Despite what the residents of Bikini Bottom might have you believe, jellyfish don’t really produce delicious jelly. In fact, jellyfish aren’t really fish at all.

Jellyfish are very simple sea creatures that have lived in the oceans since before the time of the dinosaurs. Found in every ocean around the world, some jellyfish live in deep, cold water, while others prefer warm, shallow coastal waters. Scientists believe jellyfish have been around at least 500 million years.

Members of the scientific phylum Cnidaria, jellyfish are considered to be plankton and are closely related to sea anemones and corals. Since jellyfish aren’t really fish, many scientists prefer to call them “jellies” or “sea jellies” instead.

Floating along on an ocean current, jellyfish can be mesmerizing to watch. Their elegant and colorful bodies seem so mysterious.

Take them out of the water, though, and they become boring blobs. Why? Their bodies are more than 90 percent water!

Jellyfish don’t have bones, brains, hearts, blood or a central nervous system. Instead, they sense the world around them with a loose network of nerves called a “nerve net.”

Jellyfish consist of three basic layers. The outer layer, called the “epidermis,” contains the nerve net.

The middle layer is made of “mesoglea,” the thick, elastic stuff that looks like jelly. The final, inner layer is called the “gastrodermis.”

Inside their bell-shaped bodies, jellyfish have a large “mouth” where food comes in and waste goes out. Jellyfish eat fish, shrimp, crabs, small plants and sometimes even other jellyfish. They squirt water from their mouths to move themselves through the water.

The most recognizable feature of a jellyfish is its tentacles that hang down from its body. Fascinating to look at, these tentacles can be dangerous to touch.

Jellyfish can sting with their tentacles. They use them to stun prey before they eat them.

Jellyfish don’t purposefully attack humans. Most jellyfish stings occur when someone accidentally touches a jellyfish. Even a dead jellyfish can sting!

How harmful a jellyfish sting is depends on the type of jellyfish. Some jellyfish stings have little or no effect on humans, while others may cause minor discomfort to extreme pain.

The sting of a few types of jellyfish, though — such as the Australian sea wasp, the Irukandji and the Portuguese man-of-war — can be potentially fatal.

WONDERful jellyfish facts:

  • A group of jellyfish can be called a “bloom,” a “swarm” or a “smack.”
  • The lion’s mane jellyfish might be the longest animal in the world. Its thin tentacles can reach up to 120 feet long.
  • The Nomura’s jellyfish might be the largest jellyfish. Average specimens weigh 330 pounds, and the largest can reach 440 pounds.


32 Join the Discussion

1 Star2 Stars3 Stars4 Stars5 Stars  (22 votes, avg. 4.64 out of 5)
    • Thanks for letting us know how much you enjoyed today’s Wonder of the Day® about jellyfish, Sandi! We’ve watched the video lots of times…it’s so relaxing! Some parts are so “other worldly” that they almost look animated! We’re glad you stopped by Wonderopolis today! :-)

  1. Dear Wonderopolis,

    Wow! We learned a lot from this article. We did not know that jellyfish could be so big. We had no idea that the tentacles could be 100 feet. We also didn’t know that they don’t have brains or blood.

    Gabby wants to know if people can keep jellyfish in an aquarium as pets. Mason wonders how a person would catch a jellyfish.

    Thanks again!

    Mrs. Johnson’s third grade

    • We’re so glad you explored today’s Wonder together and learned some really cool new facts about jellyfish, Mrs. Johnson’s third grade! We WONDER what it would be like to see a 330-pound jellyfish floating by in the ocean? That is one BIG jellyfish!

      We found a neat article to answer Gabby’s question about people keeping jellyfish as pets! It looks like it might be a possibility sometime in the near future! Here is a link to the article:

      Now for Mason’s question…we think people accidentally catch jellyfish when they are fishing for other things, but we would caution our Wonder Friends to be VERY careful if they were going to really try to catch a jellyfish! It is super important to have a grown-up help and use a REALLY BIG net! It’s not as easy as Spongebob and Patrick make it seem on television, and it is VERY dangerous! Remember what we learned from today’s Wonder…even a dead jellyfish can sting you! We think we’ll just stick to visiting jellyfish in videos on Wonderopolis and at special sea life aquariums, how about you guys? :-)

  2. Dear Wonderopolis,
    Wooooooooooooooooow!!!! What an amazing video. I knew that jellyfish were not made out of jelly. I learned lots of new things. I think this wonder was the best. I cannot wait to see tomorrow’s wonder.

