Piranha teeth have the tough task of crunching bone and shredding flesh and, like well-used kitchen knives, they dull over time. Since they have no way to sharpen their teeth, piranhas lose and regrow all of the teeth on one side several times over their lifespan.

A new analysis of piranha teeth revealed that a new set of teeth wait in a "crypt" below the current teeth so that the fish are never without a pair. This way, they can happily keep munching on what the study refers to as an "anatomical potpourri of fish parts, ranging from scales and fin rays, chunks of flesh and whole fishes."

The study published in the journal Evolution and Development.

Previous research showed that piranhas lose one row of teeth at a time and replace them with sharp new ones. But scientists didn't find any examples of piranhas missing teeth.

But piranhas aren't the only fish that experience this. They have plant-eating relatives called pacus that lose and regrow their teeth in the same way.

"I think in a sense we found a solution to a problem that's obvious, but no one had articulated before," said Adam Summers, study author and professor of biology and of aquatic and fishery sciences at 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."

The researchers used new imaging technology on fish specimens at different museums, using 3D examination and analyzing 40 different species. This enabled them to actually see the rows of teeth just waiting beneath the surface of the existing teeth inside the jaw.

The detailed look also allowed the scientists to understand why the teeth are replaced this way. The rows of teeth on each side of the mouth act like interlocking blocks.

"When one tooth wears down, it becomes hard to replace just one," Matthew Kolmann, study author and postdoctoral researcher at George Washington University. "Once you link teeth together, if one wears too much, it becomes like a missing link in an assembly line. They all have to work together in a coordinated way."

Interlocking teeth can also handle the stress of chewing better, the researchers said. In piranhas, the teeth act like saw blades and in pacus, the teeth are more like clasps that can crop vegetation.

"With interlocking teeth, the fish go from having one sharp tooth that can crack a nut or cut through flesh to a whole battery of teeth," said Karly Cohen, study co-author and a University of Washington biology doctoral student. "Among piranhas and pacus there's a lot of diversity in how the teeth lock together, and it seems to relate to how the teeth are being used."

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