Leeches don’t suck!

Bog Blog guest post by Minnesota Master Naturalist H. Bradford as part of 2023 North Woods, Great Lakes Biome Course in the Sax-Zim Bog!

Leeches aren’t the most loveable creatures. Leech is used as an insult towards someone who is selfish or lazy, parasitizing off of others.  In society, leeches invoke fear, such as the scene from the classic movie, The African Queen.  As for myself, I hadn’t really given them much thought until a recent trip to Borneo, wherein I became aware of the existence of terrestrial leeches. I didn’t realize that there were land leeches, which opened up the door to learning more about leeches. This newfound curiosity about this misunderstood creature led me to enter the wormhole of leech knowledge for my Master Naturalist capstone. The purpose of the capstone project was to learn more about leeches, but also share our findings. As it turns out, leeches are fascinating and important creatures.

For my capstone, I partnered with fellow Master Naturalist student Laura P. to learn more about leeches and educate our classmates on what we learned. To cover the basics, leeches are annelids, or a type of worm belonging to the subclass Hirudinea. They are closely related to Oligochaetes, or the class that includes earthworms. They have 34 body segments, one to five pairs of eyes, two suckers (usually one on the front end and larger, a posterior sucker), and are hermaphrodites (male and female parts).  The Guardian, quite sensationally, noted that they have 10 stomachs, 32 brains, and 18 testicles. For full disclosure, neither of us are “worm people” or well versed in the world of annelids, but it was surprising to find out that both ends have suckers. One end is mostly used for latching and moving, the other for feeding. Leeches have been around for over 250 million years since the Permian Period. It is fascinating to think of the long extinct creatures they might have fed on. Was there a Dimetrodon leech? Probably not, but leeches survived the Permian-Triassic Extinction and the K-T Extinction to survive and thrive in many environments today. Presently, there are over 650 species of leeches in the world, though the exact number is unknown on account of lack of leech studies and debates over leech taxonomy. Leeches occur all over the world, with freshwater, land, and marine species. Leeches can even be found in desert pools and Antarctic waters. In North America, there are 63 known species. Overall, the vast majority are fresh water dwellers, followed by marine, and lastly terrestrial. 

Leeches are everywhere, yet, largely absent in our social imagination, except for the fear and discomfort they bring to swimmers. Nevertheless, they have a long history as well as important ecological and economic roles. For thousands of years, leeches were used in Chinese, Indian, Arab, and European medicine. They were depicted on the walls of tombs during the 18th dynasty (1567-1308 BC), Chinese medicinal texts by the 3rd century BC, and are mentioned by Galen and Hippocrates. Millions of leeches were used in European and American hospitals through the mid-1800s, after which they fell out of favor due to changes in medical knowledge, decline of horses (their preferred host) and over harvesting. The name leech itself comes from the Anglo-Saxon word laece, which means physician. In 2004, the FDA approved the use of leeches for medical purposes, on account of the anticoagulant properties of their saliva.  Macrobdella decora, the North American medicinal leech, can be found in Minnesota waters and is characterized by five pairs of eyes, olive coloration with orange spots, and three jaws. To confuse matters, other Macrobdella species are also called North American medicinal leeches. In other parts of the world, other species of leeches are used for medicine, such as Hirudo medicalis in Europe and Hirudo manillensis in Asia. Because of its overuse in Europe, it is a protected species and considered near threatened by the IUCN. Beyond leech medicine, in 1850 (the aptly named) George Merryweather constructed a leech barometer called the Tempest Prognosticator. The device used live leeches in glass jars to predict storms. Although he conducted trials of the device, it never caught on and remains a historical curiosity. Unlike the return of leeches to medicine, it is unlikely they will make a comeback in meteorology.   

