It turns out that there are jellyfish that don't age. Instead, they revert to an immature polyp stage which then produces more adults. It seems unlikely for a number of reasons that memory is preserved-- in any case, it isn't being checked on for that particular species. I don't know that jellyfish of any size can remember anything (the ones that revert are quite tiny), and it turns out that something made from jellyfish is sold to help with human memory, so there's no obvious way to track down research on jellyfish memory.
Anyway, for some years I've thought that it would be a good sf premise to have a technology of regeneration for humans where the way to undo aging is to revert to an infant, then mature at the normal rate, with memories from earlier lives coming back at around age 15.
The result would be societies where the arrangements for being taken care of when reverted would be extremely important, and there'd probably be a bunch of different systems-- blood family, chosen family, commercial..... There'd be long loyalty and long revenges.
The real world is potentially more dramatic than the idea I started with, since severe trauma is one of the things which can start jellyfish reversion. I assumed there'd just be a medical procedure.
Link thanks to thnidu.
andrewducker sent a link suggesting that jellyfish may have some ability to remember.
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