The thesis presents facultative symbiotic relationships of different species of fish and medusae from the order Schypozoa. Relationships between fish and jellyfish can be mutually beneficial (mutualism), beneficial only for one species (commensalism), and in some cases the relationship is beneficial for one species and harmful for the other (parasitism). In order to achieve such relationships, fish and medusae have developed different recognition mechanisms with the lateral line system of the fish and the chemical compounds released by the medusae into the environment. Medusae as hosts provide fish with protection from predators, food (plankton, parts of jellyfish tissue, and parasitic shrimps that live on jellyfish) and a safe habitat for the development of larvae and juvenile stages of fish, which greatly contributes to the success of the recruitment of these fish species. A potential benefit of jellyfish from this relationship is the removal of ectoparasitic amphipods and detritus from the folds of the mouth.