Black Circle Reflections in Rio Coco

Black Circle Reflections, Rio Coco, Puriscal
A few days ago I decided to walk up the Rio Coco and came across something very unusual. I saw a string of black circles flowing in the water. I took a few pictures but it didn't do the phenomenon justice so I took a short video. It appeared to be some type of reflection, however I was so memorized by what I was seeing that I didn't bother to find the source. I did go back the next day, however it was a slightly different time of day with different lighting and there were no black circles swimming in the river.