Jellyfish: No Brain, No Heart, No Problem

Jellyfish are among the most ancient animals on Earth, drifting through our oceans for over 500 million years — predating dinosaurs, sharks, and even trees. They have no brain, no heart, no eyes, and no bones, yet they are highly effective predators that have outlasted countless other species.

Do Jellyfish Have Brains?

No — jellyfish have no brain, no heart, no blood, and no bones. Instead of a brain, they have a loose network of nerves called a nerve net spread throughout their body. This allows them to detect light, gravity, touch, and chemicals in the water — enough to hunt and survive.

How Do Jellyfish Sting?

Jellyfish tentacles are covered in tiny cells called cnidocytes, each containing a coiled, harpoon-like thread called a nematocyst. When triggered by touch or a chemical signal, the nematocyst fires in 700 nanoseconds — one of the fastest mechanisms in the animal kingdom — injecting venom into prey or a threat.

How Big Are Jellyfish?

Most jellyfish are small, but the lion's mane jellyfish is the world's largest, with a bell up to 7 feet wide and tentacles stretching up to 120 feet — longer than a blue whale. The smallest is the Irukandji jellyfish at less than an inch — yet it carries one of the most venomous stings on Earth.

Can Jellyfish Live Forever?

One species — Turritopsis dohrnii, the "immortal jellyfish" — can revert to its juvenile polyp stage after reaching adulthood when stressed or injured. It can repeat this cycle indefinitely, making it technically biologically immortal under the right conditions.

Amazing Jellyfish Facts

• Jellyfish are 95% water.
• A group of jellyfish is called a bloom or a smack.
• Some jellyfish are bioluminescent — they glow in the dark.
• Jellyfish are thriving as oceans warm and become more polluted — they are climate winners.
• Sea turtles eat jellyfish as a major part of their diet — and often mistake plastic bags for jellyfish.