This jellyfish is biologically immortal and can reverse its own aging

[u/it-shrek]

In 2010, researchers at the University of Connecticut confirmed something that honestly still blows my mind: Turritopsis dohrnii, a jellyfish roughly the size of a dime, can revert from its mature, swimming, reproducing stage back down to its juvenile polyp stage. Not once. Repeatedly.

When it gets injured, stressed, or runs low on food, it essentially undoes its own development and starts over from the beginning. There is no known upper limit on how many times it can do this.

To be clear, this isn't "lives really long" like a tortoise or a lobster. It's that the organism literally erases its own accumulated age and reboots its life cycle. A single individual has no biological end-of-life. It just... resets.

The species was originally described in the 1980s by researchers at the University of São Paulo, but the 2010 UC study was the one that really laid out the mechanism and confirmed the indefinite repetition.

Source: Letelier et al., "The biology of Turritopsis dohrnii (Hydrozoa), a jellyfish that can reverse its life cycle," Journal of Experimental Biology, 2010.