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Aurelia aurita
The moon jelly is one of the jellies most often found washed up on the sand: a clear, dinner-plate-sized disc with four clover-shaped gonads showing through the bell like petals. Its tentacles are only a short fringe, and its sting is very mild or goes unnoticed, so it is a good animal to learn on.
Its life cycle is the classic jellyfish story. Larvae settle on or near the seafloor and grow into tiny polyps, which can bud new polyps and later release baby jellies called ephyrae that grow into the drifting adults. One settled larva can therefore become many jellyfish.
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Portuguese man o' war
The Portuguese man o' war looks like a jellyfish but is a siphonophore: a colony of specialized, genetically identical individuals working as one animal. The blue, violet or pink balloon on top rides up to six inches above the waterline, while the tentacles below average about 30 feet.
The sting is rarely deadly but very painful, and it leaves welts. Two things matter on the beach. A stranded float can still sting weeks after washing up, so leave the pretty blue balloons alone. And vinegar, the remedy for box jellyfish, can make this sting worse; hot water is the better relief.
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Chironex fleckeri
The Australian box jellyfish is the most feared animal on this list, and the reason northern Australian beaches post stinger warnings. Its pale, boxy bell reaches about 30 cm and trails up to 60 tentacles in four clumps, each up to 3 m long, and it is found most often in summer in inshore waters.
It lives along tropical coasts from Western Australia through the Northern Territory to Queensland; in one Northern Territory study, 92% of stings came during the stinger season of October 1 to June 1. Most stings are not life-threatening, but people who die usually go into cardiac arrest within minutes. Flood the sting with vinegar for at least 30 seconds and get medical help at once.
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Carukia barnesi
Irukandji jellyfish show why size says nothing about danger. Carukia barnesi, the species best known for it, is a tiny box jellyfish with a transparent bell only 1.5 to 2.5 cm across, and its sting causes only minor skin irritation at first.
The trouble comes later. Some 5 to 40 minutes after the sting, Irukandji syndrome can set in, bringing generalized pain, often severely high blood pressure, nausea, vomiting and distress. Several small box jellyfish are suspected of causing it, but Carukia barnesi is the only one conclusively known to. Anyone stung in tropical Australian waters should have vinegar poured on the sting for at least 30 seconds and an ambulance called, even if the sting seems mild.
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Lion's mane jellyfish
The lion's mane is the giant of cold northern seas, with a reddish or yellowish-brown bell and eight clusters of tentacles, each cluster holding 70 to 150 or more. It is usually 30 to 50 cm across in British waters, but the largest have been reported at around 2 m, which makes it a contender for the world's largest jellyfish.
Accounts of its sting range from moderately painful to very severe, with blisters and muscle cramps. Its biggest risk on a beach is that it keeps stinging long after it strands, and tentacles that break off in the water can still sting swimmers who never saw the bell. Larger animals are most common from June to September.
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Chrysaora fuscescens
The Pacific sea nettle is the yellowish to reddish-brown jelly with long, frilly mouth-arms, found in coastal waters from Alaska to California and across to Japan and the Bering Sea. Its bell can reach about 45 cm, and the ruffled mouth-arms trail 12 to 15 feet below it.
Not every jelly stings, but this one does: its tentacles and mouth-arms are covered with stinging cells, so admire it from a boat or through aquarium glass rather than in the water. Sea nettles are abundant enough to matter in the plankton food chain, and changes people make to coastal waters are linked to more frequent jelly blooms.
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Chrysaora chesapeakei
The bay nettle is the summer jelly of the Chesapeake Bay, where it is the most common species and can be around from May to October. Until recently it was filed under the Atlantic sea nettle, Chrysaora quinquecirrha, so older guides use that name.
An adult has a bell up to 8 inches across with 24 tentacles several feet long and four long, ruffled oral arms. Color is a useful clue: nettles in the fresher upper bay are usually milky white, while those in saltier water often carry reddish-brown to purplish stripes. The sting is moderately painful but not dangerous unless someone has an allergic reaction.
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Velella
By-the-wind sailors are the teacup-sized blue ovals that sometimes carpet West Coast beaches by the thousands. Each one is a floating colony of specialized polyps rather than a single jellyfish, and it travels by a stiff, clear sail standing about an inch above the water, so the wind decides where it goes.
Most West Coast strandings happen in spring, and big strandings have followed warmer winters that let their numbers spike offshore. Their short tentacles catch plankton and larval fish, but their stinging cells cannot harm people, which makes a blue tide line of them one of the easier mass strandings to enjoy. Look rather than handle them all the same.
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Porpita porpita
The blue button looks like a small, bright blue jellyfish fringed with tentacles, but it is a colony of many small hydroid animals rather than a true jelly. It drifts at the surface and now and then washes up in numbers, as it did on Texas beaches such as Galveston Island in the summer of 2023.
