The candiru (Vandellia cirrhosa) lives in the rivers of the Amazon and feeds exclusively on the blood of larger fish, entering their gills and biting through their blood vessels. It satisfies its hunger within a minute and swims away in search of new prey, which it finds by the smell of ammonia emitted from its prey's gills as it breathes. 1)
There are cases when candiru enters the urinary tract of large mammals, including humans who urinate in the water. Heading upstream of the flowing urine, the fish enters the urethra and falls into a trap from which it is impossible to escape because of the sharp spines. The fish dies but can only be removed surgically. 2)