A few years ago, in a laboratory at the University of Wisconsin at Madison, I walked into a mostly dark room, with a single light illuminating a plastic cup. Within the cup were dozens of tiny white blobs, each smaller than a pea. They were baby Hawaiian bobtail squid, and they were adorable. Their diminutive arms trailed behind them as they bobbed in the water, and the pigment cells that would eventually allow their adult selves to change color gave their infant faces a freckled appearance.
In the wild, shortly after hatching, these squid would normally be colonized by microbes. But they are selective about their partners: Of the thousands of species of microbes in the ocean, only one—Vibrio fischeri—is allowed to enter the squid’s body. Once inside, it begins to glow. And that glow, it is said, perfectly matches the moonlight welling down on top of the squid, masking its silhouette from predators looking up from below. The bacteria provide the squid with a kind of luminous invisibility cloak.
But they do much more than that. For decades, Margaret McFall-Ngai and Ned Ruby, now at the University of Hawaii at Manoa, have been studying the partnership between the bacterium and the squid, and they’ve shown that the former remakes the latter. As V. fischeri enters its host, it triggers a series of changes that turn the squid’s body into a more suitable environment, much like an explorer terraforming a distant world. This transformation is also part of the squid’s maturation. It only reaches its full adult state if it takes up the right microbial partner.