A tiny marine squid has developed a symbiotic relationship with a luminescent bacterium, trading "room and board" for a slightly improved chance of not being eaten. The bobtailed squid (left) is no more than a few centimetres long and makes a tasty bite for bottom dwelling fish that can spot it as a dark blob against the brighter ocean surface.
Researchers from the University of Hawaii have discovered, however, that the squid has a purpose-built mechanism for attracting and trapping luminescent bacteria that brighten it up, making it more difficult for hungry fish to find.
Symbiosis, the co-operative joining together of two separate species for mutual benefit, is not uncommon in the animal and plant world. Certain leguminous plants secrete substances that capture nitrogen-fixing bacteria with which they live symbiotically. Our own gut is populated with bacteria that help us to digest food.
Co-authors on the report, published in the US Proceedings of the National Academy of Sciences were Dr Margaret McFall-Ngai and Dr Ned Ruby. They found that the bobtailed squid has a system that allows it to trap comparatively rare bacteria which can help it avoid becoming fish food. The luminescent bacterium, Vibrio fischeri, comprises less than one tenth of 1 per cent of all the bacteria in sea water, but the presence of Vibrio fischeri causes the squid to secrete a mucus-like substance which forms a matrix tethered to and suspended above its "light organs".
Tiny hair-like structures called cilia create water currents which draw the bacteria into this matrix, where they accumulate over several hours. They eventually migrate down into the light organs and multiply in the empty space.
Their natural luminescence creates patches of light and these mimic the light and dark patches seen at the water's surface from down below. While it doesn't guarantee the squid an easy life, it does give it an advantage over other creatures which can't produce a light source and which become the first off the buffet table when the fish start to arrive.
The researchers believe that the squid and symbiotic plants might share an ancient molecular mechanism for facilitating symbiosis. In both cases it involves an active system for allowing symbiosis to occur and one which enhances survival.