You’ve definitely heard of jellyfish, but how much do you really know about them? They’re sort of simple (read on to learn about that!), but also incredibly complicated! Read on to learn how old they are, the most unusual place they’ve ever lived, and why that ONE myth about jellyfish stings really isn’t true! Don’t forget to check out our other amazing animal facts when you’re done – we’ve got facts on orcas, pufferfish, and goblin sharks to get you started!

1. They’re not fish

Just like how seahorses aren’t horses, jellyfish aren’t fish at all! Jellyfish are creatures in the phylum Cnidaria, which also includes sea anemones and coral. The name comes from the Greek word for “sea nettle”, because cnidarians have stinging cells! Over 2,000 species of jellyfish have been discovered but there are almost definitely lots more – some are very tiny, and the ocean is absolutely huge!

2. They have no brains, hearts, or lungs

Jellyfish are VERY different in their bodily structure than mammals. They’re known as simple organisms, which means there isn’t very much to their anatomy – like minimalism, but for a body! Instead of a centralised brain (which is what we have) they have a decentralised nerve net, which helps them move and react to external forces. They’re so thin they can absorb oxygen without the need for lungs or blood, so a heart is also unnecessary. Some species do have eyes (sight receptors that are less complicated than other animal eyes), or teeth (thin hairs that pull in food). They do have muscles though – they move by pulsing their bell-shaped bodies.

3. They’re 95% water!

The defining feature of jellyfish is their jelliness, and jellyfish are more jelly than most creatures in the world! Just over 95% of their body mass is water. For comparison, the human body is about 60% water!

4. They can be tiny or huge!

There are so many species of jellyfish and they come in a staggering variety of sizes! The lion’s mane jellyfish (pictured above) is one of the largest species – the biggest recorded specimen had a bell 7 foot across and tentacles 120 foot long! Another whopper is the Nomura jellyfish, which can grow up to 6.7 foot and weigh up to 200kg! On the other end of the scale we have teeny tiny jellies like the Irukandji jellyfish, which is only 1cm to 2cm long, but packs a deadly sting! The smallest known jellyfish are in the Staurocladia and Eleutheria genera, and are less than a millimetre in length.

5. They’ve been around for a LONG time

Jellyfish have been around for at least 500 million years and possibly as long as 700 million years. That’s before there were dinosaurs, trees, or even fungi on Earth! That makes them the oldest multi-organ animal, but you won’t find many jellyfish fossils because they have no hard body parts! A few HAVE been discovered though, because jellyfish can make “soft fossils” – imprints in rock. A 505 million year old fossil was found in Canada in 2023!

6. Their tentacles have little venomous harpoons

Ever wondered what makes a jellyfish sting? It’s all in their tentacles! They’re covered with cells called cnidocytes, which have even smaller cells called nematocysts. These are teeny tiny harpoon-shaped barbs that inject venom into prey, or into potential threats.This helps the jellyfish feed and stay safe from predators, but it can also affect humans who get tangled in their tentacles. Jellyfish stings vary from slightly sore to very painful and extremely deadly – more on THAT later!

7. Turtles love to snack on them

Although jellyfish are usually near the top of the food chain where they live, they are still prey for several creatures including sea urchins, penguins, sharks, swordfish, tuna and sea turtles! This is another reason to keep plastic out of the sea – turtles have been known to mistake plastic bags for jellyfish and try to eat them, which can be fatal.

8. Some people eat them!

And speaking of snacks – believe it or not, jellyfish are widely eaten in parts of East and Southeast Asia! It’s cleaned, processed and dried before being cooked in a variety of dishes. Jellyfish salad is popular in several places, and you can also experience jellyfish sushi and candy in Japan and crunchy jellyfish noodles in Thailand!

9. Box jellyfish are very dangerous

The most venomous creature in the world is a jellyfish – the Box Jellyfish, which can be found floating in the ocean of the Indo-Pacific area. There are about 61 species and some aren’t harmful to humans, but others are VERY deadly. The sting of these deadly jellies is capable of killing an adult human in minutes. Australian swimmers have to be particularly wary of these gelatinous fiends, but don’t worry too much – they don’t chase humans, so as long as you keep your eyes peeled for long tentacles and do what lifeguards tell you, you should be all right! Box jellyfish are especially interesting because they actually have “eyes” – that is to say 24 visual sensors, two of which can see in colour! The eyes are on all four sides of their bell, meaning they have 360 degree vision.

10. Their mouths are also their bums

Jellyfish are carnivores and love to snack on plankton, crustaceans and small fish. Jellyfish feed by drifting and catching prey in their net-like tentacles, where their venom paralyses it and makes it easier to eat. Here’s something gross – due to their simple anatomy, they actually eat through the same hole they poop through!

11. Portuguese man o’ war are NOT jellyfish

You might have heard of the Portuguese man o’war, a sea creature which looks a lot like a jellyfish. They’re also in the phylum Cnidaria, but they’re in a different order called siphonophore. The really interesting thing about them is that a Portuguese man o’ war is not a single animal – it’s actually a colony! Seven smaller organisms called zooids make up the man o’ war, and they work together to trap prey, move through the water and reproduce. Like some jellyfish they hunt by trawling, and they also pack a painful sting, – so if you see one, stay away from its tentacles!

12. They can produce light

Bioluminescence is when a biological organism emits light through chemical reactions, and several species of jellyfish can do it! It has various purposes, including attracting prey, scaring predators, and sending messages to other jellyfish!

13. A group is called a smack

A collective noun is the word used to describe a group of things or animals – like a herd of cows, or a flock of birds. There are several collective nouns for jellyfish, and one of the most common is “smack”! “Swarm” and “drift” are also sometimes used, and when there’s a big increase in a jellyfish population in one area it’s called a “bloom”! They don’t live in groups though, preferring to drift alone unless several get caught in a current or near a food source.

14. They’ve been to space!

In 1991, NASA sent some jellyfish into outer space! This wasn’t just for the giggles, of course – it was done to study the effect of zero-gravity on living creatures. But why jellyfish in particular? Well, even though jellyfish are so different from us they actually share some of the same gravity-sensitive calcium crystals that are used for orientation. Ours are in our ears, theirs are on the bottom part of their bell-shaped bodies. So it’s actually a useful way to develop techniques to keep astronauts from getting dizzy! About 60,000 jellyfish ended up in space, and when they got back they had trouble swimming. Scientists concluded that the trip gave the jellyfish severe vertigo, and they had difficulty moving in water when they returned. This suggests that any babies born in space might have trouble with dizziness and balance when they came to earth!

15. No, you shouldn’t pee on a jellyfish sting

Ever seen that one episode of Friends? Well, it’s actually a total myth – pee will NOT help your jellyfish sting! In fact it might actually make it worse, because the pee could release more venom. Vinegar or a seawater and baking soda solution would be better, though you’ll still need to remove the bits of tentacle afterwards. It’s a good idea to seek medical advice immediately if you get stung!