Trichoplax, one of the simplest animals on Earth, lives in a highly specific and intimate symbiosis with two types of bacteria. The first, Grellia, is related to parasitic bacteria that cause typhus and Rocky Mountain spotted fever. Intriguingly, Grellia does not appear to harm Trichoplax. The second, Ruthmannia, sits inside the cells Trichoplax uses to digest its food. The Trichoplax symbiosis provides a window into the microbial dark matter – poorly described groups of bacteria. The study by scientists from the Max Planck Institute for Marine Microbiology and the University of Hawaii has now been published in the journal Nature Microbiology.