alligator gar teeth An alligator gar. Rogelio V. Solis/AP

Over the last four decades, Asian carp have marched up more than a thousand miles of rivers toward the Great Lakes — the largest reservoirs of freshwater in the United States.

And ecologists have anxiously watched them, trying to find a way to stop the invasive fish's progress.

Now they're calling in a fearsome, once-loathed creature that they hope will become a powerful ally: alligator gar.

Alligator gar are huge — the second-largest fish native to American freshwater. They're not going to win a beauty pageant any time soon, thanks to a mouth full of sharp teeth, but if they live up to ecologists' hopes they'll win a popularity contest.

That's because they seem willing to sit down to the Asian carp buffets in the Midwest's rivers.

flying asian carp fish The terror of Asian carp isn't limited to what's in the water. John Flesher/AP

The Midwest's problem with Asian carp began in 1973, when the fish were brought into Arkansas to serve as water filtration systems at fish farms (by eating all of the junk that sinks to the bottom).

From there they snuck into nearby rivers, where they began muscling out native fish. Because they can leap out of the water, they've also been known to smack into and injure boaters. They've even been nicknamed "garbage fish."

Ecologists have been cautious to turn to alligator gar, since introducing species to eat unwanted species has a long history of backfiring. (The poster child of pest control gone wrong is the cane toad in Australia — the toad turned its nose up at the beetles it was supposed to eat and is highly poisonous to native predators.)

alligator gar babies fish Tank-raised alligator gar waiting to be released into the wild. Rogelio V. Solis/AP

But the alligator gar has a fin up here, so to speak: It actually belongs in these rivers.

Until the middle of the 20th century, the alligator gar reached as far north as Illinois, although now it's stuck in the southern reaches of the Mississippi and other rivers that feed the Gulf of Mexico.

That's because competitive fishers thought alligator gar were bad for the fish they wanted to catch (modern ecology suggests the opposite is true), so they went after the gar with guns and explosives.

The gar are big enough to hold their own against the flood of Asian carp. They can grow more than 8 feet long — twice the length of Asian carp.

"What else is going to be able to eat those monster carp?" asked Allyse Ferrara, who studies alligator gar at Nicholls State University in Louisiana, talking to the Associated Press.

Check out alligator gar and Asian carp in action below.