The Jellyfish: Older Than Dinosaurs, Made of Almost Nothing

A jellyfish moves by squeezing its bell, pushing water out behind it in a slow pulse. It has no brain to plan with and no heart to pump with. Oxygen and food simply diffuse through a body so thin and watery that it barely needs plumbing. Its hunting gear is a set of tentacles armed with nematocysts, microscopic capsules that fire tiny harpoons on contact. Those harpoons are what sting you, and they are also how a creature with no brain catches dinner: plankton, small fish, whatever drifts too close. The family is enormous, thousands of species, found from icy Arctic water to the tropics and along every coastline. Most snorkelers will never meet the dangerous few. The ones below are the encounters to seek out, plus the honest safety notes for everywhere else.

The golden jellyfish of Palau's Jellyfish Lake carry algae inside their tissues, and the algae need sunlight. So every day the jellyfish follow the sun across the lake, turning slowly so their passengers get even light, then sink deeper at night to feed on nutrients below. You float among them while they do their rounds, bells the colour of amber, close enough to see the pulse. Moon jellies are the other gentle classic: a clear saucer with four horseshoe shapes visible in the bell, drifting through temperate coastal waters around the world. Their sting is mild, often barely felt. Then there is the upside-down jellyfish, which does everything backwards. It rests bell-down on the seabed in shallow mangrove lagoons, arms pointing up, farming sunlight for the algae inside it. It looks like a flower lying on the sand. It is not sick or stranded. That is simply how it lives. And one small marvel deserves a line of its own: the immortal jellyfish, a creature only a few millimetres across, can reverse its own life cycle when damaged or starved, returning from adult back to polyp and starting again. It can still be eaten or fall ill. It is not truly immortal. But no other animal we know bends the rules quite like that.

Most jellyfish stings are painful but not dangerous: an immediate burning feeling, red welts or stripes where the tentacles touched, and itching that can linger. A few kinds are a different matter, and snorkelers should know the difference before they get in. The box jellyfish, found in tropical northern Australia and parts of Southeast Asia, can deliver a sting severe enough to be life-threatening. The Irukandji, a tiny jellyfish of tropical Australian waters with a bell barely the size of a thumbnail, can produce severe symptoms that arrive after the sting itself. The Portuguese man o' war is not a true jellyfish at all but a colony of organisms working together, and its long tentacles give a famously painful sting. Dead jellyfish washed up on a beach can still sting, so admire those from a distance too.

1. Get out of the water and stay calm. Ask a lifeguard or someone with first aid training for help.
2. Rinse the area with seawater, never fresh water. Fresh water can trigger unfired stinging cells to discharge and make the sting worse.
3. Remove any tentacles still on the skin with tweezers or the edge of a bank card. Do not use bare hands, and do not rub the area.
4. Soak the sting in very warm water, as hot as you can tolerate, for at least 30 minutes. Hot flannels or towels work if you cannot soak it.
5. Take a simple painkiller if you need one.

What not to do: do not pee on the sting, do not rub it with sand or a towel, and do not reach for vinegar by default. Vinegar is the exception that proves the rule: for a box jellyfish sting, dousing the area with vinegar for 30 seconds before removing tentacles is the recommended step, because it stops unfired stinging cells from discharging. For most other jellyfish, including the moon jellies and barrel jellies common in European waters, vinegar can trigger more stinging cells to fire and make things worse. If you do not know what stung you, seawater and warm water are the safe defaults, and local advice beats any general rule. Get emergency help straight away if there is difficulty breathing, chest pain, seizures, severe swelling, vomiting, or lightheadedness, if the sting is on the mouth, throat, or near the eyes, if pain is severe and will not ease, or if a child has been stung. A sting in a tropical country deserves prompt medical attention even when it first seems mild.

Float, stay still, and let it drift. Never touch a jellyfish, chase one, or try to hold one up for a photo. Their bells tear easily, and a careless hand is the fastest way to turn a gentle encounter into a sting. Give every animal a wide berth, and give a whole bloom even more: swim around the edge, not through the middle. In Palau's lake, that rule is absolute. No touching, no sunscreen in the water, no fins kicking through the drift. At Kakaban, stay to the viewing edge — no swimming for now. You are a guest in their nursery, and the encounter is better for leaving it exactly as you found it.