# 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, although there is one disjunct population in northern Mexico. M. decora is a parasite of vertebrates, including humans, and an aquatic predator of eggs, larvae, and other invertebrates. Macrobdella decora is a medium-sized leech with a spotted greenish-brown back and a reddish 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 ten 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 sister taxon is believed to be the species Macrobdella diplotertia.

## Taxonomy

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. When Addison Emery Verrill erected the genus Macrobdella in 1872, he transferred Say's species into his new genus.$^{:138}$ A common name for the species is the North American medicinal leech.
Macrobdella decora is 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.$^{:587}$$^{:563}$
Cladogram illustrating species closely related to M. decora :
M. decora
M. diplotertia
M. mimicus :

## Description

Macrobdella decora is a medium-sized leech, growing between 5 and 8.5 cm (2.0 and 3.3 in) long, and weighing from 1.48 to 3.69 grams (0.052 to 0.130 oz).$^{:67}$$^{:155}$ It has a dark green, brown or olive-green back with a line of 20 or so small orange or red dots down the middle, and two corresponding sets of black dots on its sides. Its underbelly is reddish with black spots dispersed irregularly across it.$^{:67}$$^{:160}$ Its back is rounded but its belly is flattened.$^{:160}$$^{:230}$ All leeches have 32 segments, but they are all also covered with external rings called annuli; the body of M. decora has between 90 and 94 annuli total.

### Anatomy

The head of M. decora is rounded and has ten ocelli 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.$^{:138}$ They are arranged in the shape of a horseshoe.$^{:160}$ There are twenty-one ganglia in the leech's body.$^{:166}$ A jawed leech, Macrobdella decora has three long jaws which are semicircular and laterally compressed, each with one row of about sixty-five "extremely sharp" teeth.$^{:67}$ Each jaw moves like a saw to cut open the skin.
M. decora has a large muscular pharynx which accounts for the first tenth or so of the leech's digestive tract. The stomach, a large pouch composed of smaller sacs, is not nearly as muscular as the pharynx, but it occupies about five sixths of the leech's whole body 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.$^{:163–5}$ The anus is located on the leech's back, above its circular acetabulum.
Leech saliva is known to contain several compounds, including hirudin, an anti-coagulant. The saliva of M. decora is 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.
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 and found that it could not tolerate hypertonicty, and, when compared with the European Hirudo medicinalis, the North American species was relatively inefficient at the swift removal of surplus water and salt.$^{:337}$

#### Reproductive anatomy

All leeches are hermaphrodites whose male reproductive organs mature first and the female ones later. 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".$^{:155}$$^{:510}$ The male and female gonopores, which are external openings to the internal reproductive organs, are usually separated by five annuli, or external rings.$^{:67}$ The male one, 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.$^{:509}$

## Ecology
### Distribution and habitat

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.$^{:587}$ Leeches of the species have been found as far west as Alberta, North Dakota, Nebraska, Colorado, and New Mexico. They are also found on Prince Edward Island.$^{:68}$ The leeches may be panmictic – that is to say, mixing and breeding randomly – across much their range. Some populations in Ontario and New England have refrained from panmixia, and, furthermore, a 2024 paper that studied the leeches' phylogeography concluded that it should not be assumed "that M. decora is truly panmictic" of the study's large range, and that more research into the topic was needed.
Macrobdella decora is a freshwater species that is found in still or slow-moving water bodies such as streams, temporary ponds, ditches, and wetlands.$^{:67}$ They are particularly common in temporary ponds; the leeches are able to burrow into the bottom when the pond dries up.$^{:417}$ In lakes, the leeches are most active in the epilimnion, or top-most layer.$^{:3}$

### Parasitism and diet

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.$^{:67–8}$ However, M. decora is also preyed on by its own kind: Haemopis grandis, a predator and scavenger, is known to eat M. decora.$^{:62}$
The leeches engorge themselves with blood before mating. One or two months after feeding, the leeches 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.$^{:68}$ They take several years to become fully mature.

### Gut microbiome

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.$^{:1}$ The intraluminal fluid – that is, fluids found in the gut – 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.$^{:5}$

### Interactions with humans

Macrobdella decora does parasitize humans and is often found by people swimming in Canada and the northern United States. Sometimes swimming areas have had to be restricted or even closed due to the leech's presence.$^{:67}$ Historically, M. decora was in fact not used very much in bloodletting, despite its common name as a "medicinal leech".$^{:2}$ However, there is some evidence of their medicinal use as a replacement to the European medicinal leeches, specifically in Philadelphia during the 19th century. 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".$^{:132}$ 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.

#### Conservation

In Ontario, NatureServe lists the species as "Secure", but no assessment has been completed for other regions. M. decora was described as unendangered in 2021 paper proposing its use as a model for digestive tract symbioses.$^{:1}$