I identified the ribbon worm as it was swimming on the air-water interface. Based on it swimming and sinking behavior, i am guessing it is negatively buoyant, but it is a mystery to me how it kept itself attached to the air-water interface.
Unfortunately, I did not take an image before, but on touching it to bring it to a glass slide, i found it to shrink approximately 250-350 % of its body size. You can confirm this yourself, by comparing the foldscope videos, with this final image, after releasing it back to the water.
Here are two videos:
It’s not clear what the flow inside the worm is – is it sucking the water and filter-feeding? Or is it particles inside its gut? The head darting around made it challenging to track.
Here’s another video.