Happy Valentine's Day! Though, I doubt this post fits well into your lovely romantic images of the day...
I promised another vertebrate, so here it is, a parasite so cruel it's sure to make you cringe: The Candiru.
It looks innocent enough. It's a little catfish. Heck, it's so little it's known as the "toothpick fish", and yet it's more feared in the Amazon River than the fierce Piranha.
To understand why, you have to understand what this parasitic catfish does. It's intended hosts are other fish. When it finds a host, it burrows its head in to its gills and eats the blood, tissues, or mucus of the unwitting meal. 
What's most amazing is how this little fish, which only grows up to 6 inches long, is able to parasitize such nimble prey. It lies in wait in the depths, 'sniffing' the water for fish waste products like ammonia. It follows these scent trails to their creator, and launches itself as hard as it can into the gill cavity, sticking in place with its spines. It then eats inward, usually towards a major blood vessel, and when it's had enough to eat leaves the usually fatally-injured fish to digest its ill-gotten gains and wait for the next meal.
Now, there's one problem with this plan - fish aren't the only producers of ammonia-based waste products. People produce a similar compound, which the Candiru mistakes for its beloved fish excretion. Now replace "gill cavity" with urethra, vagina, or anus of a person, and...
Do I really need to explain the reason it's so feared where it lives?
Unfortunately, the fish can't survive in this accidental host... and, of course, the accidental host isn't too happy about its presence, either. Removing the Candiru from human tissues generally requires very delicate surgery, and I imagine the pain before and after is excruciating.
Talk about a nasty reminder not to pee in the pool - or at least, not to pee in the Amazon...
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