The theoretically immortal moon jellyfish

Besides its regular life cycle that culminates in reproduction (as well as cloning), once a medusa "dies", it can theoretically regenerate its body as a baby jelly (polyp) forever through a process called transdifferentiation. It reverses its age–if it is not first eaten. Also, while it is living, tear off a part of a jellyfish and it will grow back.

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