Naturalist Greg Karofelas of Davis, an associate of the Bohart Museum of Entomology, has not only seen them, he has photographed them. See his truly spectacular photo below.
The mites look like a cross between pomegranate kernels and salmon eggs. They are hitchhikers!
It's a good case of phoresy, or the symbiotic relationship in which one organism transports another organism of a different species.
Scenario: Say a damselfly is laying her eggs in a fish pond or the wetlands. Say some mites are waiting for her. They seek free meals and a free ride to the next pond to find mates and reproduce.
The damselfly dips down. They jump up.
What a load!
We wrote about these mites in a Bug Squad blog on July 25, 2013.
As for the image of the water mites that Greg Kareofelas captured, he suspects they may be Arrenurus mitoensis.
All we can say is "Wow!" Great image, Greg!