- This is the story about the leech and the alligator.

We all know about alligators and how it's best to keep our distance.

But then again-- - Who wouldn't be excited about sticking their hand in the mouth of an alligator?

- OK, maybe a few of us wouldn't be that excited.

Then there's the leech, known for their blood-sucking preferences in the nearby creek when we were kids.

[screaming] We just don't get much good press.

They're not great at public relations.

- When you say leech, I think the knee-jerk reaction is generally one of revulsion and disgust.

- What do these two beings have in common?

In a state where leeches quietly support the ecosystem, it's remarkable that one species out of hundreds worldwide has chosen alligators as its preferred host.

- Kind of giving a bad rap to an entire group of organisms that's not necessarily well-deserved.

But about half of the world's leeches feed on blood.

Only about 10 to 15 of those go for human blood.

Bronwyn Williams is an expert.

- We probably have over 30 species known of leech, known from North Carolina.

Only two of which are known to feed on humans.

This is still a small-ish one.

They get bigger then.

- Williams has spent more than 20 years studying crayfish and the small organisms that live on them.

Tiny invertebrates that led her into the wider world of leeches, a group found in freshwater habitats almost everywhere on Earth.

Hey, that's your name on this.

- It is.

I collected that.

Yep.

Awesome.

I think one of the favorite things about my job is that I can just walk across the hall, come back into the collection, pick up a jar, look at it and go, what is this?

- Literally next door to Williams' workspace at the Research Extension of the Museum of Natural Resources is one of the largest research collections in the Southeast.

A long-running record of the region's reptiles, amphibians, and freshwater invertebrates.

And leeches, of course.

- All right.

- Oh, man.

- There you go.

So this narrow end that's up in the air, questing around, is actually the head end.

So it's attached to your-- or semi-attached to your hand by its butt sucker.

Its butt sucker?

- Its butt sucker.

Oh, that's nice.

Yeah.

I'm glad it's secure.

- It is very secure.

- Williams lives for this kind of stuff.

And so does Miriam Boucher, a South Carolina researcher who studies what alligators eat.

When Boucher started finding leeches clustered on the tongues of local alligators, including some in the southern part of our state, she reached out to Williams for help figuring out what they were.

- She followed that initial message up with, well, we're finding large leeches on the outside of the alligators.

So largely on the back or kind of between the toes.

And we're finding all these little leeches in the mouths that seem to be clustering on the tongue of the alligators.

And so she sent me these specimens to look at.

And they appear to be two different species, which was kind of wild.

So we've got kind of these two species that thought one was thinking that the back of the alligator was a great place to call home.

And one thought that the mouth was a really nice, safe place to call home, as it were.

And that's a little mind blowing.

- But why is that mind blowing?

Well, part of it is that because leeches all have their preferences of hosts-- amphibians, birds, mammals.

And it looks like finding a leech on an alligator is weird.

- Then we were actually able to use genetic methods to show that these were actually the same species.

So those little leeches that were found on the tongue of alligators are actually the little juvenile leeches of the big ones that we find on the back of the alligator.

More of a heart shape, kind of flared.

- Back in the lab, William showed us how they look at and distinguish one leech from another.

That's actually its blood meal.

So that's alligator blood still in the leech.

Blood meal?

- Blood meal.

That's a curious word.

- As you scan around, see, they all look the same.

The pattern is almost-- they have a single row, if you want to look in the microscope here.

- Yeah, please.

Thanks.

- Sort of a single row that looks like a checkerboard down the middle of the back.

And then they've got several lines of tubercles, or these bumps on the back.

- t's like a mosaic on the back.

- It is, yes.

- North Carolina marks the northern edge of the American alligator's range, a boundary where animals and the species that live on them can behave differently than they do farther south.

- It was a 700-pound alligator, so there were-- - Dr.

Stewart is an expert in this area.

- The idea of there being a leech on an aquatic reptile, or a semi-aquatic reptile, is not a far-fetched idea.

But it's just amazing to me that our largest species of reptile, such a conspicuous parasite, would go undetected for so long.

- For Williams, every specimen is a clue, part of a larger story about how life adapts, survives, and sometimes surprises even the people who study it.

- This is a jar of white shrimp.

This is the oldest specimens in the invertebrate collection here.

These were collected in Charleston, South Carolina in 1855.

They're in pretty good shape for being that old.

Basically, a species is somewhat equal parts philosophy and biology, right?

It's a human construct.

But that is the critical foundation for conservation and management.

So there is a lot that's riding on what we call species.

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