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The candiru is a parasite of fish gills and a catfish has nowhere to hide. A trail of urea and ammonia streams from its gills giving signals to the candiru, which simply swims in circles until it picks up the scent. The signal becomes stronger as is gets closer to the catfish and it slips inside the gills unnoticed. Spines in the candiru's head stick into the fish, securing it while it sucks blood from the gills. Humans urinating in the water send the same cues of current and urea and a candiru can end up in the wrong host, causing great pain to the person concerned.