10 Intriguing Facts About Leeches

Leeches often receive a negative reputation, but they are fascinating creatures once you understand them—and we continue to learn more about them even today. Recently, a team led by Anna Phillips, curator of parasitic worms at the Smithsonian National Museum of Natural History, identified a new species of medicinal leech (shown above) in a Maryland swamp. We consulted parasite expert and curator at the American Museum of Natural History, Mark E. Siddall, to uncover some surprising details about these often-misunderstood worms.

1. Not all leeches consume blood.

Hematophagous, or blood-feeding, leeches are just one category. “Most species are hematophagous,” Siddall explains to Mytour, “but it varies by environment. In North America, there are likely more freshwater leeches that do not feed on blood than those that do.” Even among blood-feeding species, few target humans. “Very few leeches are interested in human blood,” Siddall notes. “They may feed on it if given the chance, but it’s not their primary food source.”

2. Leeches can be found almost everywhere.

“Leeches inhabit every continent except Antarctica,” Siddall explains. “Even in Antarctica, marine leeches thrive in its waters.” Humans have lived alongside leeches for so long, Siddall notes, that nearly every language has a term for them.

3. Leeches have regained their role in modern medicine.

While bloodletting for general health has lost favor among Western doctors, medicinal leeches still play a vital role. Surgeons now use them in operating rooms as natural tools to remove excess blood. “This is a practical and effective use of leeches,” Siddall states. However, he criticizes other applications: “Using leeches in naturopathy to eliminate ‘bad blood’ or cure unspecified ailments is nonsense. How could leeches distinguish between good and bad blood? It’s absurd.”

4. Author Amy Tan has a leech species named after her.

Land-dwelling leeches featured in Tan’s 2005 novel Saving Fish from Drowning, earning her admiration from leech researchers. “Few novels include terrestrial leeches,” Siddall notes. When he and his team discovered a new species of small land leeches, they named it after Tan. The author was delighted. “I’m honored to be immortalized as Chtonobdella tanae,” Tan remarked in a statement. “I’m planning a trip to Queensland, Australia, to stroll through the jungle while my namesake leeches feast on my ankles.”

5. Leeches can grow to impressive sizes.

The giant Amazon leech (Haementeria ghilianii) can reach lengths of up to 18 inches and live for 20 years. This species is indeed a blood-feeder. Like other hematophagous leeches, H. ghilianii uses its proboscis (which can extend up to 6 inches) to attach to a host, feed, and then detach. Thought to be extinct, the species was rediscovered in the 1970s by a zoologist who named one of the specimens Grandma Moses. This is a true story.

6. Leeches are excellent fishing bait.

Many walleye fishermen highly recommend leeches. “Leeches are more dynamic than other live baits,” professional angler Jerry Hein explained to Fishing League Worldwide. “I’ve used them since I started fishing, and they’re the most effective bait, no matter the location.” The demand for leeches has spawned an entire industry. One year, unfavorable weather conditions disrupted leech habitats, making them unavailable for collection and sale. A tackle shop owner described the shortage to CBS News as “the bait industry’s worst nightmare.”

7. Leech researchers sometimes use themselves as bait.

Siddall and his team spend hours exploring leech habitats to collect and study wild specimens. “Whether we’re wading through water or hiking in bamboo forests,” Siddall explains, “we rely on the fact that leeches are naturally drawn to us.” Do the leeches bite them? “Absolutely. We try to catch them before they latch on, but sometimes it’s unavoidable.”

8. Leech mating behavior is fascinating.

Like many worms, leeches are hermaphrodites. Mating methods vary by species, but most entwine their bodies and exchange sperm packets. (The two leeches in the video above are both named Norbert.)

9. Certain leech species exhibit remarkable parental care.

“There’s a family of leeches that protect their eggs by covering them with their bodies,” Siddall says. “They lay the eggs, shield them, and fan them to prevent fungal or bacterial growth. Once the eggs hatch, the young attach to the parent. They aren’t feeding on the parent but simply hitching a ride. When the parent leech finds its next blood meal, it carries its offspring along. This level of care is extraordinary, especially among invertebrates.”

10. You could be the next person to identify a new leech species.

Despite coexisting with leeches for millennia, much about them remains unknown. Scientists have identified around 700 species but believe many more exist. “Here’s my wish,” Siddall says. “If a leech ever feeds on you, instead of removing it and discarding it, take a moment to observe its color patterns. There’s a genuine chance it could be a new species. Let it finish feeding—it won’t take much blood. Who knows? Your observation might contribute to science.”