Jellyfish
Medusozoa
Quick answer
Jellyfish are gelatinous marine animals that have drifted through the oceans for over 500 million years, longer than the dinosaurs. They have no brain, heart, or bones, are about 95% water, and capture prey using stinging tentacles. Sizes range from a few millimeters to the lion's mane jellyfish with tentacles over 30 m (100 ft) long, and the box jellyfish is among the most venomous animals on Earth.
Jellyfish facts at a glance
| Scientific name | Medusozoa (subphylum) |
|---|---|
| Diet | Carnivore (plankton and small fish) |
| Habitat | Oceans worldwide, surface to deep sea |
| Lifespan | A few months to a few years |
| Composition | About 95% water |
| Top speed | Slow drifters; weak swimmers |
| Conservation status | Varies by species (most not assessed) |
| Kingdom | Animalia |
|---|---|
| Phylum | Cnidaria |
| Class | Scyphozoa |
Life without a brain
Jellyfish have no brain, heart, blood, or bones. They sense the world through a simple net of nerves and react to light, touch, and chemicals in the water. Made of around 95% water, their bell pulses gently to move, though they mostly drift with the currents.
Stinging tentacles
A jellyfish's trailing tentacles are armed with thousands of microscopic stinging capsules called nematocysts that fire venom-tipped threads on contact, paralyzing plankton and small fish. Some species are harmless to people, while the box jellyfish carries venom potent enough to be life-threatening.
Ancient survivors
Jellyfish are among the oldest animals on the planet, drifting through the seas for more than 500 million years, long before dinosaurs or even trees existed. They survive in every ocean, from warm surface waters to the freezing deep, which is part of why they have endured through mass extinctions.
Blooms and the 'immortal' jelly
Under the right conditions jellyfish can multiply into vast swarms called blooms, sometimes linked to warming seas and overfishing of their predators. One species, Turritopsis dohrnii, can revert to an earlier life stage when stressed, earning it the nickname the 'immortal jellyfish'.
Frequently asked questions about the Jellyfish
Do jellyfish have brains?
No. Jellyfish have no brain, heart, or bones. They use a simple network of nerves to sense light, touch, and chemicals and to coordinate the pulsing of their bell.
How do jellyfish sting?
Jellyfish sting using tiny capsules called nematocysts on their tentacles, which fire venom-tipped threads when touched, paralyzing prey. Some stings are harmless to humans while others, like the box jellyfish, can be deadly.
What do jellyfish eat?
Jellyfish are carnivores that feed mostly on plankton, tiny crustaceans, fish eggs, and small fish, capturing them with their stinging tentacles.
How long have jellyfish existed?
Jellyfish have existed for more than 500 million years, making them far older than dinosaurs and among the most ancient animals still alive today.
Is there really an immortal jellyfish?
Yes, sort of. The species Turritopsis dohrnii can revert from its adult form back to an earlier polyp stage when stressed or injured, potentially restarting its life cycle, which is why it is nicknamed the immortal jellyfish.
