Recall, jellyfish usually consume organisms such as zooplankton, copepods, cladocerans, larvaceans, and bivalves.  They use their cnidocytes which eject nematocysts to catch and kill their prey.  This same mechanism is used as a defense against their predators.

Common predators against Moon Jellyfish are Ocean Sunfish, Loggerhead Sea Turtle, the hydromedusae, and the scyphomedusa.  They are also sometimes consumed by birds.  In some countries, such as China, Phillipines, Thailand, Malaysia, and Indonesia, Jellyfish are considered a delicacy. 

An importance of jellyfish recognized by humans is their role in transformations of pelagic organic material which is important for the productivity of the ocean. 

Some fishermen do not appreciate the presence of jellyfish, though, because of their role in depleting the population of larvae of commercially important fish.  Also, they can cause an outbreak of algae blooms which are frowned upon by fishermen tourists as well.