The Japanese pinecone fish searches for food with living headlights. This hand-sized fish harbours colonies of light-producing bacteria in two organs on its lower jaw. The beams from these organs shine forward, and when night falls and the fish goes searching for food, its jaw-lamps light the way.
Elsewhere in the Pacific Ocean, the Hawaiian bobtail squid also uses luminous bacteria, but theirs act as a cloaking device. They produce a dim glow that matches the strength of moonlight from above, hiding the squid’s silhouette from hungry fish below. It’s a mutual relationship; the squid gets protection and it pays its residents with sugars and amino acids.
The glowing bacteria of these two animals may have different uses, but they are actually the same species – Vibrio fischeri, a free-swimming bacterium found in almost all of the world’s oceans. V.fischeri isn’t inherited; instead, it colonises the light organs of both fish and squid when they are young. Its challenge is to recognise the right partners among the myriad of species in the ocean, and not end up in the wrong body.
But its potential hosts, the bobtail squid and the pinecone fish, are incredibly different animals, separated by over 550 million years of evolution. How does one bacterium manage to form tight alliances with such disparate hosts?.
Incredibly enough, it does so with a single gene. Mark Mandel from the University of Wisconsin found that the strains of V.fischeri in the squid contain a gene called RscS that is missing or very different in those found in fish. RscS was the genetic innovation that allowed a fishy bacterium to set up shop in the body of a squid.