Moon jellyfish have no natural predators, but can harm the reef if there are too many of them. Moon jellyfish eat zooplankton (tiny animals), who in turn eat tiny plants in the sea called phytoplankton. If the moon jellyfish eat all the zooplankton, then there is nothing to eat the tiny phytoplankton and they grow out of control! They multiply so greatly that they can start to block out the sun!! This can then start to kill the corals and plants in the reef.