12 Fascinating Facts About Jellyfish

Jellyfish are some of the Earth's most ancient extant animals. They're also immensely diverse—most organisms that are called jellyfish are part of the phylum Cnidaria, which includes more than 10,000 species. Needless to say, jellyfish occur in abundant numbers all over the world, in cold and warm water, deep in the sea, or near the surface. Most people fear swimming alongside jellyfish because of their painful sting, but not all of these creatures threaten injury.

Find out 12 facts about the oddly charismatic, gelatinous, and ocean-dwelling jellyfish, from its literal lack of heart to the rumor that it's immortal.

1. Jellyfish Could Be Older Than Dinosaurs

Jellyfish have no bones, so fossils are hard to come by. Nevertheless, scientists have evidence these creatures have been bobbing along in the world's oceans for at least 500 million years. It's likely the jellyfish lineage goes back even further, possibly 700 million years. That's roughly three times the age of the first dinosaurs.

Some of the oldest known jellyfish fossils have been found in Utah, dating back to when the entire Western United States was under the Pacific Ocean.

2. They're Adapting Well to Climate Change

Unlike most marine creatures, jellyfish thrive in our oceans despite marine heat waves, ocean acidification, overfishing, and other human influences. While corals, oysters, and any marine organisms that build shells are considered the biggest losers of increasingly acidic oceans, jellyfish don't seem to be as susceptible.

But, as the climate crisis worsens, experts expect jellyfish populations to increase in some areas and decrease in others. Mostly, scientists think we'll see an imbalance between jellyfish and other organisms. For example, because jellyfish are more resilient to low-oxygen environments, they could soon far outnumber plankton, which need more oxygen and make up the bulk of the jellyfish's diet.

3. They Aren't Really Fish

One look at a jellyfish and this might seem rather obvious, but they aren't fish. They are invertebrates from the phylum Cnidaria and are so varied as a taxonomic group that many scientists simply refer to them as "gelatinous zooplankton."

Jellyfish don't have scales, gills, or fins like fish. Instead, they swim by opening and closing their "bells."

4. Jellyfish Are 98% Water

The human body is comprised of 60% water, whereas the jellyfish's body is made of 98% water. When jellyfish wash ashore, they can disappear after just a few hours, their bodies promptly evaporating into the air. They have a rudimentary nervous system, a loose network of nerves in the epidermis called a "nerve net," and no brain. They also don't have a heart; their gelatinous bodies are so thin they can be oxygenated solely by diffusion.

5. They Can Have Eyes

Despite their simple body design, some jellyfish have vision. In fact, a few species have surprisingly complex vision. For instance, the box jellyfish has 24 "eyes," two of which can see in color. It's also believed this animal's complicated array of visual sensors makes it one of the few creatures in the world to have a full 360-degree view of its environment.

6. Some Jellyfish Might Be Immortal

At least one species of jellyfish, Turritopsis nutricula, may be able to cheat death. When threatened, this species can undergo cellular transdifferentiation, whereby the organism's cells essentially become new again.

This jellyfish is colloquially called the "immortal" jellyfish and it inhabits the warm waters of the Caribbean and Mediterranean. Its unique transdifferentiation ability is a major subject of research, as it could help the medical field understand how to turn cancerous cells into noncancerous cells like muscles, nerves, or skin.

7. They Eat Where They Poop

It might not sound very appetizing, but jellyfish don't use separate orifices for eating and pooping. They have one orifice that does the job of both the mouth and the anus. The jellyfish is known as a simple or "primitive" animal, and its lack of a dual-hole system—which developed way down the line of evolution—is proof of that.

Still, the multifunctional gut opening is being constantly studied. In 2019, scientists discovered that one jellyfish species sprouts a new anus every time it poops.

8. They Rarely Travel in Groups

Many refer to a group of jellyfish as a bloom or a swarm, but they can also be called a "smack." In any case, seeing a group of jellyfish is rare considering these animals are mostly lone drifters. They're solitary animals, only clumping together when following a singular food source or traveling in the same water current.

9. They Are Among Earth's Deadliest Creatures

All jellyfish have nematocysts, or stinging structures, but the power of their stings can vary widely depending on the species. The most venomous jellyfish in the world is probably the box jellyfish, capable of killing an adult human in just a few minutes with a single sting. Each box jellyfish reportedly carries enough venom to kill more than 60 humans.

To make matters worse, their stings are excruciatingly painful—it's said the pain could kill you before the venom does. On the bright side, that knowledge has helped Australian researchers to develop a potential antidote for box jellyfish stings.

10. Jellyfish Can Be Tiny or Enormous

Some jellyfish are so tiny they're practically invisible floating in the ocean's currents. The smallest are those in the genera Staurocladia and Eleutheria, which have bell disks from just 0.5 millimeters to a few millimeters in diameter.

By contrast, the lion's mane jellyfish, Cyanea capillata, can extend its tentacles as far as 120 feet. The world's largest jellyfish by weight and diameter is thought to be the titanic Nomura's jellyfish, Nemopilema nomurai, which can dwarf a human diver. These beasts can have a bell diameter of 6.5 feet and weigh as much as 440 pounds.

11. Some Are Edible

You won't find them on many restaurant menus, but jellyfish are edible and are eaten as a delicacy in some places, such as in Japan and Korea. The Japanese have transformed jellyfish into candy: A sweet and salty caramel made out of sugar, starch syrup, and jellyfish powder is thought to be a good use for jellyfish that often plague the waters there. More often, jellyfish find their way into salads, are fried into crunchy noodles, or are eaten with soy sauce like sushi.

12. They Have Been to Space

NASA first sent jellyfish to space aboard the Columbia space shuttle in the early 1990s to test how the creatures might adapt to a zero-gravity environment. Why? Interestingly, humans and jellyfish rely on specialized gravity-sensitive calcium crystals to orient themselves (these crystals are located inside the inner ear in the case of humans and along the bottom edge of the jellyfish bodies). So, studying how jellyfish manage in space can reveal clues about how humans might also fare.