It is worth recognizing because a blue drifter on the sand could also be a Portuguese man o' war, which stings far worse. The blue button's white-tipped tentacles carry stinging cells, but it is not deadly to humans; the usual result of handling one is skin irritation. Look, and leave it on the sand.
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Cannonball jellyfish
The cannonball jelly is a firm, round dome somewhere between a softball and a volleyball in size, usually white with a reddish-brown lower edge. It has no tentacles around the outside of the bell, which gives it its smooth, ball-like look as it pulses along.
Its sting is mild if felt at all, so this is one of the jellies a beachgoer can relax about when one drifts past. In the Chesapeake Bay it is only an uncommon winter visitor.
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Cassiopea xamachana
The upside-down jellyfish spends its life lying bell-down on the bottom of mangrove shallows, frilly arms facing the sun. It does that to feed its partners: algae living in its tissues photosynthesize, and what they make fuels the jelly's growth and reproduction.
It also solved an old puzzle. Snorkelers in mangrove waters long complained of "stinging water" without touching any animal. A 2020 study found the likely cause: these jellies release mucus carrying tiny, mobile balls of stinging cells, named cassiosomes, that can kill small prey. If the water prickles in a lagoon full of these pale discs, that mucus is the probable reason.
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Pelagia noctiluca
The mauve stinger is a small open-ocean jellyfish of the northeast Atlantic and the Mediterranean, and one of the most abundant jellies in the western Mediterranean. Its bell grows to about 10 cm and ranges from pale red to mauve-brown or purple.
Its life is unusual for a jellyfish: it has no polyp stage at all, and adults release juvenile medusae directly in the autumn, so it never needs a seafloor to settle on. The sting is potent and painful but limited in time and extent. Treat a swarm near a European beach as a good reason to sit that swim out.
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Chrysaora hysoscella
The compass jellyfish is easy to name once you have seen one: brown V-shaped markings radiate around a central spot on its pale bell, like the face of a compass. It lives in the northeast Atlantic, including British waters, as well as the Mediterranean and off South Africa.
Its life has a twist. Each one matures as a male and later switches to producing eggs, changing from male to female over its lifespan. The sting is painful but rarely severe, so treat it with respect without panicking.
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Rhizostoma pulmo
The barrel jellyfish is the big, rubbery one that alarms swimmers around Britain and Europe, with a mushroom-shaped bell up to about 1 meter across and eight frilly arms instead of trailing tentacles. Swarms gather in warmer coastal waters from late spring, and it can be seen from May to October.
Its size is out of all proportion to its threat: the sting is not normally harmful to people. It is a favorite food of leatherback turtles, the world's largest sea turtle. Some British guides treat the barrel jellyfish of UK waters as a separate species, Rhizostoma octopus, while others call it Rhizostoma pulmo, so expect both names.
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Mnemiopsis
Sea walnut, Mnemiopsis leidyi
The sea walnut is a comb jelly, a relative that is often lumped in with jellyfish but works differently. Shaped like the meat of a walnut and as clear as glass, it swims with rows of tiny hairs that look like the teeth of a comb, and those rows scatter light into pulsing rainbows along its body.
Comb jellies do not sting at all. Instead of stinging cells they catch food with sticky cells, so a handful of sea walnuts in a bucket is perfectly safe. At night they can flash with their own light when disturbed, and are often seen glowing in boat wakes.
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Turritopsis dohrnii
The immortal jellyfish is about half the size of a pinky nail, and you will almost certainly never notice one on a swim. It is here for what it can do. When an adult is aging, damaged or stressed, it sinks, shrinks into a ball of tissue and turns back into a polyp, the stage a jellyfish normally leaves behind, then starts the usual polyp-to-medusa cycle again.
That is why it is called biologically immortal: it does not die of old age. It is not invulnerable, since predators and disease still kill it. A 2022 genome study found it carries extra copies of genes for repairing and copying DNA compared with a close relative that cannot reverse its life cycle.
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Aequorea victoria
The crystal jelly is a delicate, see-through hydrozoan jelly that drifts with the currents off the west coast of North America, and it may be the most useful jellyfish ever studied. Green fluorescent protein was first observed in it in 1962, and Osamu Shimomura, who isolated the protein, found that it glows bright green under ultraviolet light.
That protein became one of the most important tools in modern bioscience, a glowing tag that lets researchers watch genes and cells at work, and it won Shimomura, Martin Chalfie and Roger Tsien the 2008 Nobel Prize in Chemistry. It is the jelly to know for anyone curious about why scientists keep studying these animals.
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Nomura's jellyfish
Nomura's jellyfish is a giant of East Asian seas, reaching about 6 feet across and weighing up to 450 pounds. Researchers traced its breeding grounds to China's coastal waters, from where blooms drift into the seas around Japan.
In bloom years those swarms have hit Japanese fishing hard: the jellies clog and rip nets, and their venom can taint or kill the fish caught alongside them, ruining a whole day's catch. Swimmers rarely meet one, but it belongs on any list of jellyfish worth knowing as the clearest example of how a jelly bloom can upend a coastline's economy.
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