They needn't have been afraid. Researchers couldn't test whether one candiru had got itself stuck inside one man, but they could test the notion that candiru were urine-seeking missiles. For some time, people who lived by the Amazon believed that urine attracted the fish. There is some scientific evidence to back up the claim. Candiru are vampires; they suck the blood of other fish, usually by attaching themselves to the inside of the fish's gill. The prey fish of the candiru give off urea from their gills. It made sense that candiru would pursue the chemical.