Wikipedia

Macrobdella decora

Macrobdella decora, also known as the North American medicinal leech, is a species of freshwater leech found in much of eastern North America in freshwater habitats. M. decora is a parasite of vertebrates, including humans, and an aquatic predator of eggs, larvae, and other invertebrates. It is a medium-sized leech with a spotted greenish-brown back and a dull orange underbelly. It has ten ocelli, or simple eyes, arranged in a horseshoe shape, as well as three long jaws. Internally, a pharynx takes up a tenth of its digestive tract; a stomach, the majority of its body length. The stomach connects to an intestine, followed by a colon, a rectum, and finally an anus located on the leech's back. M. decora, like all leeches, is hermaphroditic, and has twenty testisacs and two ovisacs, in addition to male and female genital pores. First described by Thomas Say in 1824, the species is now placed in the genus Macrobdella. Its closest relative is believed to be the species Macrobdella diplotertia.

North American medicinal leech
A large, unflattened leech on a white background. The dorsal side is olive-brown with a series of orange points; the ventral side is dull orange.
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Annelida
Clade: Pleistoannelida
Clade: Sedentaria
Class: Clitellata
Subclass: Hirudinea
Order: Arhynchobdellida
Family: Macrobdellidae
Genus: Macrobdella
Species:
M. decora
Binomial name
Macrobdella decora
(Say, 1824)
  Observed
Synonyms[1]
  • Hirudo decora Say, 1824

Macrobdella decora is found in North America east of the Rocky Mountains: in southern Canada and the neighbouring parts of the United States. There is, however, one disjunct population of leeches living in northern Mexico. The species may be mix and breed randomly across most of its range, but the question remains unanswered and further research into the topic is needed. It is not considered to be endangered. The saliva of M. decora contains a blood thinner dubbed "decorsin" which may be unique to the species. A comparison of the saliva of M. decora and that of European species has lead researchers to the conclusion that blood-sucking in jawed leeches likely evolved from a single origin.

Taxonomy

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Macrobdella decora was originally placed in the genus Hirudo by Thomas Say, who described it in 1824 in an appendix to a book about an expedition up the Minnesota River. Three other species were described in the same pages: Placobdella parasitica, Nephelis lateralis, and Haemopis marmorata. Besides a brief physical description, Say noted that the species was "much smaller" than the other leeches he had described and was "comparatively rare."[2] When Addison Emery Verrill erected the genus Macrobdella in 1872, he transferred Say's species into it.[3]: 138  Macro simply means big, while bdella means "leech" in Greek.[4] It is commonly known as the North American medicinal leech.[5]

Macrobdella decora is most closely related (the sister taxon) to Macrobdella diplotertia; the genus Macrobdella is believed to be a monophyletic grouping. Macrobdella ditreta was previously believed to be sister to the decora / diplotertia clade, but a new species, Macrobdella mimicus, was discovered in 2023 and placed as the sister taxon to said clade.[6]: 587 [7]: 563 

Cladogram illustrating species closely related to M. decora

M. decora

M. diplotertia

M. mimicus

After Phillips et al., 2019.[6]: 594 [a]

Description

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Macrobdella decora is a medium-sized leech, growing between 5 and 8.5 cm (2.0 and 3.3 in) long, and weighing from 1.5 to 3.7 grams (0.053 to 0.131 oz), based on six specimens.[8]: 155  It has a dark green, brown or olive-green back with a line of 20 or so small orange or red dots down the middle, and two corresponding sets of black dots on its sides. Its underbelly is reddish with black spots dispersed irregularly across it.[9]: 67 [2][10][11]: 160  Its back is rounded but its belly is flattened.[11]: 160 [12]: 230  All leech species have 32 segments, but they are all also covered with external rings called annuli;[13] M. decora has "from 90 to 94 annuli" in total.[12]

Anatomy

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The head of M. decora is rounded and has ten simple eyes on the front of its body: one pair between segments two and three; a second pair on segment three; a third on four; a fourth on six; and a fifth and final pair on segment nine.[3]: 138  They are arranged in the shape of a horseshoe.[11]: 160 [12] The nervous system includes twenty-one neuron clusters[14] called ganglia and a nervous cord running the length of the body, comprising two parallel fibres enclosed for most of their length in one sheath. The first and last of the ganglia are larger than the others, and connect to five and seven nerves respectively, whereas the other nineteen connect to four or fewer nerves each. There is also brain located above the pharynx. The brain is two-parted, with each half connecting to the five ocelli on that side of the body with corresponding optic nerves.[11]: 166–7  Leech jaws are chitinous blades with sharp, serrated edges.[15] Macrobdella decora has three long jaws which are semicircular and laterally compressed, each with one row of about sixty-five "extremely sharp" teeth.[12][16][9]: 67  Each jaw uses a saw-like motion to cut open its victim's skin.[17]

