ELI5 - How do Jellyfish Survive?

Jellyfish survive by being very simple and very clever at the same time. They do not live like people, dogs, or even fish. They are more like floating survival balloons that use the ocean to do most of the work for them.

First, jellyfish are made mostly of water. About ninety five percent of their body is water. This means they do not need strong bones or muscles to hold themselves up. The ocean holds them. Because of this, they use very little energy just to exist. Using less energy makes it easier to survive for a long time. Jellyfish do not have a brain, a heart, or lungs. This sounds bad, but it actually helps them. They do not need to pump blood or breathe air. Oxygen moves straight through their soft, thin bodies from the water around them. This works because they are so simple and see through.

To eat, jellyfish use their tentacles. The tentacles have tiny stinging cells. When a small fish or plankton touches them, the stingers fire. The sting stops the prey from moving. Then the jellyfish pulls the food into its mouth. The mouth is also the stomach. Food goes in, gets digested, and waste comes out the same hole. Jellyfish do not chase food very much. They mostly drift with ocean currents. This saves even more energy. If food comes to them, great. If not, they can wait. Some jellyfish can survive long periods without eating.

One amazing trick jellyfish have is how they handle danger. Many animals need to heal wounds carefully. Jellyfish can heal quickly because their bodies are simple. Some species can even go backward in their life cycle. They turn from an adult back into a baby form when times are bad. Then they grow up again when things improve. This makes them very hard to wipe out. Jellyfish have been around for over five hundred million years. They survived dinosaurs, ice ages, and huge changes in the ocean. They did this by not needing much. No bones. No brain. No busy lifestyle. Just floating, eating when they can, and letting the ocean carry them along.

In simple terms, jellyfish survive because they are easy to keep alive. Nature made them simple, flexible, and patient. That turns out to be a very powerful way to live.