Leeches are beneficial beyond their uses in medicine. Terrestrial leeches can also be used in biodiversity research, as the blood that they have fed on can indicate the types of animals present in an area. They are an important food source for fish, birds, reptiles, and amphibians. Because they are a popular food source, they are also important to the bait industry. Leeches for bait is a $1.5 million industry (a number that is outdated but still cited) providing income to those who catch and sell them. Bait leeches are also important for recreation, as they are a popular bait for catching a variety of fish and especially associated with walleye fishing. The number of bait leeches harvested each year is not often updated, but old estimates put the number at 97,000 lbs a year. The species used for fishing is the ribbon leech, Erpobdella obscura. Specific data on environmental impacts of the leech bait industry is not readily available, but it can be inferred from DNR licensing language that there are concerns regarding the overharvesting of leeches, the transport of leeches from out of state, and the dispersal of invasive species through improper bait management. Leeches are kept as pets and a cursory glance at pet leeches available online shows some medicinal leeches marketed as “Dracula leeches”for sale. The terrestrial Tiger leech from Asia is also being sold as a pet, though having had a few on me in Borneo, I don’t think they are a pet I would like to keep. The Buffalo leech (Hirudinaria manillensis) is one of the medicinal leeches for sale as a pet. While it is certainly an unconventional pet, hopefully they bring joy to those who keep them.               

Besides being useful, leeches are interesting in their own right. For instance, leeches in the family Glossiphoniidae are known for their rare parental care among annelids. Their young adhere to the underside of the parent leech and are brought to feed on their first meal. Members of this family feed on the blood of a variety of animals such as snails, turtles, amphibians, and of course, sometimes humans. Members of the family Piscioloidae are fish parasites. Species in the family Theromyzon are known for being bird parasites and lodging themselves in the nasal cavities (or generally mucus membranes) of birds ranging from swans to penguins. Although many leeches do not cause lasting harm to their host, these ones can be injurious to birds. Globally, there are 30,000 species of blood eating animals, though not all leeches feed on blood. Those which feed on blood host bacteria in their stomachs, which provide them with Vitamin B. Their tissue binds with iron to make it less toxic for them. Although 75% of leeches are parasites, others are carnivores and some also consume detritus. For example, leeches in the genus Helobdella feed on such things as insect larvae, snails, decaying matter, and worms. The role of leeches as detritivores is another overlooked area of study.      

Although leeches are useful in many ways, we quickly discovered that there is a poverty of knowledge when it comes to popular science about leeches. For example, there is a key to identifying leeches from UMN-Morris from 1959 and a leech survey from 1912. I-Naturalist lacks robust sightings or data on Minnesota leeches. Therefore, we were uncertain how many leech species are in Minnesota. Michigan has about 40 leeches and combining various lists, we found over 30 species listed for Minnesota, but there is no field guide (that we are aware of) or recent or comprehensive list of species. Online sources also differed in making generalizations about leeches, such as the number of body segments (ranging from 32-34). We briefly looked for specimens on day three of our Master Naturalist training and likely saw two species. However, it was difficult to identify the leeches due to lack of user friendly resources. Because many resources are outdated, facts and figures are difficult to trust. For instance, at least two sources noted that some leeches can be detritivores, but most other sources did not mention this feeding habit, only mentioning leeches as carnivores and parasites. Nevertheless, it can generally be said that Minnesota has a wide variety of leeches with an array of sizes, appearances, and feeding habits, with many specialized in feeding fish and turtles and few which interact with humans at all.

An overall takeaway from the capstone is to start paying attention to leeches. I would encourage anyone who reads this to pay attention when fishing, swimming, or looking at rocks and start collecting and sharing more data. What is out there? What is left to be uncovered? How can we make this more accessible to everyone? The world of leeches is vast, mysterious, and misunderstood. Prior to this summer, I grew up thinking that there were only two kinds of leeches which were either “bloodsuckers” that fed on humans or the larger “leeches” used for bait. I had no reason to think otherwise or ponder what might be out there. That is the magic of learning. It uncovers a whole world which always existed, but was never perceived. Learning about leeches taught me that all along, in plain sight, on the shells of turtles or gills of fish, there is a whole universe of leeches. Leeches in the nostrils of penguins. Leeches in the watering holes of camels. Leeches eating snails. Leeches in hospitals. Most are brown or dark, but upon closer inspection, they can have colorful spots or stripes. Some are pale and smaller than a fingernail. The giant Amazon leech is almost a foot and a half long. Leeches are everywhere, living quietly, but globally, for millions of years, through extinctions, into this one.    

–H. Bradford, August 24, 2023