REWARD: ID Mysterious Thingy and you win!

Anyone out there in the life sciences? If you know (or have a good guess of) what this thing is depicted in several images below, PLEASE enlighten me! I think it might be a pollen grain of some sort, but I know a lotta pollen, and this doesn't look like anything I know. My other idea was a planula larvae, but it doesn't appear to be ciliated. It's about 60 microns long. It's been everywhere in my zooplankton and fish gut samples all summer and autumn long and no one in our lab knows what it is. I am offering a reward (you'll get to Hug a Limnologist Today) to whoever can confirm for me what this is.