Meet the candiru. "Candiru" literally means "toothpick" in Spanish. It's a very small fish. It lives in the Amazon River. It is translucent, so its presence is hard to detect in the Amazon's turbid waters. It is a parasite. It sucks blood from the catfish's gills. How does it find its host? By detecting the ammonia that the catfish's gills exude as the catfish swims close by.

Ammonia is abundant in urine. So what happens when a human swims in the Amazon River and then pees underwater?


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