Mutualistic symbioses are key to evolutionary and ecological novelties and can be found throughout the tree of life. One prominent example is the symbiosis between corals and photosynthetic dinoflagellates – the foundation for coral reefs, the most biodiverse marine ecosystems on the planet. We use the marine sea anemone Aiptasia as a model system to study fundamental principles of this animal-algal endosymbiosis at the molecular, cellular and organismal level within the context of ecology and evolution.