The question is this: What is the evolutionary advantage of a fish that does this?
In the case history documented here, the fish died. Was that just a “fluke” incident? Or, in fact, is the interior of the male human appendage simply not a survivable environment for this critter? And if that is the case, how did this fish evolve to behave like this?
ETA: Link to column: Can the candirú fish swim upstream into your urethra (revisited)? (Sept. 7, 2001; re-published March 5, 2015.)
Given that candiru fish can grow to lenghts of up to 40 cm (roughly 16 inches), it transpires that any candiru that manages to fit inside a human urethra is going to be a rather young and immature specimen.
I chalk this incident up to a confused young fish who is trying to attack something that is most definitely not a fish. Either that, or it was a rebellious teen candiru.
The wikipedia article offers a little more skepticism on the event.
And an earlier thread showed some of were (and for myself still am) skeptical of exactly what happened.
Elsewhere on this board I have commented on this myth.
I included comments about the actual casereported in the medical literature by a Brazilian physician and one other as well as the book by Stephen Spotte.
The candiru does not swim up male urethras, although the first medical case report does show a candiru in a man’s urethra. (Pencils don’t swim up urethras either, despite having been found there.)
But it’s just too juicy a legend not to have fun with.
Much more fun to think a boy cleaning his fish tank was holding his candiru while peeing when it escaped into his penis than it is fuss over whether or not, if candirus were so inclined, they would prefer vaginas.