Daphnia are the center of our 11th day of Microbial Advent Calendar! Those tiny crustaceans are commonly found as inhabitants of ponds and lakes throughout the world. This zooplankton is a key primary consumer in freshwater ecosystems. A recent set of studies highlighted the fact that when you sterilize Daphnia embryos the emerging population has reduced fecundity. The author identified the main bacteria involved in this symbiosis as a Betaproteobacteria from the Limnohabitans genus, although it is unclear where the bacteria is located or how it contributes to the host reproductive fitness.