I know that the population of rainbow jellies waxes and wanes and seems to be linked to both the new moon and the volcano. But I'm not sure they're meant to vanish and I'm pretty sure they shouldn't then reappear the next time I log on.
Whatever the plan is with the rainbow jellies, I hope there's some way in the final release to make it possible to keep them in an landlocked pond/lake. I don't think this is any more OP than an open-ocean jelly, and it's not even enough light to serve as an alternate light source unless you get on a raft and row out to them. But it is a really cool thing to have in your base late game, especially when their color changes sync up. It gives veteran players something to strive for--both finding an island to base on with lakes, and then decorating them.
Captured rainbow jellies and dropped them into land-locked lake near base. Sometimes they disappear while I am away from base, then are there again when I log out and back on. Other times they disappear permanently.
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