My dissertation focuses on the development of Cassiopea as a model organism to study symbiosis. Cassiopea, similarly to corals, has a mutualistic relationship with Symbiodiniaceae. Albeit, is a more amicable system when compared to hard corals, as this upside-down jellyfish can be easily maintained in laboratory conditions, has a closed sexual reproductive cycle, and short generation time. Transcriptome profiling of aposymbiotic and symbiotic Cassiopea are aiming to identify genes related to symbiosis. A cross-comparison with data available for the sea anemone Exaiptasia, will allow to identify if the symbiosis in Cassiopea is analogous to those reported in sea anemones or corals. Moreover, genes of interest will then be targeted for knockout using CRISPR-Cas9 for testing their functionality related to endosymbiosis. The project aims to have implications and benefits for the scientific community by developing Cassiopea as a model organism for symbiosis and by providing a platform to conduct transgenic and gene knockout experiments.