10 Incredible Jellyfish Facts

Deadliest Jellyfish - Box Jellyfish
© Nuttawut Uttamaharad/Shutterstock.com

Written by Kellianne Matthews

Published: September 10, 2022

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One of the ocean’s most fascinating creatures is the jellyfish. Jellyfish are part of the subphylum Medusozoa, with more than 4,000 species. However, scientists believe there are many jellyfish species we have not discovered yet—possibly even as many as 300,000! Most jellyfish are marine animals that live in saltwater, but there are a few species that actually live in freshwater. These mesmerizing creatures float through every ocean on earth, in both warm and cold climates, as well as shallow waters and the deep sea. Let’s take a closer look at 10 incredible jellyfish facts!

1.      Jellyfish Might be Even Older Than Dinosaurs

Yellow Jellyfish in ocean

Jellyfish swim through the water by pulsating their bells.

©Dan90266 / Creative Commons – Original

Fossils of jellyfish are difficult to find because jellyfish do not have any bones! However, those that have been uncovered are extremely old. For example, many well-preserved jellyfish fossils were discovered in Utah that date back to 505 million years ago when it was covered by ocean waters. Many scientists believe that jellyfish may have lived even earlier, however, possibly up to 700 million years ago!

2.      Jellyfish Are Not Fish

Weakest Animals Jellyfish: crystal jellies

Crystal jellies (

Aequorea victoria

), are a bioluminescent hydrozoan jellyfish, or hydromedusa, that is found off the west coast of North America.


Although we call them “jelly-fish,” these unique ocean-dwellers are not actually fish at all! Whereas fish have backbones, jellyfish are invertebrates without scales, fins, or gills. “Jellyfish” (or sea jellies) describes several groups of water-dwelling, gelatinous animals like “true” jellyfish, comb jellyfish, box jellyfish, and sometimes siphonophores. Jellyfish move through the ocean by pulsating their bells and absorb oxygen through their thin membranes.

3.      Jellyfish Comes in All Shapes and Sizes

Lion’s Mane Jellyfish, Cyanea capillata, at Coll island, Scotland.

Lion’s Mane Jellyfish (

Cyanea capillata

) at Coll island, Scotland. This giant stinging jelly can grow huge with tentacles reaching over 100 feet long.

©Martin Prochazkacz/Shutterstock.com

There are countless varieties of jellyfish in the ocean. “True Jellyfish”, for example, have the iconic “jellyfish” shape, with an umbrella-shaped bell on top, and tentacles and/or oral arms streaming out below. Tentacles are straight and slender, but they can be short or long. Oral arms are much thicker than tentacles and can be short, long, bushy, embellished, or branching. True jellyfish cannot control their movements very well, so they rely instead on the ocean’s current for transportation.

Box Jellyfish, on the other hand, look like gummy boxes with long streaming ribbons. They have one or more stinging tentacles that hang from the four corners of their cube-shaped bell. Box jellyfish have much more control than true jellyfish and can swim through the water much quicker.

One of the largest jellyfish species is the Lion’s Mane Jellyfish (Cyanea capillata). The bulk of this large jellyfish comes from the enormous, hair-like tentacles that flow from its bell like a lion’s mane. The largest lion jellyfish ever recorded had a bell that was 7 feet in diameter, and tentacles that were 120 feet long!

The smallest jellyfish species is the Common Kingslayer. Measuring less than half an inch, this tiny jelly is smaller than a fingernail! However, don’t let its small size fool you. Common kingslayers are one of the most dangerous jellyfish, and one of the most venomous animals on earth.

4.      Jellyfish Do Not Have Hearts or Brains

Animals that don't poop – jellyfish

Jellyfish expel waste through the same hole where they take in food.

©Chai Seamaker/Shutterstock.com

The body of a jellyfish is made of 95% water and does not have any of the organs usually found in animals. In fact, a jellyfish lacks a brain, heart, and stomach, as well as bones, blood, intestines, lungs, and eyes! With so little making up the body of a jellyfish, how does this curious creature live and breathe?

The body of a jellyfish has two main layers: the epidermis layer on the outside, and the gastrodermis layer on the inside. In between these two layers there is a watery, gelatinous material called mesoglea. Jellyfish also have a very simple digestive cavity that works like a combined stomach and intestine. And here’s a somewhat disgusting jellyfish fact: jellyfish have a mouth in the middle of their bodies that allows them both to eat food as well as to expel waste.

In addition, jellyfish also have a sensitive “nerve net”. This is a unique nervous system that helps them to perceive light, smell, and respond to other external sensory activity. Since they do not have lungs or gills, jellyfish absorb oxygen by diffusion through their thin, gelatinous membranes.

5.      Only One Species of Jellyfish Has Eyes

10 Most Venomous Animals -Box jelly fish photographed in aquarium

Box jellyfish have twenty-four eyes, and two of them help it to see color.

©Daleen Loest/Shutterstock.com

Here’s a rather strange jellyfish fact: the Box Jellyfish is the only jellyfish species that has eyes. In fact, these jellies have 24 eyes! Box jellyfish have clusters of eyes arranged around all four sides of their cube-shaped bell. Because of the unique placement of their eyes, the box jellyfish is one of the few animals in the world that can view its surroundings in 360 degrees.  

