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Dip a toe into the wrong lake in the Amazon, and it may get bitten off. Here, gangs of piranhas swarm almost anything that moves. Anything, that is, except the arapaima. This humungous fish swims unchewed, even in piranha-infested lakes and rivers. A new study reveals how: The arapaima's unique scales are tough enough to deflect a piranha's razor-sharp bite.

Study co-author Marc Meyers, a mechanical engineer at the University of California, San Diego, is no stranger to the Amazon's deadly waters. He's spent several months sport fishing in Brazil and once reeled in an arapaima (Arapaima gigas). It was a whopper. These fish—often known as "living fossils" because they still harbor lungs and need to breathe air—frequently stretch to 2.5 meters and weigh 200 kilograms.

To snare an arapaima, Meyers says, fishermen wait until one of the flat-snouted creatures comes up for air, then they toss a large hunk of meat near its mouth. If the arapaima misses the meal, "that bait will survive 5 minutes," he says. In other words, piranhas get it. During the dry season, hundreds of piranhas cram lakes off Brazil's Araguaia River and similar waterways. Meyers got curious about how the arapaima, and its succulent flesh, could survive in the lakes if his bait lasted only minutes.

The obvious answer was its scales. The arapaima's silver-and-black shingles, which can grow to 10 centimeters long, certainly look like serious protection. So Meyers and his colleagues tried a simple test. To mimic an actual Amazonian chomp, they cemented a piranha tooth to a machine that delivered a bite to several arapaima scales. The fang didn't just ricochet off the armor, it shattered, the team reports online in Advanced Engineering Materials.

Big scales or no, that's not an easy feat. Piranha teeth are as sharp as kitchen carving knives, making them difficult weapons to deflect, Meyers says. So the researchers took a closer look at the scales using a scanning electron microscope and similar tools. The scales, they discovered, are double layered. On the inside, they're made from sheets of collagen, a tough-yet-springy material found in joints and bones. But their exteriors, or the sides exposed to piranha bites, are rock hard. These surfaces are also made from collagen, the team realized, but those fibers have been cemented together with a mineral also popular in bone: calcium.

The hard-on-soft pattern does the trick, Meyers says. If you shatter a uniformly rigid material—say, a ceramic bowl—cracks will spread from the site of impact all the way through, he explains. But because the hard outside of an arapaima scale is backed by more squishy material, "the cracks will stop." It's a common armoring strategy, he notes. Take our teeth, which are covered in enamel but have a core made from more pliant dentine.

The study is "very nice work," says Markus Buehler, a materials scientist at the Massachusetts Institute of Technology in Cambridge. Meyers and colleagues demonstrate a common feature in nature, he says. Different materials combine to make a structure that takes on new traits.

Francois Barthelat, a mechanical engineer at McGill University in Montreal, Canada, agrees. "It's pretty impressive." Arapaimas are hardly alone in the fish world, he notes. His team previously discovered the same layered patterning in the scales of several other fish, even many "fish market" species. No, sea bass scales can't fend off a piranha attack, but they're still surprisingly tough, given their flexibility, he says. Barthelat has taken the concept to the lab, where he and colleagues design glass that directs cracks and fractures just as fish armor does. Such designs could one day be used to make lightweight body armor, meaning that soldiers could charge into combat wearing fish scales.

Meyers's next venture revolves around piranha's, too. He's writing a novel set in the Amazon. It includes a scene based on a supposedly true story he heard from an old acquaintance. Mercenaries execute a bloodied man by forcing him to swim across a piranha-infested river. Unlike the arapaima, he doesn't make it.