Read time: 2 mins
Forget the ethereal glowing world of James Cameron’s Avatar, here on Earth we have plenty of our own bioluminescent creatures and the effects can look magical. Organisms have evolved this ability for various reasons like attracting mates, escaping predation, scanning for prey and even to communicate. Creatures with this ability can be found on land (like fireflies, mushrooms and worms) but most bioluminescent organisms exist in the ocean.
Many ocean dwellers have devised extraordinary methods of disguise in order to hide from predators in the deep blue, and the Hawaiian Bobtail Squid is no exception. This tiny squid (only about 3cm in length!) uses bioluminescence as self-preservation: a nocturnal animal, it hides in the sand during the day and emerges as night falls, emitting a blue glow similar to light coming from the moon and stars. For bigger fish looking up from below, the squid is camouflaged against the starlight as it doesn’t cast a silhouette.
This invisibility cloak comes with the help of Vibrio Fischeri, a bacteria that lives inside the squid. Hawaiian Bobtail Squid are born without this bacteria, and must recruit it from surrounding seawater. Juvenile squid ‘invite’ it inside their body shortly after hatching: they do this by flushing bacteria laden ocean water through their body, towards a special light organ inside. This jumble of bacteria is filtered by many obstacles, sort of like a microscopic carwash: the bacterial cells have to swim through a long duct, being doused with anti-microbial mucus and battered by cilia (finger like appendages that extend from cells, wafting away material or directing it somewhere else). Vibrio Fischeri reach the light organ and live inside what microbiologists call ‘crypts’: amazingly, it is the only bacteria that manages to reach the end goal and ‘set up shop’ within the squid. This incredible feat is still a mystery to scientists, who liken finding Vibrio Fischeri to finding a needle in a haystack.
The bacteria live inside the squid, but not inside the squid’s cells: one researcher likened it to ‘a sort of cave, this kind of nest that the squid makes for the bacteria’. The squid house their bacteria partners in a special light organ on their underbelly, where they receive nutrients in the form of sugar and amino acid solution. In return, the bacteria’s presence enhances the squid’s defences by producing the cloaking blue glow. But interestingly they don’t stay there all the time: every morning as the squid retreats back into the sand, it ejects 90% of the bacteria back into the seawater. As the squid rests in the sand, the bacteria grow up again in time for nightfall – so that when the squid emerges in the moonlight, it is once again equipped with a glowing blue invisibility cloak.