No species on this planet is living in total isolation from others and these associations are crucial for ecosystem equilibria. Among these interactions, symbiotic associations are characterized by long-term physical interactions between a host and one to several symbionts. Such associations could occur during a short period or during the entire span live of the two partners.  They could be beneficial for both (mutualism), or at the opposite lethal for the host (parasitism). Between these two extrema, all interaction levels are possible. This web-site is dedicated to these poorly known associations in aquatic systems (marine and freshwater).