Moon Jelly

The moon jelly, also called the common jellyfish or saucer jelly, is an almost entirely translucent true jellyfish, usually about 25 to 40 cm across. It can be recognized by the four horseshoe-shaped gonads visible through the top of the bell. It is endemic to the northeastern Atlantic, including the Baltic Sea, with established populations in the northeastern United States and Argentina. Its short, fine tentacles collect plankton, copepods, mollusks and fish eggs, which are carried into the body for digestion. It is capable of only limited motion and drifts with the current. Its sting is mild, and most people have little or no reaction.

About the moon jelly

The moon jelly is a common, nearly transparent jellyfish with a saucer-shaped bell and four horseshoe-shaped gonads visible through the top. It is a true jellyfish (class Scyphozoa) in the genus Aurelia. Moon jellies drift in temperate coastal seas worldwide, and their sting is too weak to bother most people.

The name has a complicated history. For most of the 20th century, scientists treated Aurelia aurita as a single species found almost everywhere. Genetic studies in the 2000s and 2020s showed that animals once called A. aurita actually belong to at least 28 species, and the genus now has 25 accepted species with more still undescribed. In the strict sense, A. aurita lives only in the north Atlantic and Argentina. Because the species look so alike, older research on "A. aurita" may describe a relative, and identifying one usually takes genetic sampling.

A moon jelly has no respiratory, excretory or circulatory system. It takes in oxygen by diffusion through the thin membrane that covers its body, helped by tiny hairlike cilia that move fresh water through its central gut cavity. A simple nerve net controls swimming and feeding. Sense organs called rhopalia, set around the edge of the bell, let it tell light from dark and up from down, and balance organs inside them help it stay upright.

The adult bell is translucent. The four gonads glow through it in a ring of horseshoes, which is the easiest way to recognize the animal. Its gut can take on the color of its meals, turning pink or lavender after certain crustaceans or orange after brine shrimp. Unlike many jellyfish, it has no long trailing tentacles. Instead, hundreds of short, fine tentacles fringe the bell, and four frilly oral arms hang beneath it.

Behavior and diet

Moon jellies are weak swimmers. They pulse their bells to stay level near the surface, but they mostly drift wherever the current takes them. Staying flat near the surface spreads the tentacles wide for catching food.

They are carnivores that eat almost anything small enough to stick to the bell or tentacles. Their main food is zooplankton, including copepods, small mollusks, fish eggs and larvae, and sometimes other jellyfish and comb jellies. Stinging cells on the tentacles and oral arms catch the prey, mucus binds it, and cilia sweep it into the gut, where enzymes digest it.

They are hardy in poor water. Moon jellies tolerate low oxygen far better than most fish, and their feeding rate stays steady even when oxygen drops. In some bays in summer, moon jellies are most abundant exactly where bottom oxygen is lowest, which reduces competition from fish for the same plankton. In Japan's Seto Inland Sea, swarms have been estimated to eat all of the mid-sized zooplankton in the water during July and August.

Many animals eat moon jellies, including ocean sunfish, leatherback sea turtles, other large jellyfish and seabirds. Their transparency makes them hard to see from above or below.

The life cycle has two very different stages. Adult jellies, called medusae, are male or female. Males release strings of sperm into the water, females draw them into their gastric pouches, and the fertilized eggs are released. Each egg becomes a tiny swimming larva, which settles on a hard surface after a day or more and grows into a polyp about 1.6 cm (0.6 in) tall. The polyp clones itself by budding and then, triggered mainly by seasonal temperature changes, splits into a stack of tiny disks. Each disk swims off as a young jellyfish called an ephyra, up to about 1 cm (0.4 in) across, and grows into an adult over roughly four to six months.

Adults usually die after a few months of breeding. A 2015 study found that A. aurita can also reverse its life cycle, re-forming polyps from adult tissue, much like the so-called immortal jellyfish.

Range and habitat

A. aurita in the strict sense is native to the northeastern Atlantic, including the Baltic Sea, where the type specimen was collected in 1758. It has established populations in the northeastern United States and Argentina, probably introduced, though natural spread to the U.S. cannot be ruled out. A single specimen has turned up in the Pacific Northwest.

The genus as a whole is found in the Atlantic, Arctic, Pacific and Indian oceans. Moon jellies are most common in temperate waters, such as off northern China, Japan, Korea, New Zealand, northern Europe and both the northeastern and northwestern coasts of the United States. They are an inshore group that lives in estuaries, bays and harbors. They handle water temperatures from 6 to 31 °C (43 to 88 °F) and salinity as low as 6 parts per thousand.

Conservation status

Moon jellies have not been evaluated for the IUCN Red List, so they carry no status. That is different from being rated Least Concern or Data Deficient; no assessment has been made at all. They are not on any international or federal protection lists.

With moon jellies, the concern usually runs the other way. Large blooms clog fishing nets, compete with fish for plankton, eat fish larvae and block the water intakes of power plants and desalination plants. Where moon jellies arrive in new waters with plenty of food, their numbers can grow quickly, and in some places they are removed with nets and trawlers.