Candiru Fish

The area most populated by this fish is at the junction of the Amazon River and the Rio Negro, near Brazil's inland city of Manaus. Here they thrive as the low pH, brown, largely organic-material based Amazon river churns with the conversely high pH (basic), oligotrophic (with very low nutrient content, i.e., organic material), tannin-saturated flows of the Rio Negro. This mixing point provides a rich diversity of sustained fauna.
Attacks on people
Although lurid anecdotes of attacks on humans abound, there is only one documented case of a candiru entering a human orifice. In this instance, the victim had a candiru swim into his urethra as he urinated while thigh-deep in a river. Jeremy Wade, a British biologist, investigated this incident for the Discovery Channel's River Monsters. The victim underwent a two-hour urological surgery to remove the candiru. Dr. D. Scott Smith, Chief of Infectious Diseases at Kaiser Permanente, described the candiru as having spikes to assist it clinging to its host.