Just as a seahorse is not a horse, a jellyfish is not a fish. These misunderstood creatures play an important role in the aquatic ecosystem, and while there are a few species that can inflict a painful sting, the vast majority harmlessly float by on the ocean's currents. As one of the oldest forms of life on our Blue Planet, they can be found throughout the world's oceans (and even a few lakes). Here are ten jellyfish facts to dispel your preconceptions of these bizarre, intriguing creatures.

Jellyfish

1. Jellyfish Are Millions of Years Old

The likelihood of finding jellyfish fossils is rare. After all, these soft-bodied creatures are mainly composed of water. However, the US state of Utah defied the odds with the discovery of a 505-million-year-old jellyfish fossil. This means that jellyfish have been riding ocean currents for 250 million years longer than dinosaurs and, if we were to hedge a bet, will likely outlive us landlubbers, too.

2. Jellyfish Can Glow in The Dark

Some jellyfish have developed the ability to create their own light. Called bioluminescence, this light is a result of the energy released by a chemical reaction between luciferin and luciferase, occuring within the cells and tissues of jellyfish. Bioluminescence enables them to lure prey, camouflage themselves from predators and even communicate among other jellyfish.

Jellyfish

3. No Brain, No Blood, No Heart

As one of the planet's simpler life forms, jellyfish do not have hearts, blood or brains. In fact, they have no organs at all. Jellyfish have bodies formed of two different layers of cells: the internal gastrodermis and the external epidermis. They eat food, expel waste products and procreate using a solitary opening in the gastrodermis.

4. Jellyfish Have Been To The Final Frontier

In May 1991, scientists made one small step for jellyfish, one giant(ish) leap for mankind, by launching moon jellyfish polyps into space. Once the polyps matured into jellyfish, they were returned to Earth to study the effects of gravity on jellyfish. Turns out that developing without gravity had affected their ability to tell up from down.

Box Jellyfish

5. Jellyfish Can Pack a Punch

Jellyfish are best known for their stingers. However, only a few species, like the Irukandji and box jellyfish, are deadly. The good news is that jellyfish don't chase humans, and even if they did, they're so slow that they can be easily avoided. We also hate to break it to you, but peeing on a sting won't relieve the pain.

6. Jellyfish Are Passive Carnivores

Jellyfish are opportunistic eaters that feed on anything drifting in the ocean, including plankton, tiny crustaceans, other jellyfish, fish eggs and larval fish. Bonus jellyfish fact: they eat and defecate through the same hole in the middle of the bell.

Turtles eating jellyfish

7. Some Jellyfish Are Edible

Believe it or not, you can eat jellyfish. In some Asian countries - like Myanmar, China, Indonesia, South Korea, Malaysia, the Philippines and Thailand - jellyfish is a delicacy. In fact, some environmentalists are actively encouraging eating them to keep their population in check. But it's not just humans who eat jellyfish. Turtles and mola mola also enjoy gorging on sea jellies, which you can see in action during thejellyfish blooms in Jellyfish Lake in Palau and Nusa Penida in Indonesia.

8. From Tiny to Huge

There are 2,000 known species of jellyfish. The smallest jellyfish (the creeping jellyfish) has bell disks ranging from 0.5 millimetres to a few millimetres in diameter, while the largest jellyfish in the world (Nomura's jellyfish) can reach two metres in bell diameter and can weigh up to 200kg.

Jellyfish Lake

10. Jellyfish Also Inhabit Lakes

While most jellyfish live in the ocean, there are a few that have taken up residence in brackish lakes. The most famous example of this is Jellyfish Lake in Palau, which hosts millions of jellyfish. Unlike other jellyfish, they don't drift in currents but migrate around the lake chasing the sun. With no predators in the lake, these jellyfish have evolved without stingers, presenting the unique opportunity to snorkel alongside millions of harmless jellyfish. For the best jellyfish encounters, enter the western side of the lake at sunrise when these pulsating sea jellies start their daily migration. For the best visibility, visit during the dry season (December to April).

9. Some Jellyfish Are Immortal

The aptly-named immortal jellyfish (Turritopsis dohrnii) has fascinated scientists with its apparent ability to, under certain circumstances, transform its cells from medusa (adulthood) back to its polyp stage (childhood). Much like Brad Pitt in Benjamin Button, these jellyfish will revert back to young jellyfish after reproduction, thus starting the whole life journey again.