Aurelia aurita (also called the moon jelly, moon jellyfish, common jellyfish, or saucer jelly) is a widely studied species of the genus Aurelia. All species in the genus are closely related, and it is difficult to identify Aurelia medusae without genetic sampling; most of what follows applies equally to all species of the genus.
The jellyfish is translucent, usually about 25–40 cm in diameter, and can be recognized by its four horseshoe-shaped gonads, easily seen through the top of the bell. It feeds by collecting medusae, plankton, and mollusks with its tentacles, and bringing them into its body for digestion. It is capable of only limited motion, and drifts with the current, even when swimming.
When washed up on shore they look like a round disk-shaped pad of hardened jelly. Most of the moon jellyfish you find are about 4 or 5 inches in diameter. The disk that washes up on shore is the most durable remnant of this creature.
Moon jellyfish tend to stay close to the water surface, in large part because this is where there is the highest concentration of plankton. This behavior makes the jellyfish themselves susceptible to being eaten. Moon jellyfish are preyed upon by various large fish, turtles and even some birds. Another consequence of living close to the water surface is that the jellyfish are moved around by water currents. Moon Jellyfish can manipulate their bell shaped body to thrust water away from them and propel themselves forward.
This jellyfish is a carnivore. It eats plankton that it catches while floating in the water column. Their prey includes a wide variety of small organisms like juvenile mollusks and crustaceans, rotifers, young polychaetes, protozoans, diatoms, tunicate larvae and fish eggs. Tentacles that hang down from the underside of the animal sweep the water, entangling whatever it encounters. Stinging nematocysts are effective at killing their prey and secreted mucus helps hold onto ensnared plankton. Captured food is moved via flagellar movement towards canals along the underside of the disk, then moved by other
Fortunately the stingers of this species are relatively mild. Most people who come into contact with moon jellyfish have little reaction. Some may feel a bit of a stinging sensation but even this can be relatively mild compared to what can occur with more troublesome jellyfish species. Many moon jellies you find no longer have any stingers attached to them.
Moon jellyfish’s stinging cells are relatively benign to us. In some cases a person may experience some mild stinging sensation if stung. The discomfort is fairly localized and does not persist for a long period of time. The disks you see on the beach usually have few if any tentacles remaining attached hence there are no stinging nematocysts present.