Do Moon Jellyfish Sting? Are They Dangerous to Swimmers?

How strong is a moon jellyfish sting?

The moon jellyfish catches plankton, not fish, so it has no need for powerful venom. Its stinging cells sit mainly on its short fringe of tentacles and on its four oral arms, and they are too small to pierce thick skin easily.

When a sting does happen, it usually feels like a mild tingle or itch. A small red patch or a few bumps may appear where the jellyfish brushed the skin and usually fade within minutes to hours. Thin skin on the face, lips, neck or inner arms is more likely to react than the palms of the hands, and children and people with sensitive skin may notice it more.

Serious reactions to moon jellyfish are very rare. As with any sting, though, a small number of people can have an allergic reaction.

How jellyfish stinging cells work

All jellyfish, including moon jellies, carry stinging cells called cnidocytes. Each one holds a tiny capsule, the nematocyst, with a coiled, hollow thread inside. When the cell is touched by prey, often together with a chemical signal, the capsule bursts open and fires the thread like a harpoon, in one of the fastest movements known in nature. The thread pierces the skin of the prey and injects venom.

The strength of a sting depends on the type of capsule, the length of the threads, the number that fire and the venom inside. Moon jellies have short threads and weak venom, which is why they barely affect people. Box jellyfish, by contrast, have long threads and extremely potent venom (see How dangerous is a box jellyfish?).

Stinging cells fire automatically, without any decision by the jellyfish. That is why they can still fire after the animal has died or a tentacle has broken off.

Are moon jellyfish dangerous to swimmers?

For most swimmers, no. Moon jellies cannot chase people, they drift with the current, and their stings are mild. The main nuisance comes in summer, when large blooms gather in harbors, fjords and bays, and swimmers may bump into dozens of them.

Blooms can also include other species, and that is the bigger concern. Moon jellies often share the water with the lion’s mane jellyfish in northern Europe and New England, or with sea nettles along the US East Coast. Because these look different but can be hard to see in murky water, the safest approach is to avoid contact with all jellyfish.

Jellyfish that sting more in Europe and North America

Several common jellyfish of European and North American waters give stings that are noticeably more painful than a moon jelly’s:

The lion’s mane is the largest jellyfish in these waters, with a reddish-brown bell up to about 2 m (6.5 ft) across in rare giants and tentacles that can trail many meters. The Portuguese man o’ war is not a true jellyfish but a floating colony of hydrozoans (see Is a Portuguese man o’ war a jellyfish?).

Can a dead or stranded jellyfish still sting?

Yes. Stinging cells can keep working for some time after a jellyfish washes up or dies, and loose tentacles in the water or on the sand can still sting. A stranded moon jelly will rarely cause more than a mild tingle, but a stranded lion’s mane or Portuguese man o’ war can still give a painful sting.

It is best to look at stranded jellyfish without touching them, and to keep children and dogs away from them on the beach.

How to avoid jellyfish stings

A few simple habits greatly reduce the chance of being stung:

• Check local warnings. Lifeguards and beach signs report jellyfish blooms. In the United States a purple flag signals dangerous marine life.
• Look before you swim. Moon jellies are easiest to spot from a pier or jetty, where their four pale rings stand out in the water.
• Avoid touching jellyfish, alive or dead, including on the beach and in rock pools.
• Cover up. A long-sleeved swim shirt, wetsuit or full-length swimsuit protects much of the skin when jellyfish are around.
• Be careful after onshore winds, which can push swarms into bays and onto beaches.

When to get medical help

Most moon jellyfish stings are mild and fade on their own, while stings from other species can be far more painful. If anyone has trouble breathing, swelling of the face or throat, chest pain, a spreading rash or feels faint after any jellyfish sting, seek emergency medical help immediately: call 911 (US), 112 (EU) or 999 (UK). Lifeguards on patrolled beaches can also give advice about local species.