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= Macrobdella decora =
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Introduction
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'Macrobdella decora', also known as the North American medicinal
leech, is a species of 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 reddish or orange underbelly with black
spots. 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'. It is not considered to be endangered.
'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 able to mix and breed randomly
across most of its range, but further research into the topic is
needed. 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 led 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." When Addison Emery Verrill
erected the genus 'Macrobdella' in 1872, he transferred Say's species
into it. 'Macro' simply means 'big', while 'bdella' means 'leech' in
Greek. It is commonly known as the North American medicinal leech.
'Macrobdella decora' is most closely related (the sister taxon) to
'Macrobdella diplotertia.' 'Macrobdella ditreta' was previously
believed to be sister to the clade comprising 'decora' and
'diplotertia', but a new species, 'Macrobdella mimicus', was
discovered in 2019 and placed as the sister taxon to that clade. The
genus 'Macrobdella' was formerly believed to be a monophyletic
grouping, but the 2019 paper that described 'M. mimicus' concluded
that 'Macrobdella' was in fact a paraphyletic taxon. 'Macrobdella
decora', 'M. diplotertia', and 'M. mimicus' form one monophyletic
clade. However, 'Macrobdella ditreta' is more distantly related, and
the grouping 'Macrobdella' is paraphyletic as it excludes the
monophyletic 'Philobdella' clade. The 2019 researchers, led by Anna J.
Phillips, used both maximum likelihood and maximum parsimony
techniques to work out the phylogeny of the Macrobdellidae, and the
techniques yielded slightly different results, with parsimony
supporting the genus 'Philobdella' as sister to the 'decora',
'diplotertia', and 'mimicus' clade and the maximum likelihood analysis
putting it sister to 'M. ditreta'.
|1=**'M. decora**'
|1=**'M. decora**'
|{{cladogram|title=Parsimony analysis |cladogram= {{clade
|label1=Macrobdellidae |1= {{clade |1='Oxyptychus' |2= {{clade |1='M.
ditreta' |2= {{clade |1='Philobdella' |2= {{clade |1= }} }} }}
}} }} |align=center|caption=After Phillips et al., 2019.}}
|{{cladogram|title=Maximum-likelihood analysis |cladogram= {{cladeR
|reverse=yes |label1=Macrobdellidae |1= {{cladeR |1='Oxyptychus' |2=
{{cladeR |1= |2= }} }} }} |align=center|caption=After Phillips et
al., 2019.}}
Anatomy
=========
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. They are arranged in the shape of a
horseshoe. The nervous system includes twenty-one neuron clusters
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. The brain is
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. Leech jaws are chitinous blades with
sharp, serrated edges. 'Macrobdella decora' has three long jaws which
are semicircular and laterally compressed, each with one row of about
sixty-five "extremely sharp" teeth. Each jaw uses a saw-like motion to
cut open its victim's skin.
'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. The anus is located on the leech's back, above its
circular acetabulum, a large posterior sucker.
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) and found that the nephridia 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.
Reproductive anatomy
======================
All leeches are hermaphrodites whose male reproductive organs mature
first and the female ones later. 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.' 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". The male and female gonopores, which are external openings to
the internal reproductive organs, are usually separated by five
annuli, or external rings; this degree of separation is an important
feature for identifying the species. 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 deeply furrowed
sides, the organ being at the tip. The leech's four copulatory glands
are arrayed in a square in an area of rough skin on segments thirteen
and fourteen.
Saliva
========
Leech saliva is known to contain several compounds, including hirudin,
an anticoagulant. 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 expressed in 'M. decora' saliva (its
transcriptome) was described in 2010. 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
blood-sucking among jawed leeches evolved from a single origin.
Distribution and habitat
==========================
'Macrobdella decora' is the most widely distributed 'Macrobdella'
species, and it 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. 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. Leeches of the
species have been found as far west as Alberta, North Dakota,
Nebraska, Colorado, and New Mexico. At the eastern end of their range,
they are also found on Prince Edward Island. 'M. decora' is possibly
panmictic (i.e., mixing and breeding randomly) across much of its
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 (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.
'Macrobdella decora' is a freshwater species that is found in still or
slow-moving water bodies such as streams, temporary ponds, ditches,
and wetlands. They are particularly common in temporary ponds; the
leeches are able to burrow into the bottom when the pond dries up.
They are able to survive some time on land, and have been found up to
away from aquariums in which they were kept. They may also attach to a
terrestrial host such as a mammal or bird and follow the host onto
shore. Nonetheless, their overland biological dispersal ability "is
assumed to be limited". In lakes, the leeches are most active in the
epilimnion, or top-most layer.
NatureServe lists the species as Secure within Ontario, but no
assessment has been completed for other regions. 'M. decora' was
described as unendangered in a 2021 paper on its gut microbiome. As of
October 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'.
Parasitism and diet
=====================
'Macrobdella decora' is both parasitic and predaceous. Using its teeth
to pierce the host's skin, it sucks the blood of many vertebrates,
including humans but also amphibians, fish, turtles, wading birds, and
cattle. The leech can remain attached to its host for up to two hours.
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. The leeches have
also been recorded hunting amphibian larvae: in 2020, a leech was
found preying on tiger salamander larvae in Minnesota. However, 'M.
decora' is also preyed on by 'Haemopis grandis', a predator and
scavenger leech. The leeches are also themselves parasitized by
trematodes, a new species of which, 'Alloglossidum hamrumi', was
described from the intestine of a specimen of 'M. decora' in 1976.
Reproduction
==============
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 long, emerge. They will take several years to become fully
mature.
Gut microbiome
================
The gut microbiome is simply the collection of microorganisms living
in an animal's digestive system. 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. 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.
Interactions with humans
======================================================================
'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. In Europe, 'Hirudo medicinalis' was traditionally used for
bloodletting. Despite its common name as a "medicinal leech", 'M.
decora' was historically not used very often for this purpose.
However, there is some evidence of their medicinal use as a
replacement to European 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". It is possible that human leech-trading helped move
leeches between water bodies. According to a 2024 paper, they are
sometimes used by fishers for fishing bait, and they may be
transported for that reason. However, they are not a preferred choice,
due to a tendency to get free of collection traps. Fish are also
claimed anecdotally by fishers to find them less attractive than other
leech species.
License
=========
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License URL: http://creativecommons.org/licenses/by-sa/3.0/
Original Article: http://en.wikipedia.org/wiki/Macrobdella_decora
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