Freshwater Jellyfish?

Yes, a thing. Common Freshwater Jellyfish/Craspedacusta sowerbii.

Introduced, all of the world now. They just showed up in Green-Wood’s Sylvan Water.

On the plus size, the largest are the size of US quarters. They do sting, that’s how they capture their tiny prey, but their stinging cells are so small they don’t seem to have the capacity to pierce human skin.