16.5D: Squid-Aliivibrio Symbiosis
- Explain the symbiotic relationship of squid and aliivibrio
Bioluminescence
A special category of symbiotic relationships involve bioluminescence, where light producing bacteria are hosted by another organism. One of the best studied examples of bioluminescence is the Hawaiian bobtail squid ( Euprymna scolopes ) and its mutualistic bacteria, Aliivibrio fischeri . Aliivibrio fischeri inhabits a special light organ in the squid’s mantle. The bacteria are fed a sugar and amino acid solution by the squid. In return, they produce light to hide the squid’s silhouette when viewed from below, allowing the squid to match ambient light conditions.
Bobtail squid hatchlings do not have Aliivibrio fischeri naturally in their bodies. They are born with a special light organ structure, with cilliated cells at the opening designed to trap passing A. fischeri , but must obtain the bacteria from sea water. To do this, the squid secretes a special mucus whenever its cells detect peptidoglycan (which is found in the cell walls of bacteria). The mucus collects near the opening of the light organ which traps passing bacteria. The squid weeds out unwanted bacteria in several ways. For instance, A. fischeri is able to survive in the mucus better than other species. It is also a very mobile bacteria, and is able to swim against the current created by the cilia at the mouth of the light organ.
The squid also creates a hostile environment at the entrance to the light organ by secreting an enzyme that splits hydrogen peroxide, creating a toxic environment for most bacteria. Aliivibrio fischeri can capture hydrogen peroxide before the squid can use it as a toxin, and thus can survive in the hostile chemical environment. Once A. fischeri has passed these hurdles at the opening of the light organ, it can colonize chambers of the light organ and begin enjoying the benefits of symbiosis.
Despite all the effort that goes into obtaining Aliivibrio fischeri, the squid ejects 95% of its bacteria every day. It not fully understood why the squid cleans out its light organ, but the bacteria require a great deal of sugar and amino acids, so it may be most useful to the squid to host bacteria only when they are needed. It may also provide a supply of bacteria for squid hatchlings.
Key Points
- Squid rely on Allivibrio bacteria to generate light that allows them to blend in with the light coming from above. Animals below them cannot see their shadow when they view the squid from below.
- Squid use mucus to attract many species of bacteria into their light organ, but they sort out Aliivibiro in several ways. Ciliated cells in the light organ create a current that expels most bacteria, and the squid uses hydrogen peroxide to create a hostile environment that Aliivibrio can resist.
- Once inside the light organ, the Aliivibrio bacteria receive sugars and amino acids from the squid. However, this is costly to the squid, and the squid clears out its light organ during the day so that it does not have to constantly maintain a colony of Aliivibrio bacteria.
Key Terms
- cilia : Organelles found in eukaryotic cells. Cilia are slender protuberances that project from the much larger cell body.
- BIoluminescence : The emission of light by a living organism.