The Invisible Squid
The Hawaiian Bobtail Squid, Euprymna scolopes, has a clever way of duping predators during its nightly activities. It uses a symbiotic luminescent bacteria, Vibrio fischeri, to light up its underside, so that upwards-looking predators don't see a dark, edible form silhouetted against a moonlit or starlit sky. Instead, hungry sharks or other fish see only sky. The squid is invisible. And while this is very cool, there's a whole lot more to this symbiosis story.
What the glowing bacteria get out of this arrangement is a comfy place to live, food and even help reproducing. But because the squid does not need the bacteria except as a camouflage, it can live happily without it in a laboratory. "The squids are not physically compromised without the bacteria," said University of Wisconsin biologist Margaret J. McFall-Ngai, who has studied the squid and its symbiotic bacteria for two decades. This makes it possible to look at the relationship of the two organisms in great detail. The same cannot be said of a human's gut bacteria, she said. They are not so good at living apart, and there are so many kinds of them. The one squid, one bacteria model simplifies the study tremendously, she said.
Among the things that have been discovered by studying this relationship is that the bacteria and the squid operate together on a daily rhythm, said biologist Spencer Nyholm of the University of Connecticut. Every morning the squid spits out 95 percent of the glowing bacteria, along with some of its own cells, perhaps to feed the bacteria. These expelled bacteria are then taken in and grown by other young squids. After expelling the bacteria, the squid buries itself in sand and rests for the day, growing a new batch of glowing bacteria, which only glow when they reach a certain concentration.
How do the bacteria "know" when they reach that concentration? That's another important mystery that needs to be found out. "You see similar things in the guts of humans and other animals," said Nyholm, the second author on a paper on a recent issue of the Proceedings of the National Academy of Sciences reporting on the biochemical mechanisms behind the daily rhythms in both organisms.
What About Us?
Understanding the inner workings of these rhythms could lead to new ways to treat disease. Among the big questions that the squid and bacteria could answer, for instance, is how the two organisms communicate so they don't harm each other. "I'm interested in specifically how the immune system reacts," said Nyholm. "How can they tell the good from the bad bacteria?" Indeed, how does the human body know the good gut bacteria from the bad bacteria? And why don't our gut bacteria just keep growing and kill us? "That doesn't happen (in the squid) and that doesn't happen in us either," said McFall-Ngai. Discovering why could, among other things, lead to new ways to fight bacterial infections, since the molecules involved in the process are, remarkably, the same in squids and humans.
The very fact that the molecules at work are the same in such very different animals suggests something else too, she said. Unraveling the fine details of this symbiotic relationship could open a window into some very fundamental and ancient processes that date back to the earliest life on Earth. That's some big science from a small glowing squid.