Velella! Also known as the sea raft and the purple sail, it’s a free-floating cnidarian that goes where the wind takes it, courtesy of that little sail up there. Much like the Portuguese man o’ war, it has no way of directing itself and occasionally winds up in huge piles on beaches.
Above: Huge fucking pile of velella on a beach.
Unlike the Portuguese man o’ war, it’s not a war-crime.
Above: Oh god somebody do something.
Above: Two Glaucus atlanticus doing something.
Velellae are predatory, but they feed mostly on plankton and aren’t harmful to humans. They’re technically a huge colony of tiny individuals rather than an independent organism, but that’s cnidarians for you.
They also have a completely wackypants life cycle, where the harmless little velellas go around just constantly shedding teeny tiny jellyfish, which spend a month developing into sexual maturity and then release their eggs and/or sperm into the water column. The planktonic larvae that result from successful hits turn into conaria, each of which will grow into a new velella colony if they survive that long. This would be more work if everybody wasn’t just rolling along in the waves, I’m sure.
Basically this is one of those jellies that gets mad props for not having it in for humanity. Well done, you weird little frisbee. Well done.