M. decora has a large muscular pharynx which accounts for the first tenth of the leech's digestive tract. The stomach is not nearly as muscular as the pharynx, but it occupies about five sixths of the leech's total body length and is subdivided into eleven chambers. The intestine extends from behind the stomach and narrows towards the anus. The last part of the intestine is the colon, followed finally by a small rectum.[11]: 163–5  The anus is located on the leech's back, above its circular acetabulum[12], a large posterior sucker.[18]

Nephridia are understood to be the primary organs handling the balance between salt and water in leeches. A 1987 study examined how M. decora withstood osmotic shock (a shock caused by sudden alteration in the concentration of a given solute, resulting in dehydration via osmosis)[19] and found that it could not tolerate hypertonicity (overly salty solutions), and, when compared with the European Hirudo medicinalis, the North American species was relatively inefficient at the swift removal of surplus water and salt.[20]: 337 

 
An individual from Eastman, Quebec

Reproductive anatomy

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See also: Leech § Reproduction and development

All leeches are hermaphrodites whose male reproductive organs mature first and the female ones later.[21] Testisacs are separate sperm-producing parts of the main body cavity; similarly, ovisacs are parts of the cavity which contain tissues specialized for oogenesis. The ovisacs connect to the leech vagina via oviducts, while epididymes connect the vas deferens with the ejaculatory ducts.[22][16]: 510  M. decora has ten pairs of testisacs located from segments thirteen to twenty-three, with large, compact, and much-coiled epididymes, and crescent-shaped or globular ovisacs which are located in the thirteenth segment. A 2023 paper described the leech's oviducts as "thin" and "torturously folded".[8]: 155 [16]: 510  The male and female gonopores, which are external openings to the internal reproductive organs,[23][24] are usually separated by five annuli, or external rings; this degree of separation is an important feature for identifying the species.[25][9]: 67  The male gonopore, when withdrawn, appears as nothing more than a hole in between segments eleven and twelve; however, when the male gonopore and its surrounding parts are everted, they appear as a small cone, with the organ at the tip and having deeply furrowed sides. The leech's four copulatory glands are arrayed in a square in an area of rough skin on segments thirteen and fourteen.[16]: 509 

Saliva

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Leech saliva is known to contain several compounds, including hirudin, an anti-coagulant.[26][27] The saliva of M. decora is also known to contain several substances not previously all identified from the same leech, as well as an anticoagulant dubbed decorsin which might be unique to M. decora. The set of all mRNA experssed in M. decora saliva (its transcriptome) was described in 2010.[27][28][29] A 2019 paper published in the Journal of Parasitology compared hirudin and decorsin from M. decora, as well as hirudin and "hirudin-like factors" – substances which resemble hirudin but are not known to act as anticoagulants – obtained from European species. The authors concluded that that blood-sucking among jawed leeches evolved from a single origin.[30]: 423–4 

Ecology

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Distribution and habitat

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Being the most widely distributed Macrobdella species, M. decora is found in North America east of the Rocky Mountains in southern Canada and the neighbouring United States. There is, however, one isolated population in Mexico, in the state of Nuevo León.[6]: 587  For the most part, M. decora does not occur south of Virginia, Macrobdella ditreta being the dominant leech species in the southern United States. However, M. decora has been found in the southern Appalachian Mountains in Georgia and South Carolina.[25] Leeches of the species have been found as far west as Alberta, North Dakota, Nebraska, Colorado, and New Mexico.[31] At the eastern end of their range, they are also found on Prince Edward Island.[9]: 68  M. decora may be panmictic – that is to say, mixing and breeding randomly[32] – across much their range. Some populations in Ontario and New England have refrained from panmixia, and, furthermore, a 2024 paper that studied the geographic distribution of the species' genetic diversity[33] (phylogeography) concluded that it should not be assumed "that M. decora is truly panmictic" across the study's large range, and that more research into the topic was needed.[31]

Macrobdella decora is a freshwater species that is found in still or slow-moving water bodies such as streams, temporary ponds, ditches, and wetlands.[9]: 67 [4][34] They are particularly common in temporary ponds; the leeches are able to burrow into the bottom when the pond dries up.[35]: 417  In lakes, the leeches are most active in the epilimnion, or top-most layer.[36]: 3 

In Ontario, NatureServe lists the species as "Secure", but no assessment has been completed for other regions.[37] M. decora was described as unendangered in 2021 paper on its gut microbiome.[5]: 1  As of August 2025, M. decora was the most-observed species of leech in Canada on the citizen science platform iNaturalist, and the second-most-observed species in the United States, after Placobdella parasitica.[38][39]

