They're gooey, they sting and they're probably not the first thing that comes to mind when you think of seafood, but jellyfish are surprisingly fascinating creatures. They can mesmerize aquarium crowds like few other organisms, and they're some of Earth's most ancient animals that are still alive today. Take a few moments to revel in these fun facts about jellyfish. You might be surprised at what you don't know about these oddly charismatic gelatinous creatures.

They predate the dinosaurs by hundreds of millions of years

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

They aren't actually fish; they're gelatinous zooplankton

jellyfish swimming with fish

Despite their name, jellyfish aren't fish at all. (Photo: Shutterstock)

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

They are 98 percent water, without a brain or a heart

Jellyfish seem to blend in to their environment, undulating gently with the ocean's currents, and with good reason: Their bodies are actually made up of as much as 98 percent water! When they wash ashore, they can disappear after just a few hours as their bodies evaporate into the air. They have a rudimentary nervous system, a loose network of nerves located in the epidermis which is called a "nerve net," but no brain. They also don't have a heart; their gelatinous bodies are so thin that they can be oxygenated solely by diffusion.

But some have eyes...

box jellyfish with eyes

Some box jellyfish species are among the most venomous creatures in the world. They also have several sets of "eyes." (Photo: Shutterstock)

Despite their simplistic body design, some jellyfish do still have the ability to see. In fact, for a few species their vision can be surprisingly complex. For instance, the box jellyfish has 24 "eyes," two of which are actually capable of seeing in color. It is also believed that this animal's complicated array of visual sensors make it one of the few creatures in the world to have a full 360-degree view of its environment.

Some might be immortal

At least one unique species of jellyfish, Turritopsis nutricula, may never actually die. When threatened, this species is capable of undergoing "cellular transdifferentiation," a process whereby the organism's cells can essentially become new again. In other words, this jellyfish has a built-in fountain of youth. It is theoretically immortal!

They poop where they eat

It might not sound very appetizing, but jellyfish have no need for separate orifices for eating and pooping. They have one orifice that does the job of both the mouth and the anus. Yuck! But that's also beautiful in a minimalist sort of way.

A group of jellyfish is called...

group of jellyfish

A very large group of jellyfish is often referred to as a "bloom." (Photo: Shutterstock)

A group of dolphins is called a pod, a group of fish is called a school, and a group of crows is called a murder. But what is a group of jellyfish called? Many refer to groups of jellyfish as blooms or swarms, but they can also be called a "smack."

They are among Earth's deadliest creatures

All jellyfish have nematocysts, or stinging structures, but the power of their stings can range widely depending on the species. The most venomous jellies in the world are probably the box jellyfish, capable of killing an adult human with a single sting in just a few minutes. To make matters worse, their stings are excruciatingly painful. It is said that the pain alone could kill you before the venom does.

They range widely in size

lions mane jellyfish

Possibly the world's longest jellyfish, the lion's mane jellyfish has a poisonous sting. (Photo:Shutterstock)

Some jellyfish are so tiny that they are 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 world's largest jellyfish are true monsters. The lion's mane jellyfish, Cyanea capillata, might be the world's longest, with tentacles that can extend as far as 120 feet! But the perhaps the world's largest jellyfish by weight and diameter is the titanic Nomura's jellyfish, Nemopilema nomurai, which can completely dwarf a human diver. These beasts can have a bell diameter of six and a half feet across and weigh as much as 440 pounds.

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. In fact, in Japan jellyfish have been transformed into candy. A sweet and salty caramel made out of sugar, starch syrup and jellyfish powder has been produced by students in an effort to make use of the jellyfish smacks that often plague the waters there.

They have been to space

Though they look rather alien, jellyfish are indeed from planet Earth. Nevertheless, they have been to space. NASA first started sending jellyfish to space aboard the Columbia space shuttle back in the early '90s to test how they might get along in a zero-gravity environment. Why? Interestingly, both humans and jellyfish rely on specialized gravity-sensitive calcium crystals in order to orient themselves (these are located inside the inner ear in the case of humans, and along the bottom edge of the mushroom-like bodies of jellies). So studying how jellyfish manage in space can reveal clues about how humans might also fare.

Related on MNN: