Also called a medusa, the jellyfish isn't technically a fish - it is a free-swimming non-vertebrate found in every sea across the world. Jellyfish consist of a gelatinous bell with trailing tentacles capable of stinging prey. Jellyfish stings are in most cases harmless to humans, but some can be extremely serious, even lethal.
(Note: From "in most cases harmless" to "can be lethal." That's comforting. - JM)