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.
