Aurelia aurita (also called the common jellyfish, moon jellyfish, moon jelly or saucer jelly) is a species of true jellyfish in the family Ulmaridae and the type species of its genus, Aurelia. Before the 2020s, the species was considered cosmopolitan, but the taxon has since been split into at least 28 species. The species now identified as A. aurita is only found in the north Atlantic and in Argentina, and is not dangerous to humans.
Like other species in the genus, the jellyfish is almost entirely translucent, usually about 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 short, fine tentacles, and bringing them into its body for digestion. The sting has a mild effect on humans, with most having little or no reaction. It is capable of only limited motion, and drifts with the current, even when swimming.
A. aurita is endemic to the northeastern Atlantic Ocean, including the Baltic Sea where the 1758 type specimen was collected. Established populations are also found in the Northeastern United States and Argentina, where they were introduced (although natural dispersion to the United States can't be ruled out). A single specimen has also been found in the Pacific Northwest.
The life cycle of Aurelia aurita is similar to other species in the genus, consisting of two main stages: the free-swimming medusa and the benthic polyp, also known as a scyphistoma. During the medusa stage, A. aurita reproduces sexually, as opposed to the asexually reproducing polyp stage. The scyphistoma stage can survive for months without feeding, and have shown to have relatively low feeding selectivity. In this stage, A. aurita can adapt to several different environmental conditions.
Most species in the genus are very similar, and it is difficult to determine the species of Aurelia medusae without genetic sampling. In the 1990s, only two species were recognized in the genus, including A. aurita, and the species was considered to have a worldwide distribution. In the 2020s, reexamination of morphology and analysis of genetic samples revealed that specimens previously identified as A. aurita belong to at least 28 different species. Much of the research published about the species before this split may instead describe one or more related species.