Updated on March 25 at 6:30 pm ET
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 in the video isn't good enough to determine exactly what type of fish they are. Another fish biologist concluded the same thing but suspected they might actually be catfish, according to news reports.
"Really almost any fish, especially any predatory fish, under the right circumstances, will feed like that," says Hogan.
In the case of piranhas, the toothy, omnivorous fish are found throughout South America, where people often wade or swim around them without being bitten, says Hogan.
“But piranhas can be dangerous if they are trapped in a backwater without food, or [are] somehow concentrated in an area and they are hungry,” he says.
Piranhas can be dangerous if they are trapped in a backwater without food.
Such a situation was famously chronicled by Theodore Roosevelt during an expedition to the Amazon in 1913-14, when starved piranhas trapped in a small pool shredded a whole cow carcass in seconds. (Watch piranhas devour an egret chick.)
Fishermen are occasionally bitten, and there have been some recent reports of attacks by piranhas and related fish in South America, including one death, “but these events are extremely rare,” says Hogan.
Whatever they may be, the fish in the video might be particularly aggressive if they've been fed regularly by people, Hogan says. Piranhas or catfish congregating to catch an easy meal would build up in large numbers, causing them to act just as they do when trapped in a small pool.
The area filmed might be a fish-cleaning station, or perhaps a planned act to amuse tourists, Hogan speculates. (Learn about the man who smuggled 40,000 piranhas.)
Piranhas play an important ecological role as scavengers and predators in their native rivers, says Hogan. They will often resort to cannibalism if food gets scarce. It's unknown how many species of piranhas exist, with estimates ranging from 30 to 60.
“I've been swimming dozens of times in rivers where piranha were very abundant and I've never been bitten,” adds Hogan.
Hogan will speak at National Geographic headquarters in Washington, D.C., on March 26, following the opening of the new exhibit "Monster Fish: In Search of the Last River Giants." Nat Geo Wild is showing a Monster Fish marathon on March 22 from 12 p.m. to 7 p.m. EST.