Leeches are segmented parasitic or predatory worm-like animals that belong to the phylum Annelida and comprise the subclass Hirudinea. They are closely related to the oligochaetes, which include the earthworms, and like them have soft, muscular, segmented bodies that can lengthen and contract. Both groups are hermaphrodites and have a clitellum, but leeches typically differ from the oligochaetes in having suckers at both ends and in having external annulations that do not correspond with their internal segmentation. The body is relatively solid, and the spacious body cavity found in other annelids, the coelom, is reduced to small channels.
The majority of leeches live in freshwater environments, while some species can be found in terrestrial and marine environments. The best-known, such as the medicinal leech, Hirudo medicinalis, are hematophagous, attaching themselves to a host with a sucker and feeding on blood, having first secreted the peptide hirudin to prevent the blood from clotting. A minority of leech species are predatory, mostly preying on small invertebrates.
In aquatic species, the eggs are enclosed in a cocoon which is usually attached to something solid, but terrestrial species often conceal the cocoon under a log or in a crevice. Almost 700 species of leech are currently recognised, of which some 100 are marine, 90 terrestrial and the remainder freshwater.
Leeches were used in medicine from ancient times until the 19th century to draw blood from patients. In modern times, leeches find medical use in microsurgery, while hirudin is a valuable drug for some blood-clotting disorders.
Temporal range: Silurian –Recent
|Hirudo medicinalis sucking blood|
Almost 700 species of leech have been described, of which some 100 are marine, 90 terrestrial and the remainder freshwater. The name for the subclass, Hirudinea, comes from the Latin hirudo, hirudinis, a leech.
The more primitive Acanthobdellidea are often included with the leeches, but some authors treat them as a separate clitellate group. The Branchiobdellida include cleaning symbionts such as Xironodrilus appalachius, which lives on the crayfish Cambarus chaugaensis. Both groups live in freshwater.
The phylogenetic tree of the leeches and their annelid relatives is based on molecular analysis (1999) of mitochondrial cytochrome c oxidase. Both the former classes "Polychaeta" (bristly marine worms) and "Oligochaeta" (including the earthworms) are paraphyletic.
The most ancient annelid group is the free-living polychaetes that evolved in the Cambrian period, being plentiful in the Burgess Shale about 500 million years ago. Oligochaetes evolved from polychaetes and the leeches branched off from the oligochaetes. Both the oligochaetes and the leeches, having no hard parts, do not fossilise well. The first leech fossils are found in the Jurassic period around 150 million years ago, but an annulate fossil found in Wisconsin, that appears to have a large sucker, will extend the groups evolutionary history back to the Silurian, some four hundred million years ago.
Leeches show a remarkable similarity to each other in morphology, very different from typical annelids which are basically cylindrical, with a fluid-filled space, the coelom (body cavity). In leeches, the coelom is reduced to four slender longitudinal channels, and the interior of the body is filled with a solid dermis in between the various organs. Typically, the body is dorso-ventrally flattened and tapers at both ends. Longitudinal and circular muscles in the body wall are supplemented by diagonal muscles, giving the leech the ability to adopt a large range of body shapes and show great flexibility. Most leeches have a sucker at both the anterior (front) and posterior (back) ends, but some primitive leeches have a single sucker at the back.
Like other annelids, the leech is a segmented animal, but unlike other annelids, the segmentation is masked by the external annulation (ring markings), with the body surface being divided into 102 annuli, whereas its internal structure consists of 32 segments. Of these segments, the first five are designated as the head and include the anterior brain, several ocelli (eyespots) dorsally and the sucker ventrally. The following 21 mid-body segments each contain a nerve ganglion, and between them contain two reproductive organs, a single female gonopore and nine pairs of testes. The last seven segments contain the posterior brain and are fused to form the animal's tail sucker.
The body wall consists of a cuticle, an epidermis and a thick layer of fibrous connective tissue in which are embedded the circular muscles, the diagonal muscles and the powerful longitudinal muscles. There are also dorso-ventral muscles. The coelomic channels run the full length of the body, the two main ones being on either side. The coelom has taken over the function of the hemal system (blood vessels) in other annelids. Part of the lining epithelium consists of chloragogen cells which are used for the storage of nutrients and in excretion. There are ten to seventeen pairs of metanephridia (excretory organs) in the mid-region of the leech. From these, ducts typically lead to a urinary bladder, which empties to the outside at a nephridiopore.
Leeches are protandric hermaphrodites, with the male reproductive organs, the testes, maturing first and the ovaries later. In hirudinids, a pair will line up with the clittelar regions in contact, with the anterior end of one leech pointing towards the posterior end of the other; this results in the male gonopore of one leech being in contact with the female gonopore of the other. The penis passes a spermatophore into the female gonopore and sperm is transferred, and probably stored in, the vagina.
