Aurelia Aurita ( 75 cm. )
The common jellyfish or moon jellyfish is perhaps the pictogram species of a jellyfish. Found throughout most of the world's oceans, and with quite poor motion skills, it drifts with the current, and are often found beached.
The moon jellyfish is translucent, usually about 25–40 cm (10–16 in) in diameter, and can be recognized by its four horseshoe-shaped bright orange or pink gonads, easily seen through the top of the bell. It feeds by collecting medusae, plankton, and mollusks with it tentacles, and bringing them into its body for digestion. Moon jellyfish are often found in massive schools in shallow water during summer. This phenomenon is caused by the species’ strong tolerance to low dissolved oxygen conditions. Where other animals flee from oxygen depleted zones, moon jellyfish stay.
That means they have very little to no competition for food, as well as their predators have disappeared - these are optimal conditions for the moon jellyfish to bloom into thousands of animals.