Twelve jellyfish facts, fact-checked
Jellyfish are among the oldest multi-organ animals on Earth, and among the simplest. Full profile in our jellyfish guide.
1. No brain, heart, bones or blood
A nerve net coordinates the body, and oxygen diffuses directly through thin tissue.
2. They are about 95% water
Which is why a stranded jellyfish effectively disappears as it dries.
3. They predate dinosaurs
Jellyfish-like animals appear in the fossil record over 500 million years ago.
4. One species can reverse its ageing
Turritopsis dohrnii can revert from adult medusa back to its polyp stage, potentially indefinitely — though it can still be eaten or diseased.
5. Stinging cells fire mechanically
Nematocysts discharge in microseconds when triggered — one of the fastest movements in nature. They fire even after the animal dies.
6. They are not fish
Jellyfish are cnidarians, related to corals and sea anemones, not vertebrates.
7. Box jellyfish have real eyes
Twenty-four eyes including lensed ones, and they actively navigate rather than drift.
8. Some are extremely dangerous
Australian box jellyfish venom is among the most potent known, though most species are harmless to people.
9. The lion's mane can exceed 30 m
Tentacle-tip to tentacle-tip, making it one of the longest animals on Earth.
10. They can form vast blooms
Blooms clog power station intakes and fishing nets, and are linked to warming, overfishing of predators and nutrient runoff.
11. Some host algae
Species such as the upside-down jellyfish farm symbiotic algae and rest bell-down to expose them to sunlight.
12. Urine does not help a sting
It can make things worse by triggering unfired nematocysts. Vinegar is recommended for box jellyfish stings; hot water immersion helps with many others.
Read the full jellyfish wildlife guide for IUCN status, population context and sources — or donate to WARN to help fund the partner teams protecting wildlife on the ground.
