Piranha fish swap old teeth for new simultaneously

Piranha CT scan — A CT-scanned image, left, of the red-bellied piranha (Pygocentrus nattereri) shows a set of lower teeth growing below the existing teeth. An advanced imaging technique, right, of the same fish illuminates the replacement teeth on both the bottom and top of the jaw.

Piranha fish have a powerful bite. Their teeth help them shred through the flesh of their prey or even scrape plants off rocks to supplement their diet.

Years ago, scientists discovered that piranhas lose all of the teeth on one side of their mouth at once and regrow them, presumably to replace dulled teeth with brand new sharp spears for gnawing on prey. But no museum specimens have ever shown this theory to be true, and there’s no documentation of piranhas missing an entire block of teeth.

With the help of new technologies, a team led by the University of Washington has confirmed that piranhas — and their plant-eating cousins, pacus — do in fact lose and regrow all the teeth on one side of their face multiple times throughout their lives. How they do it may help explain why the fish go to such efforts to replace their teeth.

The findings were published Aug. 26 in the journal Evolution & Development.

“I think in a sense we found a solution to a problem that’s obvious, but no one had articulated before,” said senior author Adam Summers, a professor of biology and of aquatic and fishery sciences at UW Friday Harbor Laboratories on San Juan Island.

“The teeth form a solid battery that is locked together, and they are all lost at once on one side of the face. The new teeth wear the old ones as ‘hats’ until they are ready to erupt. So, piranhas are never toothless even though they are constantly replacing dull teeth with brand new sharp ones.”