For the first time in 44 years, America has a new species of so-called medicinal leech. Investigations revealed that not only has the leech been around for a long time, but specimens have been collected over a large part of the eastern United States. However, it was only when one was collected in southern Maryland, not far from the nation’s capital, that parasitologists recognized they had a new species on their hand, legs or other exposed body parts.
Medicinal leech is the term given to any species of Hirudinea worm that feed on human blood. Whether the leech was ever used to treat conditions like fevers or gout is irrelevant to the definition. On this basis, the creature named as Macrobdella mimicus in the Journal of Parasitology definitely qualifies.
Dr Anna Phillips of the Smithsonian National Museum of Natural History has conducted a long-term study of North America’s leech biodiversity. In 2015 she collected an olive-green specimen with orange spots in a Maryland swamp. On the basis of appearance, she identified it as Macrobdella decora, but the DNA didn’t match.
Ricardo Salas-Montiel, a graduate student of the National Autonomous University of Mexico examined the new find more closely and noticed its reproductive pores are spaced differently to M. decora. (Maybe try not to think too closely about the fact leeches have multiple reproductive pores.)
Leeches subsequently collected in South Carolina were found to have pores matching those of the Maryland find. “Then we sequenced [their DNA], and they all came out more closely related to the leeches we had found in Maryland than to anything else known to science," Phillips said in a statement.
At this point, it was clear the Macrobdella genus had a fifth member, and one that might be quite widespread. Phillips checked the extensive collection of leeches stored in her museum’s collection. "All of a sudden, I started finding these things everywhere," she said. For at least 80 years parasitologists had been collecting M. mimicus and thinking it was M. decora because they hadn’t looked closely enough at its reproductive pores, for which we cannot blame them.
M. mimicus has a range that extends at least from northern Georgia to Long Island. Indeed it seems to have this territory to itself, with true M. decora restricted to a more coastal band, while other species are present further west.
“We found a new species of medicinal leech less than 50 miles from the National Museum of Natural History – one of the world's largest libraries of biodiversity," Phillips said. "A discovery like this makes clear just how much diversity is out there remaining to be discovered and documented, even right under scientists' noses."
