The Loh Down On Science

Arapaima vs. Piranha

Piranha! It's not just a movie any more!

This is Sandra Tsing Loh with the Loh Down on Science.

Meet mechanical engineer Marc Meyers. He went to Brazil and caught an arapaima.

Don't worry, it's not an infection--just a very large freshwater fish.

During Brazil's wet season, rivers flood, forming lakes where piranhas and arapaimas comfortably coexist. But in the dry season? The lakes shrink--and piranhas' appetites grow.

Yet they still don't attack arapaimas. Meyers wondered why.

In his lab at the University of California, San Diego, he ran some tests. He mounted piranha teeth and arapaima scales in a vice-like machine. When he pressed the two together hard ... Snap! The teeth broke!

Oddly, though, he also found that piranha teeth are harder than arapaima scales.

So what gives? Not the scales! Meyers found they have a hard, mineralized exterior but a flexible, cross-layered collagen interior. Even if teeth penetrate the scales' exterior, which isn't easy, the springy interiors just flex without tearing.

So piranhas leave arapaimas alone. Ith justh not worth the rithk.



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