Oceangal forwarded this video on the jellyfish of Palau.
These jellyfish live in saltwater in the center of the island, trapped away from the sea. In their new little homes (there's more than one lake) they seem to have thriven. The jellies are farmers, or perhaps farms, depending on your level of anthropomorphism. They keep algae inside their bodies and live off the by-products of the plants' photosynthesis. In return, they ferry the algae from sunny area to sunny area during the day, and at night take them down to the low-oxygen level of the lake where some of the weirder bacteria live. They pick up nutrients from them for their plants and then go back up to catch rays the next morning. The jellies also catch the occasional copepod and eat it.
Although jellyfish look as though they're going somewhere, they aren't really swimming like a frog or an otter; their bell contracts and relaxes in the same way a heart does, with as much conscious thought as a heart puts into its beat.
Apparently these can barely sting, so snorkelling around in a cloud of them is a nice experience.