Simple aquatic animals like corals and sponges have it even worse. Their entire bodies are little more than layers of epithelium immersed in a bath of microbes. And yet they too can control their symbionts. How? For a start, they use mucus, the same slimy goo that clogs your nose when you have a cold. ‘You can’t go wrong with mucus, because mucus is cool,’ says Forest Rohwer.22 He should know – he has been collecting samples of the stuff from across the animal kingdom for years. Nearly all animals use mucus to cover tissues that are exposed to the outside world.
