I Don’t Swim in South American Rivers

Why, you might ask? Because of this:

Since a piranha-like fish with 1+ inch teeth isn’t scary enough on it’s own, this one has a frickin’ isopod for a tongue! Big sharp teeth, murky water, and a tongue-replacing parasite: need I say more?

This little beauty was fished from the depths by my labmate Steve Paiero while visiting Guyana in January. I can imagine reeling in this Hydrolycus (I think the fish is either Hyrdolycus armatus or Hydrolycus tatauia) would be exciting, but then to remove the hook and find a parasitic isopod looking back at you? Amazing. Or horrifying. I’m not sure.

Cymothoidae isopods¹ are one of the stranger parasites out there, and have garnered considerable attention from other bloggers. After entering the fish’s mouth through the gills², they begin to suck on the fish’s tongue, drawing all the blood from it. As the tongue withers and dies, the isopod takes its place, and helps itself to any food the fish may happen to ingest, making them an odd combination of parasite turned kleptoparasite!

I could find very little taxonomic or natural history information about these isopods, but there is apparently only 1 species of Cymothoid isopod with a penchant for tongue, Cymothoa exigua [Update: Dr. Alistair Dove (@para_sight) of the Georgia Aquarium informs me there are numerous tongue biting isopods. This is unsettling]. Oddly, all accounts I’ve been able to find list Cymothoa exigua as a marine species, and not something you’d expect to find hundreds of kilometers up a river in the mouth of a freshwater fish! Seeing as it has a few more legs than I’m used to dealing with, if someone has a better idea on what’s going on, I’d love to hear about it.

Needless to say, I’m glad I’m not a small guppy potentially staring this in the face one day!

UPDATE April 11, 2012: It seems there are a mouthful of tongue biting isopods in South American freshwater ecosystems. I’m in the process of doing a little more research and will update the identity and biology when I can!

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1- That’s right, this beastie is a cousin of the innocuous little pill bugs (sometimes called potatoe or sow bugs) that you can find in your own backyard.

2- You can see another little isopod latched on to the right side of the mouth in both photos. Whether this is an immature Cymothoa exigua or a different species I’m not sure. Either way, tough luck for the fish!