speaking of fish... i know this isn't this or that, but it is kind of interesting:
If you go swimming in the rivers in South America, like the Amazon or the Oranaco, there's this little blood-sucking parasitic fish called the Candiru (also Vampire Fish), and it is feared by South Americans a million times worse than pirhanas. The fish is attracted to nitrogenous waste eliminated through the gills of other fishes, and by following the odor, it can lodge itself into the gills of other fish and feeds off the nutrients from their lungs and in their blood.
Well, it turns out if you piss in the river, the nitrogenous waste (urea) in your urine will also attract the Candiru, since it is the same substance eliminated by fishes' gills. The Candiru will follow the trail of nitrogenous waste, find it's way into the person's urethra and lodge itself in there. It is supposedly one of the most painful things a person can ever experience. The way it lodges into the urethra is by puffing out its fins, which are friggin pointy and kind of sharp, and then it is only removable by surgery. The spanning out of the fins is a fright response, making it all the more difficult to remove the fish from the urethra. Also, these things suck your blood, so don't piss in rivers or you'll be stuck with something a lot worse than a marble or a pirhana in your dick...