A hydra, a freshwater cousin of the jellyfish, can survive being chopped in a blender. Its pieces will crawl their way back toward each other to reassemble as a complete organism.
A certain species of jellyfish has been deemed “immortal” by scientists who have observed its ability to, when in crisis, revert its cells to their earliest form and grow anew. That means that these tiny creatures, 4 mm to 5 mm long, potentially have infinite lives.
A tiny (1sq. cm) transparent Australian jellyfish has venom delivering tentacles a meter long, and whose sting causes Irunkandji syndrome, which induces feelings of impending doom, severe pain, and cardiac arrest.
There is a species of jellyfish that is biologically immortal.
Jellyfish are 95% water.