    • We’re glad you and so many other Wonder Friends enjoyed the video for today’s Wonder, Marco! Thank you for letting us know you learned some new things about jellyfish, too! :-)

    • We thought the jellyfish in the video for this Wonder of the Day® were very pretty, too, Natalia! We appreciate you leaving us another AWESOME comment! :-)

  3. I thought they were fish! Jellyfish are cool. When I was at Rancho El Chorro (and outdoor school where you stay the night), we went to the beach and we found a dead jellyfish on the sea shore.

    • Rancho El Chorro sounds like a really cool place to learn, Gracie! You are lucky that you got to stay there and experience all the WONDERS of an outdoor school! Thank you for hanging out in Wonderopolis today and for sharing your jellyfish story. :-)

  4. I read a book, and part of it was when someone who was allergic to Portuguese Man of Wars got stung by one (but didn’t die, but was really close to dying). That book was a really great book. It was called Dolphin Song (I forget who it was by)! :) :) :)

    • We’re glad you shared your personal connection to this Wonder, Missy! We think the Dolphin Song book you are talking about was written by author, Lauren St. John. Thanks for sharing about this book, too, so other Wonder Friends can learn more about it! :-)

    • We did some extra WONDERing after we got your comment yesterday, and found out about Lauren St. John! Her books seem very interesting! See, you inspired us to learn something new, too, Missy! Thanks for being a GREAT Wonder Friend! :-)

    • Thanks for always leaving us such smiley and enthusiastic comments when you visit Wonderopolis, Missy! We appreciate them very much! :-)

  5. How do you guys think of these crazy things? They are so interesting. You should do one on the most intelligent creatures (that aren’t people), which are DOLPHINS!!!!!!!!!!!!

    • That’s a great question, Anthony! Aside from the Australian sea wasp, the Irukandji and the Portuguese man-of-war mentioned in this Wonder of the Day®, we’d have to do a bit more WONDERing to find out if there were more jellyfish whose stings could be fatal. Thanks for WONDERing even more about this Wonder after you explored it! :-)

    • That sounds like a very interesting culinary creation, Andrew! We like to try new things, but we think we might have to take your WONDER word on this one! :-)

    • WOW, a 50 foot jellyfish, Evelyn! We bet that would be awesome… but we’d like to see it from a very far distance! Take a look at the excerpt below– we Wonder if you can find your answers here:

      “A group of jellyfish can be called a “bloom,” a “swarm” or a “smack.”
      The lion’s mane jellyfish might be the longest animal in the world. Its thin tentacles can reach up to 120 feet long.
      The Nomura’s jellyfish might be the largest jellyfish. Average specimens weigh 330 pounds, and the largest can reach 440 pounds.”


  6. What is the size of the smallest jellyfish? Sorry I’m asking soooooooooooooooo many question. I have to do a report on this, so I want to get all the info I can. :)

  7. I did not like this article. Most of the facts I already knew and it was very inspesific. I will see if you have anything better. :(

    • Hey there, Ranking101! We are sorry to hear that today’s Wonder wasn’t one of your favorites, but we have lots of Wonders to share! We hope you’ll find one that makes you smile! :)

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Have you ever wondered…

  • Are jellyfish made of jelly?
  • Are jellyfish really fish?
  • Can a jellyfish sting be fatal?

Wonder Gallery

jellyfish_shutterstock_56875405Vimeo Video

Try It Out

OK… so now you know that jellyfish aren’t made of jelly. The jelly you eat on your peanut butter sandwiches doesn’t come from jellyfish either.

If you’re hungry, though, there’s no need to run to the ocean. You can make your own homemade jellies and jams at home in your own kitchen.

Check out a few of these recipes, and then choose one to make with a friend or family member:


Still Wondering

Check out National Geographic Xpeditions’ Glowing in the Dark: Bioluminescence picture gallery to learn about creatures that create their own light.


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