Several times within the first couple episodes of Ad Vitam there are jellyfish. At the beginning of the first episode someone was dressed up as a huge jellyfish, and then later on Christa's parents are shown having a pet jellyfish. (There are also a couple other instances of jellyfish but those mentioned are the two major ones.)
So, what's the significance of a jellyfish? AFAIK they've not talked or hinted about it at all (and I'm on the third episode now and it doesn't seem like they're going to).