Just as a seahorse isn’t actually a horse, a jellyfish isn’t a true fish.

These fascinating creatures may be misunderstood, but they play a vital role in the aquatic ecosystem.

While some species can deliver a painful sting, most harmlessly drift along with the ocean currents. As one of the oldest life forms on Earth, jellyfish are found in oceans across the globe and even in a few lakes. Here are ten intriguing jellyfish facts to help clear up some common misconceptions about these bizarre creatures.

1. Jellyfish Are Millions of Years Old

Jellyfish have been drifting through the oceans for an astonishing 505 million years. Although their soft, water-based bodies make fossilization rare, a 505-million-year-old jellyfish fossil was discovered in Utah. This means jellyfish have been riding the ocean currents for 250 million years longer than dinosaurs—and if history is any guide, they may outlast us too.

2. Jellyfish Can Glow in the Dark

Some jellyfish have the remarkable ability to glow in the dark through a process called bioluminescence. This occurs when a chemical reaction between luciferin and luciferase within their cells releases energy in the form of light. This glow helps jellyfish attract prey, hide from predators, and even communicate with each other.

3. No Brain, No Blood, No Heart

Jellyfish are among the simplest life forms on Earth. They lack hearts, blood, and brains and in fact, no organs at all. Their bodies are made up of two layers of cells: the internal gastrodermis and the external epidermis. Jellyfish have a single opening for eating, excreting waste, and reproducing.

4. Jellyfish Have Been to Space

In May 1991, scientists sent moon jellyfish polyps into space to study the effects of zero gravity on their development. Once the jellyfish matured, they were brought back to Earth, revealing that living without gravity affected their ability to sense up from down.

5. Jellyfish Can Pack a Punch

While most jellyfish species are harmless, a few, like the box jellyfish and the Irukandji, can deliver a potentially deadly sting. Fortunately, jellyfish don’t actively chase humans, and their slow movement makes them easy to avoid. And despite popular belief, peeing on a sting won't help relieve the pain.

6. Jellyfish Are Passive Carnivores

Jellyfish are opportunistic feeders, consuming anything that drifts by, including plankton, small crustaceans, fish eggs, and even other jellyfish. Interestingly, they use the same opening to eat and expel waste, making their digestive process a little unusual.

7. Some Jellyfish Are Edible

Jellyfish are considered a delicacy in several Asian countries, including China, South Korea, and the Philippines. Some environmentalists even encourage eating them to help control their populations. In addition to humans, creatures like turtles and the mola mola fish also enjoy feasting on jellyfish, especially during jellyfish blooms, like those in Jellyfish Lake in Palau.

8. From Tiny to Huge

There are about 2,000 known species of jellyfish, ranging vastly in size. The smallest, the creeping jellyfish, has a bell diameter as tiny as 0.5 millimeters, while the largest, Nomura's jellyfish, can grow to an impressive two meters across and weigh up to 200 kg.

9. Jellyfish Also Inhabit Lakes

Though most jellyfish live in the ocean, a few species thrive in brackish lakes. One of the most famous is Jellyfish Lake in Palau, home to millions of jellyfish. These jellyfish don't float with ocean currents but migrate through the lake in search of sunlight. With no natural predators, they’ve evolved to lack stingers, offering a rare opportunity for snorkelers to swim with these harmless creatures.

10. Some Jellyfish Are Immortal

The immortal jellyfish (Turritopsis dohrnii) has a fascinating ability to reverse its life cycle. Under certain conditions, it can transform its cells from medusa form back to it's polyp stage, essentially starting it's life all over again. This incredible process allows it to potentially live forever, continuously rejuvenating after reproduction—truly a biological marvel!

Jellyfish predate dinosaurs. How have they survived so long? - David Gruber

Video by TED-Ed