Moon Jellyfish (Aurelia aurita): Facts, Life Cycle and Sting

What does a moon jellyfish look like?

The body of an adult moon jellyfish is a flat, see-through disk called the bell, or umbrella. It is usually colorless, milky or faintly bluish, and in bright water it can look almost invisible. Through the top of the bell you can see four horseshoe-shaped or ring-shaped organs arranged like a four-leaf clover. These are the gonads, the reproductive organs, and they are often pink, lilac or pale purple.

The edge of the bell carries a fringe of hundreds of short, fine tentacles, quite unlike the long trailing tentacles of a lion’s mane jellyfish. Eight small notches around the rim each hold a tiny sense organ called a rhopalium. Beneath the bell, four frilly oral arms surround the central mouth and handle food.

The color of the gut can change with diet. Moon jellies that have eaten many small crustaceans may take on a pink, lavender or orange tint in the middle of the bell. Despite its name, a moon jellyfish is not a fish at all but a cnidarian, a relative of corals and sea anemones (see Are jellyfish fish?).

How big do moon jellyfish get?

Most adult moon jellies have a bell about 10–40 cm (4–16 in) across. Size depends heavily on food and temperature, and a jellyfish that goes hungry can actually shrink. The tentacles are only a few centimeters long, so a moon jelly’s reach is little wider than its bell.

Like other jellyfish, moon jellies are made of about 95 percent water. Most of the bell is mesoglea, a stiff jelly of water and protein fibers that acts as a flexible skeleton. Out of the water a moon jelly loses its shape, and a stranded one dries out into a thin, glassy film within hours.

Where do moon jellyfish live?

Moon jellies are coastal animals. They live in bays, harbors, fjords, lagoons and estuaries as well as shallow shelf seas, usually in the upper part of the water column. They tolerate cool water, brackish water with little salt and water that is low in oxygen, which helps them thrive in places where many fish struggle.

For a long time, Aurelia aurita was thought to live in almost every ocean. Genetic studies since the early 2000s, and a major revision of the genus published in 2021, showed that “the moon jellyfish” is really a group of more than 25 look-alike species. In the strict sense, Aurelia aurita is native to the northeast Atlantic, including the coasts of Scandinavia, the Baltic and North seas, Britain, Ireland and western Europe. It also has established populations in the northeastern United States and in Argentina, which were probably carried there by ships.

Elsewhere, the moon jellies people see usually belong to related species. On the Pacific coast of North America, from northern California to Alaska, the common one is the greater moon jelly, Aurelia labiata. Many of the moon jellies on display in public aquariums may also belong to other Aurelia species, because the species are almost impossible to tell apart by eye.

What do moon jellyfish eat?

Moon jellies are carnivores that feed on zooplankton, the tiny animals drifting in the water. Their prey includes copepods and other small crustaceans, the larvae of shellfish and worms, fish eggs and fish larvae, and small jellyfish and comb jellies.

They feed in an unusual way. Instead of snaring large prey with long tentacles, a moon jelly catches plankton in sticky mucus on its bell and tentacles as it swims. Tiny hair-like cilia move the mucus and food toward the edge of the bell, where the oral arms collect it and pass it to the mouth and four stomach pouches. Stinging cells on the tentacles and oral arms help to hold small prey.

Because moon jellies eat the same plankton as young fish, and also swallow fish eggs and larvae, large swarms can compete with fish and reduce the food available to them.

How do moon jellyfish move and sense their surroundings?

A moon jellyfish swims by contracting a ring of muscle around its bell, pushing water out behind it, then letting the springy jelly relax. It is a slow swimmer and mostly drifts with currents, but it can steer up and down and keep itself near the surface. Research on moon jellies has shown that this pulsing is one of the most energy-efficient forms of swimming known in animals, because each relaxation also captures a free push from the swirling water left behind.

There is no brain, heart, blood, gills or lungs. Oxygen simply diffuses through the thin body wall, and a branching system of canals carries food from the stomach out across the bell. Swimming and feeding are controlled by a decentralized nerve net, coordinated by the eight rhopalia around the rim. Each rhopalium has simple light-sensitive spots and a tiny crystal-weighted balance organ that tells the jellyfish which way is up. Read more in Do jellyfish have brains?

