Dear Didge,
I think you already know mine...we talked about this before?
it is the Turritopsis dohrnii...the immortal jellyfish, found in the Mediterranean Sea and the Sea of Japan. Whenever it begins to feel sickish or a bit long in the tooth, it simply pulls some kind of string and reverts to its juvenile form. Starts the process of growing up all over again, in theory can live forever!