The name ‘moon jelly’ can refer to any of several jellies in the genus Aurelia that are round with a shallow bell and relatively short tentacles. In this case, we mean moon jelly to refer to the species that is common throughout both sides of the north Atlantic Ocean. Like all true jellies, the moon jelly’s tentacles are covered with specialized stinging cells, called cnidocytes. The moon jelly uses these stinging cells to hunt small pelagic invertebrates and occasionally fishes and to capture other food particles with which it might come in contact. The cnidocytes are also the source of the sting that people feel when we come in contact with a moon jelly.
Though the moon jelly lives throughout the epipelagic zone, it is most commonly found near the coast and in upwelling areas, where its prey occurs in higher concentrations. This species is not a very strong swimmer, so they are often found on beaches after strong storms or tides that push them onshore. Along with other jellies, moon jellies are the favorite prey of some open ocean predators, like the ocean sunfish and the leatherback turtle. They have very little nutritional value, however, so the predators that specialize on them must eat hundreds and hundreds of these jellies in order to maintain their required energy levels.
Like many jellies, moon jellies have an interesting life cycle that includes a combination of sexual and asexual reproduction. Sexually mature moon jellies are the animals we see swimming around the open ocean (known as medusae), with which we are most familiar. The distinct horseshoe-shaped structures at the top of the medusa’s bell are the gonads. These adults reproduce via external fertilization, where females release eggs and males release sperm into the water column. Once the egg is fertilized, a larva hatches and lives in the pelagic environment for some time. As it grows, the larva searches for a suitable place in shallow water and eventually attaches to the sea floor where it grows into an upside down medusa known as a polyp. During the polyp phase, an individual asexually buds off several clones of itself that swim away as medusae and eventually grow into sexually mature moon jellies. This alternation of sexual and asexual reproduction may be a means of quickly increasing numbers while preserving the importance of mixing genes with other individuals.
Scientists believe that moon jellies and other jellies thrive in areas that are particularly affected by human activity. Overfishing, ocean warming, and pollution are all factors that reduce moon jellies’ predators and competitors and increase their prey. These results provide a more favorable environment for this species. As people continue to increase our ocean activities, the Moon Jelly may become one of the more successful species in the open ocean.