The bridge over Thorne Gut Marsh in Nanjemoy might seem like an unlikely locale for a major scientific discovery, but that’s where an international team of biologists came across a previously undiscovered species of leech — the first new species of its kind discovered in over 40 years.
The new species, which the scientists named Macrobdella mimicus, was publicly unveiled in a research article in the Aug. 15 issue of the Journal of Parasitology.
The last time scientists identified a new species of North American bloodsucking leech, which are also called medicinal leeches, was in 1975.
Smithsonian zoologist Anna Phillips, the article’s lead author, said that the species was discovered by accident while she and a team of research scientists from the Universidad Nacional Autonoma de Mexico were attempting to solve a genetic mystery: why the geographic range of another leech species, Macrobdella decora, is so large.
Phillips said that M. decora — so named because it is decorated with bright orange spots — can be found from the east coast of the United States almost to the Rockies, and from the Canadian border to the South, a spread that she described as “almost unbelievably large.”
“It’s suspiciously large to a biologist,” said Phillips, the curator of parasitic worms at the National Museum of Natural History. “How is this all one species?”
That’s what led Phillips, biologist and co-author Alejandro Oceguera-Figueroa and two of Oceguera-Figueroa’s students to undertake an expedition deep into the wetlands of western Charles County in the summer of 2015, where Phillips had previously found an abundance of leeches to examine.
“I had been down in Southern Maryland before and I had found a leech and said, ‘Oh, it’s Macrobdella decora’ and I tossed it back,” Phillips recalled with a laugh. “’We’ll come back and get some later when Ricardo [Salas-Montiel, another co-author] and Alejandro are visiting.’”
The team collected six leech specimens from the freshwater wetlands around Nanjemoy during their research trip.
“Collectors waded into the water and periodically examined their legs for attached leeches to collect,” the paper states drily. “Investigators also used dip nets.”
Phillips said that the experience was memorable for the two students, for whom this was their first trip outside Mexico.
“We had been going out, slogging through these wetlands,” Phillips recalled. “It’s off of a highway, so there’s a little bit of trash in the water. And in the meantime we had this barrage of mosquitoes and ticks and chiggers and briars and stinging nettles and all that Southern Maryland has to offer in the summer. And bloodsucking worms at that.”
At the end of summer, as the students prepared to return to their university, Phillips asked them what they thought of their trip. Their response was a model of scientific diplomacy.
“They said, ‘D.C. was nice, but field work is much harder in Maryland than it is in Mexico,’” Phillips said.
When Salas-Montiel returned to Mexico City and analyzed genetic samples from the leeches collected in Nanjemoy, he discovered that there was between a 6% and 11% difference from samples of genetic materials taken from other M. decora specimens.
The scientists had expected to see a difference of anywhere from 2% to 6%. This wasn’t just genetic variance within a single species — this was an indication of an entirely new one.
“There were a lot of emails flying,” Phillips said. “Thank goodness for video conferencing, that’s how we shared the excitement.”
Not long after their surprise discovery, Phillips was scouring the photo website Flickr for pictures of leeches when she came across a set of photos posted by a man in South Carolina who was searching for orchids. In several photos, the man’s legs were covered with leeches.
After visiting the area and collecting several leeches for analysis, Philips found that genetic samples from the leeches matched those taken from the mystery species.
“We realized, ‘Wait a minute, this isn’t just in Southern Maryland,’” Phillips said. “Southern Maryland to western South Carolina is a big jump.”
“That’s when I said, ‘Something is happening,’” she said.
That launched Phillips on a quest to identify the geographic range of their newly discovered species. She began by examining every specimen of M. decora in the Smithsonian’s National Leech Collection in Suitland, as well as in leech collections in South Carolina, North Carolina and Virginia.
Fortunately, Phillips’ search was made easier by the discovery that the new species could be easily distinguished from other leeches by the number of circumferential rings between the various organ pores on its belly.
This was crucially important because almost all older museum specimens, including those in the Smithsonian’s collection, are stored in formalin, a chemical that gradually breaks down DNA over time. That meant that Phillips would not be able to use the same technique that Oceguera-Figueroa used to identify them.
At the North Carolina Museum of Natural Sciences, Phillips recalled, “I had to go back to them and say, ‘I’m so sorry, you don’t have Macrobdella decora in your collection. Everything is this new species.’”
As a result of her intensive research, Phillips was able to establish that the new species — which she and her colleagues named M. mimicus because it mimics the overall appearance of M. decora — can be found along a narrow corridor that runs from the Carolinas through Southern Maryland, with two other samples having been identified in Long Island in 1937 and 1940.
“I don’t know what’s going in the Pine Barrens of New Jersey,” Phillips said, because she has not identified scientific collections in that state and has not had an opportunity to mount an expedition there.
“I’m convinced it’s in Alabama,” she said. “I just haven’t found it yet.”
Phillips, who has hunted for leeches on six continents, is clearly eager to find out.
Although Phillips said there is no record of any of the known species of Macrobdella (which means “large leech”) being used for medicinal purposes in the United States in the 17th and 18th centuries, leeches are used in modern medicine to help heal skin grafts and prevent complications following reconstructive surgery.
“They are an approved medical device in the United States,” she noted. “They are not experimental.”
Although Phillips, who joined the Smithsonian in 2013, has coauthored many other scientific papers in her career, she said that this one is special because it has allowed her to come full circle on her research into medicinal leeches, which began when she was an intern at the American Museum of Natural History in New York.
“I love this paper,” Phillips said. “Any time you get to name a new species, it’s fun.”
Phillips said that it’s also exciting to her as a scientist to realize that there is unrecognized biological diversity so close to home.
“When you think about the fact that we’ve had the Smithsonian here for so long ... you’d think that everything around the D.C. area would be really well known,” Phillips said. “And it turns out that ... it’s been here waiting.”
The discovery of M. mimicus is a reminder that “you don’t just have to go to faraway places” to find new scientific discoveries, she said.
It’s also a case study in the value of local knowledge, she pointed out.
“When I first went to Southern Maryland, I didn’t know where to look,” Phillips said. “People who live there, out in the environment, [they] know it better than I do. It may be that they’re seeing patterns and trends and organisms that they know more about ... than scientists with degrees.”
Scientific discovery, Phillips said, “really comes from a place of curiosity and appreciation for what’s around you.”