Like anemones and corals, the moon jellyfish is a cnidarian, which means it has a sac-like body with a mouth and tentacles. The moon jellyfish spends its time floating near the surface of the water, waiting for food to float by. It can be as small as 2 inches (5 cm) or grow over 1 foot wide (30 cm). It can be found along reefs and all over the world, and can float by itself or in groups of hundreds!