It's unusual for leeches to swarm en masse, but as you can see in the video above, it's a terrifying possibility. This band of (possible) bloodsuckers was filmed near Green Bay, Wis., by Shedd Aquarium fish ecologist Solomon David. He's not in the habit of swimming into hordes of leeches: He brought the footage back last week from a trip focused on restoring the migratory habitats of Great Lakes fishes.

While investigating some of the so-called aquatic corridor — rivers, streams and even agricultural ditches — that fishes use to migrate, he stumbled upon something unexpected in a  nondescript ditch site.

"I noticed several leeches immediately in view," he said, "But as I zoomed out, there were just leeches everywhere, as far as the eye could see."

Luckily, David says, he was a teaching assistant in parasitology for years while getting his degree, so he wasn't too grossed out.

David is working with researchers at the American Museum of Natural History to identify the species of leech seen in what he's dubbed the leech-nado (SyFy Channel producers, if you're reading this, please, please don't make this your "Sharknado" follow-up. I beg of thee, have mercy) but it may be tricky. Just days after the video was taken, the leech swarm had disappeared. David has seen leeches of the same type in the area in smaller numbers before, so he hopes his intrepid research assistants can get some samples for further examination.

Lots of leeches live on blood, it's true. They either latch on with strong jaws full of sharp, y-shaped teeth or come at their prey super-fast with a powerful proboscis, mosquito-style. Once they're attached to prey, they use mucus and suction to stay that way until they're satiated, oozing out a special enzyme to make your blood flow more easily. Some leeches have harmless bites, but others can be quite painful. And while you can't catch leech parasites, you could very well be susceptible to whatever bacteria they're carrying.

Before you swear off swimming forever, note that swarming leeches are so rare that when I Googled "leech swarm," everything that came up was referencing the monsters in a popular role-playing game.

And hey, these leeches might not even suck blood! They might gobble down aquatic invertebrates instead. So it's all good.


David hopes his video won't make people afraid of the water. In fact, he hopes it will make people appreciate how weird and wild the natural world can be.

"In the past couple years there’s been weird stuff in this pool," David said, pointing out that the Great Lakes have experienced unusually cold temperatures as the rest of the country has gotten warmer. "One year we saw thousands of yellow perch — more than I'd seen anywhere at one time, and I've been doing this fish thing for a while now.  There are strange phenomena going on right beneath the surface, even in these areas that might not seem so exotic."

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