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.
You can find the papers from Peerakietkhajorn et al. on this topic, here and here.