6.      Some Jellyfish Change Color and Glow in the Dark

Jellyfish with neon glow light effect in sea aquarium in Sentosa, Singapore

Moon Jellyfish change color depending on what they eat.

©I Wei Huang/Shutterstock.com

There are several species of jellyfish that can change the color of their bodies, and some of them can even glow in the dark! The Moon Jellyfish (Aurelia aurita), for example, typically has a pale, translucent bell that looks like a full moon. However, depending on what the jellyfish eats, its bell can change to purple, pink, white, or blue.

Many other jellyfish species are bioluminescent, which means that they can produce their own light due to chemical reactions inside of their bodies. Scientists believe that around 50% of jellyfish species are bioluminescent and can produce their own light. These jellies mostly live in the darkness of the deep sea, where their bioluminescence helps them to attract prey, and to defend them from predators.

The Crystal Jellyfish (Aequorea Victoria) is probably the most famous bioluminescent jellyfish. Normally this jelly is crystal clear and can be difficult to see. However, when it is threatened or disturbed, the crystal jellyfish emits green and blue light. Another intriguing jelly, the Atolla or Alarm Jellyfish (Atolla wyvillei) has a red body that flashes when it is attacked by a predator.

7.      Not All Jellyfish Are Dangerous

Barrel Jellyfish

The barrel jellyfish can reach 4 feet in diameter. This jellyfish is venomous, but not deadly to




A surprising jellyfish fact is that, contrary to popular belief, not all jellyfish are actually dangerous. Jellyfish are carnivores that use their venomous tentacles to hunt plankton, fish, and crustaceans. However, most jellyfish species are not dangerous to humans, and typically only cause discomfort or pain. In fact, there are some jellyfish species that are harmless to humans, like moon jellies (Aurelia aurita), Blue Buttons (Porpita porpita), and Mushroom Cap Jellyfish (Rhopilema Verrilli). These three species can still sting, but their venom is mild and usually only causes some skin irritation or a rash.

The most venomous jellyfish is the Sea Wasp (Chironex fleckeri). This is a species of box jellyfish with a painful and potent venom that has been known to kill humans within a matter of minutes. Sea wasps have 60-foot-long stinging tentacles that can grow up to 10 feet long. However, for a sea wasp to sting you, it needs to have direct contact with the surface of your skin. Some swimmers claim that wearing nylon pantyhose will keep these killer jellies from stinging you.

But be careful, because even a dead jellyfish can still sting! After a storm it is common to find many dead jellyfish scattered along the ocean shore. In New Hampshire several broken tentacles from a lion’s mane jellyfish washed up on shore and stung nearly 100 people on the beach!

8.      Jellyfish Can Reproduce Togetheror By Themselves

Baby jellyfish in water surrounded by other jellyfish out of focus

Jellyfish polyps bud off into tiny baby jellyfish.


Although there are both male and female jellyfish, these curious sea creatures can reproduce sexually with a partner, as well as asexually by themselves. Adult jellyfish, called medusae, reproduce sexually. Male jellies release sperm and females release eggs into the water, and once fertilized, the eggs grow into tiny jellyfish larvae. These tiny jellyfish larvae eventually attach themselves to smooth surfaces and become polyps. Jellyfish polyps reproduce asexually by budding or splitting into several new jellyfish.

9.      A Group of Jellyfish is Called a Bloom

What Do Jellyfish Eat and How Do They Eat It - swarm of jellyfish

A group of jellyfish is called a “swarm,” “bloom,” or “smack.”


When jellyfish gather together in groups, they are called a “smack” or a “bloom.” These blooms can contain millions of jellyfish at a time! While many marine animals are suffering greatly because of climate change, large blooms of jellyfish are becoming more common. Unfortunately, the populations of jellyfish predators like sea turtles are declining because of overfishing and human-caused pollution. Because of this, jellyfish are reproducing in large numbers. Blooms of jellyfish can disrupt power plants that use water from the sea and destroy fisheries.

10. Jellyfish Can Regenerate and Possibly Live Forever

Immortal Jellyfish

The Immortal Jellyfish can reset its lifecycle.

©Fon Duangkamon/Shutterstock.com

On of the most incredible jellyfish facts is their shocking immortality. Not only can jellyfish reproduce on their own, they can heal themselves, regenerate, and even grow new body parts! Some jellies can also modify their existing parts to restore order and balance to a lop-sided body. Scientists observed that after losing two of its arms, a young moon jellyfish simply reorganized its remaining parts in order to restore its lost symmetry.

Then there is the Immortal Jellyfish (Turritopsis dohrnii). This jellyfish gets its name from its unique ability to live forever! When an immortal jellyfish becomes too stressed, it will revert from its adult medusa stage and become a baby polyp again. By using this process of “transdifferentiation,” the immortal jellyfish avoids aging out and dying by beginning its life cycle over from the beginning again.

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About the Author

Kellianne Matthews is a writer at A-Z Animals where her primary focus is on anthrozoology, conservation, human-animal relationships, and animal behavior. Kellianne has been writing and researching animals for over ten years and has decades of hands-on experience working with a variety of different animals. She holds a Master’s Degree from Brigham Young University, which she earned in 2017. A resident of Utah, Kellianne enjoys creating, exploring and learning new things, analyzing movies, caring for animals, and playing with her cats.

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