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The parasitic, eel-like fish called lampreys are coming back to England. While this might not seem like a good thing, environmental authorities say it's a sign that English rivers are cleaner than they've been in a long time.

Lampreys feed use their horrifying "mouths" to latch onto animals and suck blood and scales. They're considered endangered in Europe. But recently they've been spotted in some rivers with less polluted water, according to England's Environmental Agency.

They used to be a royal delicacy in medieval England. More recently, you might have seen them make an appearance on an episode of Game of Thrones, where Tyrion Lannister chows down on some lamprey pie as he plots to overthrow something or kill someone.

Actually, lamprey pies turn up quite a lot in Game of Thrones, it turns out:

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And legend has it that King Henry I's fondness for lamprey pie actually killed him in the end‚Äďas an old man, he ignored doctor's orders and ate a whole bunch of lamprey, which gave him food poisoning and resulted in his death.¬†In 2012, Queen Elizabeth II was sent a lamprey pie to celebrate her diamond jubilee, or 60th year as monarch, but the fish¬†had to be imported from the U.S., according to the¬†Guardian.

Lampreys pre-date dinosaurs by something like 200 million years, the Telegraph writes. But since the 1800s, they've been missing from English rivers because of pollution and man-made obstacles which blocked their ability to migrate up and down river systems. That was partly because of the industrial revolution, according to ITV News.

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‚ÄúThese are fascinating fish, living fossils, that have a special place in the history and traditions of this country, and we hope that with a helping hand from us they will be able to thrive in England‚Äôs rivers once again,‚ÄĚ Simon Toms, a fisheries expert at the Environment Agency, told the¬†Telegraph.