Moon jellyfish life cycle: from polyp to medusa

Like most true jellyfish, the moon jellyfish has a two-stage life cycle that alternates between a free-swimming adult, called a medusa, and a tiny polyp fixed to the seabed.

1. Adults spawn. Adult medusae are either male or female. Males release sperm into the water, and females take it in to fertilize their eggs. Each female then broods the developing larvae in small pouches on her oral arms.
2. Larvae settle. The larvae, called planulae, are covered in cilia and swim for several days before attaching to a hard surface such as rock, shell, a pier or the underside of a boat.
3. Polyps multiply. Each larva becomes a small, stalked polyp with a crown of tentacles. Polyps feed and bud off new polyps, so one larva can produce a whole colony of identical clones.
4. Strobilation. Usually in winter or spring, often triggered by a change in water temperature, a polyp’s body divides into a stack of discs. These break off one by one as tiny, star-shaped ephyrae.
5. Growth to adult. The ephyrae feed and grow into adult medusae over a few months, ready to breed in summer and autumn.

This system lets moon jellies appear suddenly in great numbers. A single polyp colony can release large numbers of ephyrae in the same season, and all of them grow up at the same time.

How long do moon jellyfish live?

The adult medusa stage is short. In the wild most moon jellies live for several months to about a year, and a medusa older than about two years is exceptional. After the breeding season many adults shrink, become damaged or diseased and die, often in late summer or autumn. In aquariums, with steady food and no predators, some live longer.

The polyps are the long-lived part of the life cycle. A polyp colony can survive for years on a pier or rock, releasing new jellyfish season after season, and polyps can go for months without food. Scientists have also seen some Aurelia medusae change back into polyps in the laboratory, a trick made famous by the so-called immortal jellyfish. For more, see How long do jellyfish live?

Blooms, predators and threats

Moon jellies are famous for blooms, sudden swarms of thousands or millions of jellyfish. Blooms build up when warm water, plenty of plankton and calm weather come together, and currents and wind then pack the jellies into bays and along shorelines. Moon jellies cope better than most fish with nutrient pollution and low oxygen, so blooms may be becoming more common in some coastal seas.

Their predators include the leatherback sea turtle, which feeds mostly on jellyfish, the ocean sunfish, larger jellyfish such as the lion’s mane, and some seabirds and fish. Small crustaceans called amphipods often ride on moon jellies and feed on them. The larvae, polyps and ephyrae are eaten by filter-feeding animals such as mussels and sea squirts, and many never reach adulthood.

Natural hazards kill huge numbers each year. Storms tear the fragile bells, and onshore winds strand whole swarms on beaches. Hunger, warm late-summer water and disease finish off many adults after they breed.

Do moon jellyfish sting?

Yes, but weakly. Like all jellyfish, moon jellies have stinging cells, called cnidocytes, but theirs are tiny and their venom is mild. Most people who touch one feel nothing at all, or only a faint prickle, itch or redness that soon fades. Thin, sensitive skin, such as on the face, lips or inner arm, may react a little more.

That makes the moon jelly one of the least harmful jellyfish on European and North American coasts, far milder than the lion’s mane or the compass jellyfish, and nothing like the deadly Australian box jellyfish. It is still wise not to handle jellyfish, alive or stranded. Anyone who has trouble breathing, facial swelling or another severe reaction after any sting should seek emergency medical help immediately: call 911 (US), 112 (EU) or 999 (UK). See Do moon jellyfish sting? for more detail.

Conservation status

The moon jellyfish has not been evaluated by the IUCN Red List, and it is not protected by any international agreement. It is abundant across its range, and in many places it is seen as a nuisance rather than a species in need of protection. Its future is tied to the health of coastal seas: warming water and nutrient pollution can favor jellyfish over fish.

Moon jellyfish and people

Moon jellies are among the most familiar animals in public aquariums, where their slow pulsing under soft blue light draws crowds. They are relatively easy to raise from polyps, so many aquariums keep their own colonies instead of collecting jellies from the sea.

Big blooms can cause real trouble. Masses of moon jellies clog fishing nets and the cooling-water intakes of coastal power stations. In 2011, for example, the Torness nuclear power station in Scotland had to shut down its reactors for several days after a swarm blocked its seawater intake. Moon jellies have even been used in space research: in 1991, moon jellyfish polyps flew aboard the Space Shuttle Columbia so scientists could study how young jellyfish develop their sense of gravity in weightlessness.