math., of DeHaan Services. Well written and well researched freelance articles; ghost writing for clients.
