Isopods are crustaceans (like lobsters). Some isopods are parasitic (like the one staring back at you in the picture up top). If you're having trouble making out what, exactly, this isopod has parasitized, the answer is: a fish. This little monster wriggled its way in through the gills of its host, implanted itself inside the fish's mouth, and gorged itself on its victim's tongue — which it has now effectively replaced. Permanently.
Researchers have known about these stomach-turning parasites for decades, but there's still much that we don't know about them. Only recently, for example, did we learn that fish with parasites in their mouths tend to have lower blood counts than those that are non-parasitized, suggesting that these isopods do more than just eat their hosts' tongues — they actually behave like "blood-drinking mouth leeches." (A description I'm borrowing from Carl Zimmer, because it made my hair stand on end when I read it.)
Now, a new study published in the Biological Journal of the Linnean Society has shown that these tongue parasites are rampant among a species of Mediterranean fish known as striped sea bream. In protected, unfished waters, the scientists found that 30 percent of the bream were harboring tongue-eating, blood-drinking mouth leeches. But it gets worse. In a second, heavily fished population, the percentage of bream with parasitic mouth demons jumped from 30 all the way to 47. "Fishing pressure," conclude the researchers, "can exacerbate the effects of parasitism."
You all know what that means, right? It means we have to stop fishing. (For the record, no, these parasites do not attach to human tongues, nor do parasite-harboring fish cause human disease. Oddly, I've found that knowing these things has offered me very little comfort, and will probably not make getting to sleep tonight any easier.)