Like a scene right out of a Hollywood horror flick, a video published online last week shows a river in Brazil practically boiling over with feeding fish.

The video may make swimmers everywhere (or at least in South America) recoil in horror, but such fish are rarely dangerous to people, says Zeb Hogan, a National Geographic fellow and professor of biology at the University of Nevada, Reno.

The uploader of the video, who labeled them piranhas, has not responded to requests for comment. An earlier version of this story presumed the fish were piranhas, but after we received comments on Facebook to the contrary, we asked Hogan to take another look at the footage.

Hogan says the resolution