What Is a Jellyfish?

Jellyfish are believed to have existed on Earth even before the dinosaurs, which makes them one of the oldest surviving species among marine creatures. Their lifespan can vary from as little as a few days to about a year. 

A jellyfish isn't actually a fish at all. It's an invertebrate, meaning that it is a living organism without a backbone. Jellyfish are plankton made up of a gelatinous, jelly-like substance. 

They are mostly water and don't have a brain, a heart, or bones.

Jellyfish range in size from the tiny Irukandji jellyfish, which is only about one cubic centimeter in size but also one of the world's deadliest jellyfish, to the enormous lion's mane jellyfish, which can grow up to 7 feet in diameter with tentacles up to 190 feet long!

Jellyfish defend themselves and catch their prey using their tentacles to sting. The tentacles have special cells called cnidocytes. These cells contain nematocysts, which are venom-filled structures that sting their prey.

A jellyfish sting is painful and some are even deadly! You don't have to be "attacked" by a jellyfish to get stung. Simply brushing their tentacles while in the water (even a tentacle that has broken off a jellyfish) or touching those washed up on the beach can cause a sting.

Jellyfish move mostly with the ocean's current, but they can control their vertical movement by opening and closing their bell-shaped bodies. They can propel themselves by squirting water from their mouths. The mouth is also used for eating and expelling waste!

Jellyfish eat algae, small plants in the water, shrimp, fish eggs, and even other jellyfish. Sea turtles eat jellyfish. That's one reason we must take care that plastic bags don't make their way into our oceans. They look like a tasty jellyfish to an unsuspecting sea turtle who can die trying to consume the plastic bag.