A bit of marine biology in case anyone is interested~
Jellyfishes or sea jellies are a free-floating subphylum of cnidarians (except comb jellies), like sea anemones but actually distinct from them. They really do detach one at a time from sessile larval forms though, which kinda messes with my head tbh.
Plankton isn't actually a family, but rather a catch-all term for any living things that lack the self-propulsion abilities to resist currents.
Cliones or sea angels are actually swimming sea slugs xD