Found a bunch of these guys apparently living in my giant duckweed. They are about the size of a grain of salt, and when disturbed swim around visibly in random directions. When this movement observed under the microscope, it appeared to be a bivalve opening and closing a shell with a foot/tongue coming out to one side. I did have what appeared to be a fingernail clam in an aquatic plant I bought long ago, and if it's those, that's fine since they don't have a parasitic larval stage, just thrown off by the coloration here, looks more mussel-ish to me? Or is there some insect that has a weird clam-ish water larva phase I don't know about? Thanks!