The Stranger's Handshake

A bacterium arrives at the surface of a squid's light organ. It is one of a million bacteria in the surrounding seawater. It has never been here before. The squid has never met it. Neither has any reason to trust the other. Within hours, one of them will be living inside the other's body. The Hawaiian bobtail squid hunts at night in shallow water. To avoid being silhouetted against the moonlit surface, it uses counter-illumination: a light organ on its underside produces a glow that matches the ambient light from above. Predators looking up see no shadow. The squid cannot produce this light itself. It outsources the job to Vibrio fischeri, a bioluminescent bacterium. But V. fischeri is less than one in ten thousand of the bacteria in Hawaiian seawater. The squid hatches with a sterile light organ. It must find and select the right partner from an ocean of strangers. It does this through a protocol so precise that biologists have spent three decades mapping it. Every step has been chara