Just when you thought it was safe to go back in the water…

Jellyfish are some of the ocean’s most graceful swimmers. I personally think watching them swim is very soothing. All jellyfish are found in the phylum Cnidaria (meaning they are not fish at all and are actually closely related to coral), but come in lots of diverse species that can be found at every depth of every ocean on the planet. Some species even live in freshwater. They are relatively simple organisms with no specialized digestive, circulatory, nervous or respiratory systems. These fragile animals (they are 98% water) have a fun life cycle with two distinct stages...

… I’ll keep the background information to a minimum (look it up if you’re interested) and get to the more interesting part…

When you think of jellyfish, you are probably not thinking about their lifecycle. You are actually envisioning how scary it would be if you fell into the middle of a bloom of jellies (more than one jellyfish = a bloom) and got stung to death! Hopefully that will never happen to you, but if it did wouldn’t you want to know how they were stinging you?

Members of the phylum Cnidaria have specialized stinging structures called cnidae (the phylum is named after these structures). When you think of a jellyfish, chances are you are imagining a species with a specific type of cnidae called a nematocyst.

Warning: I am going to talk about 2 different things: nematocyst and nematocyte. I won’t bring up the nematoblast. Just try to keep them straight.

All along the tentacles of a jellyfish (and in some species, a few other places) are specialized cells called nematocytes. Inside these cells are the long, hollow, tubular nematocysts; all coiled up and covered in barbs. Now let’s say you are swimming and you happen to brush your leg up against a jellyfish. This is what happens…

First, by coming in contact with the tentacle, you touched a small hair-like structure (cilium) that acts as a trigger. Once triggered, the trapdoor to the nematocyte cell (operculum) will spring open and allow the nematocyst to fly out at you. Like a glove with a finger inside out, the nematocyst will pop out while twisting. This twisting motion, coupled with a forceful discharge and sharp barbs, will drill through your skin. Thousands of nematocysts can be fired into your skin within an instant of a second. Once in you, the barbs will keep the nematocysts embedded in your skin and also serve as the deposit sites of the jellyfish venom. Depending on the species, this venom may be an issue.


The toxins are not well studied and their effects range from bursting surrounding cells, to rupturing red blood cells, to shutting down the central nervous system. Stay away from the venom of a Sea Wasp or Portuguese Man-of-War.

The nematocyst remains in the victim, so jellyfish are constantly making new ones. It takes about 2 days.

It is unknown how jellyfish actually fire their nematocysts with such force. It only takes 2 milliseconds to fire these little spears with around 140 atmospheres of pressure.