What is a jellyfishes defense?

Jellyfish defend themselves using nematocysts in their tentacles. Nematocysts are roughly the equivalent of having many tiny spears on the end of many tightly wound springs. Once something touches the jellyfish's tentacles, the nematocysts are launched. If you are stung, they are very painful and depending on the species of jellyfish, they can be very dangerous or even life-threatening. Jellyfish also use this same technique to catch prey.