Immortal jellyfish

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Photo from Peter Schuchert's incredible Hydrozoa Directory

The Turritopsis nutricula is known as the "immortal jellyfish" because even once sexually mature, it can revert back to its polypoid stage, its first life stage. And then rinse and repeat. Again and again. From Mother Nature Network:

Because they are able to bypass death, the number of individuals is spiking. They're now found in oceans around the globe rather than just in their native Caribbean waters.  "We are looking at a worldwide silent invasion," says Dr. Maria Miglietta of the Smithsonian Tropical Marine Institute.
"The world's only immortal animal" (Thanks, Andrea Mangini!)