Why do jellyfish leave a mark on your skin?

Jellyfish leave marks or stings that feel hot and itchy on your skin due to the release of small sacs filled with toxins called nematocysts during contact. They are a defence mechanism used by jellyfish and other related animals to capture and paralyse prey or ward off predators. The skin of humans is much thicker, relatively speaking, but their stings can still penetrate in some cases.

The level of toxicity varies considerably among different jellyfish species, and a single jellyfish may have thousands of nematocysts. Each nematocyst is a tiny, coiled, capsule-shaped structure that contains a barbed thread and a venom reservoir. When a nematocyst comes into contact with a foreign object, such as human skin, it discharges, releasing the thread and injecting the toxin into the skin. The toxin can cause a range of symptoms, including pain, swelling, itching, and skin irritation. In some cases, jellyfish stings can be fatal, particularly if the victim is allergic to the toxin.

When you brush against a jellyfish tentacle, the nematocysts are triggered, and their microscopic, barbed stingers penetrate your skin like microscopic hypodermic needles. Nematocysts also contain venom that can cause inflammation and other symptoms, depending on the type of jellyfish.

Although the marks or stings on the skin may not be dangerous in most cases for healthy adults, they can cause greater distress and may even be deadly to children, the elderly or people with heart or lung disorders.