Beautiful but Dangerous

Stingers of the Sea

Jellyfish have drifted through the oceans for more than 500 million years — long before the dinosaurs. Most are harmless to people. A few are among the most dangerous animals alive.

Jellyfish belong to a group of stinging animals called cnidarians, which also includes corals and sea anemones. Every cnidarian carries tiny stinging capsules called nematocysts. When something brushes a tentacle, these capsules fire a thread that injects venom. Because the toxin is injected rather than eaten, scientists call jellies venomous, not poisonous — the same way a snake is venomous but a bad mushroom is poisonous.

The gentle drifters

The moon jelly is the jelly most people meet. Its clear, saucer-shaped bell drifts with the currents through New York's bays and harbors, trailing short tentacles that catch tiny plankton. Its sting is so mild that most swimmers barely feel it.

The lion's mane jelly is the giant of the group and a true Long Island local. It blooms in the cold Atlantic each spring and summer, sometimes appearing along Montauk and the South Shore. Its tentacles can stretch the length of a school bus, and its sting is painful — but it is rarely deadly.

The deadly two

The box jellyfish breaks all the jellyfish rules. It has a cube-shaped bell, true eyes that can sense shapes, and the power to actively swim and chase its prey instead of drifting. Its venom is among the most powerful on Earth and can stop a human heart within minutes. Box jellyfish live in the warm Indo-Pacific waters around northern Australia.

The Portuguese man o' war looks like a floating blue balloon trailing long threads. It looks like a jellyfish, but it is actually a siphonophore — a colony of many specialized animals living and working together as one. A gas-filled float called a pneumatophore keeps it at the surface, where the wind pushes it across the ocean. Its tentacles can hang dozens of feet down. Man o' wars ride the warm Atlantic currents and sometimes wash up on Atlantic beaches, including on Long Island, where their sting stays painful even after they wash ashore.

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The Doctor Who Stung Himself

A real scientific mystery — and the risky experiment that solved it.

For years, swimmers along the beaches of northern Queensland, Australia were struck by a strange and terrifying illness. Minutes after a swim, a person would be hit by waves of crushing pain in the back, stomach, and chest, along with sweating, vomiting, and a racing heart. Doctors called it Irukandji syndrome, but no one knew what caused it — there was never a jellyfish in sight.

Dr. Jack Barnes, a physician in Cairns, had a hypothesis: the culprit was a tiny, nearly invisible box jellyfish, so small and clear that victims never saw it sting them. But how could he prove that one specific animal caused the illness?

In 1961, Barnes did something few scientists would dare. He found one of the tiny jellyfish in the shallow water and deliberately pressed it against his own arm. To be sure the result was no accident, he also let it sting his nine-year-old son and a local lifeguard who volunteered. Then he watched, and recorded everything.

About forty minutes later, all three were in agony. Their stomach muscles clenched into rigid, board-like spasms and they struggled to breathe. They were rushed to the hospital — and, thankfully, all three recovered. Barnes had his answer: the tiny box jellyfish was the cause of Irukandji syndrome.

Barnes published his results in 1964, and the jellyfish was later named Carukia barnesi in his honor. His experiment was dangerous and would never be allowed today — but it solved a medical mystery that had puzzled doctors for decades, and it shows how far a scientist will go to test a hypothesis.