My Dad was raised along the Amazon in Ecuador by missionary parents. He told me about this fish some 50 years ago. According to him, the fish has an instinctual need to rapidly swim into warmer water. When one relieves his bladder in the water, the little fishy swims rapidly upstream--sometimes directly up the urethra. Of course, if one is wearing swim trunks, a barrier exists. Because of the structure of the little fishy's whiskers, they fold back when it is swimming into tight spaces. And, because there is no place to make a turn in the tight confines of the ureter, its little whiskers imbed when it is pushed back toward the opening. According to Dad, if you went directly to the doc with the problem, he could split the pipe open, remove the fish, and crefully stitch you closed. However, remember that you are on the Amazon and the nearest doc might be three-days-away, by which time the fishy has started to rot, you haven't been able to pass urine, and the prognosis is usually death by massive infection unless the offending appendage is cut off sometime around the end of day one.
It was enough to convince me to hold it until I got out of any water I was in. I'm over 60 now, and it still gives me the heebeejeebees just to think about that dang little fish.