Parasitism and diet

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Macrobdella decora is both parasitic and predaceous. It sucks the blood of many vertebrates, using its teeth to pierce the host's skin, including humans but also amphibians, fish, turtles, wading birds, and cattle. It also hunts voraciously, and eats oligochaete worms, snails, amphibian eggs, the larvae of insects, and even other individuals of its own species. In the spring, the leech's aggressive predation of American toad eggs may lead to up to 80% mortality.[9]: 67–8  The leeches have also been recorded hunting amphibian larvae: in 2020, a leech was found predating tiger salamander larvae in Minnesota.[40] However, M. decora is also preyed on by its own kind: Haemopis grandis, a predator and scavenger leech.[9]: 62  The leeches are also parasitized themselves by trematodes, a new species of which was described from the intestine of an M. decora leech in 1976.[41]

The leeches engorge themselves with blood before mating. One or two months after feeding, they produce spongy cocoons, which are pale yellow and elliptical in shape. About another month later, the young, only 20 to 22 millimetres (0.79 to 0.87 in) long, emerge.[9]: 68  They will take several years to become fully mature.[42]

Gut microbiome

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An individual from Buckingham, Quebec

The gut microbiome is simply the collection of microorganisms living in an animal's digestive system.[43] The gut microbiome of the North American Macrobdella decora is quite similar to that of Europe's Hirudo verbana. Bacteria of the genera Aeromonas, Bacteroides, Butyricicoccus, and Proteocatella dominate M. decora's gut microbiota.[5]: 1  The intraluminal fluid – that is, fluids found in the gut[44] – was found to be most abundant with bacteria like Aeromonas and Bacteroidales; combined, on median they represented 60% of microbiota living in the fluids, while much of the rest was Clostridiales, which on median accounted for 30% of the microbiota.[5]: 5 

Interactions with humans

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Macrobdella decora may parasitize humans and is often encountered by people swimming in Canada and the northern United States. Sometimes swimming areas have had to be restricted or closed due to the leech's presence.[9]: 67  Historically, M. decora was not used very often in bloodletting, despite its common name as a "medicinal leech".[5]: 2  However, there is some evidence of their medicinal use as a replacement to the European medicinal leeches, specifically in Philadelphia during the 19th century.[31] Indeed, Addison Emery Verrill, writing in 1872, noted M. decora's use by doctors as a stand-in for "imported leeches", and he noted the North American species was "equally efficacious".[3]: 132  It is possible that human leech-trading helped move leeches between water bodies; today, they are sometimes used as and transported for fishing bait, but they are not a very favoured choice.[31]

Notes

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  1. ^ Phillips et al. used a maximum likelihood analysis, but in the excerpt shown, there is no difference between this tree and the most parsimonious one.

References

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  1. ^ "Macrobdella decora". WoRMS. World Register of Marine Species. Retrieved 5 January 2025.
  2. ^ a b Keating, William Hypolitus; Long, Stephen H.; von Schweinitz, Lewis David (1824). Narrative of an expedition to the source of St. Peter's river, Lake Winnepeek, Lake of the Woods, &c., performed in the year 1823, ... under the command of Stephen H. Long. Vol. 2. Philadelphia: H. C. Carey & I. Lea. p. 268.
  3. ^ a b c Verrill, A. E. (1872). Dana, James D.; Silliman, Benjamin (eds.). "Brief Contributions to Zoölogy from the Museum of Yale College. No. XVII.—Descriptions of North American fresh-water Leeches". The American Journal of Science and Arts. Third series. 3. New-Haven: S. Converse: 126–139 [137]. Archived from the original on 2023-11-08. Retrieved 2024-11-29.
  4. ^ a b Garcia de Jesus, Erin I. "This Smithsonian Scientist is on a Mission to Make Leeches Less Scary". Smithsonian Magazine. Archived from the original on 2024-11-30. Retrieved 2024-11-29.
  5. ^ a b c d e McClure, Emily Ann; Nelson, Michael C.; Lin, Amy; Graf, Joerg (2021-04-27). Johnson, Karyn N. (ed.). "Macrobdella decora: Old World Leech Gut Microbial Community Structure Conserved in a New World Leech". Applied and Environmental Microbiology. 87 (10). Bibcode:2021ApEnM..87E2082M. doi:10.1128/AEM.02082-20. ISSN 0099-2240. PMC 8117757. PMID 33674439.
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  42. ^ Beckerdite, Fred Willie. The Description and Life History of a New Species of Alloglossidium (Trematoda: Macroderoididae).Alloglossidium (Trematoda: Macroderoididae) (PhD thesis). Louisiana State University and Agricultural and Mechanical College. Archived from the original on 2024-07-19. Retrieved 2024-12-20.
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