There are some nasty things that lurk in bodies of water. Whether you’re in a lake, river, or ocean, there’s always something you should look out for, always something swimming quietly in the depths to make you just that little bit nervous while you’re paddling around. Today’s animal, the candiru, is a particularly nasty critter.

Luckily, candirus are only found in the Amazon River Basin, so you don’t have to worry about them unless you’re swimming in the Amazon. Candirus have been found in Bolivia, Brazil, Colombia, Ecuador and Peru. They like slow moving, shallow areas, particularly those with muddy bottoms.

Candirus are quite small, only growing to maximum lengths of 17cm. They are not very noticeable fish, as they are translucent except after feeding. They are long and thin, with large eyes on top of their heads. The most interesting part about candirus is the backwards-facing spines that project from their gill covers. These have quite a sinister purpose, I assure you.

It used to be thought that candirus were attracted to urea released from their hosts’ gills, but this is untrue. They hunt using visual cues, hence the large eyes. Image source

You see, candirus have a taste for blood, and the way they get it is definitely creepy. They lodge themselves into a fish’s gills, where their spines pierce the thin gill membranes, letting blood flow right into the candirus’ mouths. It used to be thought that Candirus were attracted to urea released from their hosts’ gills, but this is untrue; they hunt using visual cues, The spines also keep the candirus stuck under their hosts’ gills; the backwards-facing nature of the spines means these parasites are incredibly difficult to dislodge.

Sounds pretty nasty, right? Well that’s not the worst thing about candirus, according to prevalent rumours. You see, it’s said that sometimes when people are urinating in the river, a candiru gets a little confused and swims right into a person’s urethra. And because of those spines, they are extremely hard to remove. Not pleasant experience I’m sure.

Some recently fed candirus. Image source

So yes, candirus are horrible fish. But the accounts of candirus swimming into peoples’ urethras are rare, and are not terribly reliable. The only modern case occurred in 1997, and has a few inconsistent details that makes it unlikely that a candiru did actually lodge itself in a man’s urethra. Still, I’m not going to be the one to go in the Amazon to test whether or not the tales are true.