Some jawless leeches (Rhynchobdellida) and proboscisless leeches (Arhynchobdellida) lack a penis, and in these, sperm is passed from one individual to another by hypodermic injection. The leeches intertwine and grasp each other with their suckers. A spermatophore is pushed by one through the integument of the other, usually into the clitellar region. The sperm is liberated and passes to the ovisacs, either through the coelomic channels or interstitially through specialist "target tissue" pathways.
Some time after copulation, the small, relatively yolkless eggs are laid. In most species, an albumin-filled cocoon is secreted by the clitellum and receives one or more eggs as it passes over the female gonopore. The cocoon is fixed to a submerged object, or in the case of terrestrial leeches, deposited under a stone or buried in damp soil. The cocoon of Hemibdella soleae is attached to a suitable fish host, and that of a branchiobdellid, to the shell of a crayfish on which the young will become ectosymbionts. The glossiphoniids brood their eggs, either by attaching the cocoon to the substrate and covering it with their ventral surface, or by securing the cocoon to their ventral surface, and even carrying the newly hatched young to their first meal.
When breeding, most marine leeches leave their hosts and become free-living in estuaries. Here they produce their cocoons, after which the adults of most species die. When the eggs hatch, the juveniles seek out potential hosts when these approach the shore. Leeches mostly have an annual or biannual life cycle.
About three quarters of leech species feed on blood extracted from a host while the remainder are predators. Leeches either have a protusible pharynx, commonly called a proboscis, or a non-protusible pharynx which may or may not be armed with jaws.
In the proboscisless leeches, the jaws of Arhynchobdellids are at the front of the mouth, and have three blades set at an angle to each other. In feeding these slice their way through the skin of the host, leaving a Y-shaped incision. Behind the blades is the mouth, located ventrally at the anterior end of the body. It leads successively into the pharynx, a short oesophagus, a crop (in some species), a stomach and a hindgut, which ends at an anus located just above the posterior sucker. The stomach may be a simple tube, but the crop, when present, is an enlarged part of the midgut with a number of pairs of ceca that stores ingested blood. The leech secretes an anticoagulant, hirudin, in its saliva which prevents the blood from clotting before ingestion. A mature medicinal leech may feed only twice a year, taking months to digest a blood meal.
The bodies of predatory leeches are similar, though instead of jaws many have a protrusible proboscis, which for most of the time they keep retracted into the mouth. Such leeches are often ambush predators that lie in wait till they can strike prey with the proboscises in a spear-like fashion. Predatory leeches feed on small invertebrates such as snails, earthworms and insect larvae. The prey is usually sucked in and swallowed whole. Some Rhynchobdellida, however, suck the soft tissues from their prey, making them intermediate between predators and blood-suckers.
When they are hungry, leeches use their anterior suckers to connect to hosts for feeding. Once attached, leeches use a combination of mucus and suction to stay in place while they inject hirudin into the hosts' blood streams. In general, sanguivorous leeches are non host-specific, and do little harm to their host, dropping off after consuming a blood meal. Some marine species, however, remain attached until it is time to reproduce. If present in great numbers on a host, these can be debilitating, and in extreme cases, cause death.
Leeches are unusual in that they do not produce amylases, lipases or endopeptidases. This lack of endopeptidases means the mechanism of protein digestion cannot follow the same sequence as it would in all other animals in which endopeptidases first split proteins into peptides, and the exopeptidases then degrade the peptides. However, they do produce intestinal exopeptidases which remove amino acids from the long protein molecules one by one, possibly aided by proteases from endosymbiotic bacteria in the intestine.  This evolutionary choice of exopeptic digestion in Hirudinea distinguishes these carnivorous clitellates from oligochaetes, and may explain why digestion in leeches is so slow.
Deficiency of digestive enzymes and of B complex vitamins is compensated for by enzymes and vitamins produced by endosymbiotic microflora. In Hirudo medicinalis, these supplementary factors are produced by an obligatory symbiotic relationship with two bacterial species, Aeromonas veronii and a still-uncharacterised Rikenella species. Nonbloodsucking leeches, such as Erpobdella punctata, are host to three bacterial symbionts, Pseudomonas, Aeromonas, and Klebsiella spp. (a slime producer). The bacteria are passed from parent to offspring in the cocoon as it is formed.
The nervous system of leeches is formed of a small number of large nerve cells; this has resulted in leeches being used as model organisms for the study of invertebrate nervous systems. The main nerve centre consists of the cerebral ganglion above the gut and another ganglion beneath it, with connecting nerves forming a ring around the pharynx a little way behind the mouth. A nerve cord runs backwards from this in the ventral coelomic channel, with 21 pairs of ganglia in segments 6 to 26. In segments 27 to 33, other paired ganglia fuse to form the caudal ganglion. Several sensory nerves connect directly to the cerebral ganglion; there are sensory and motor nerve cells connected to the ventral nerve cord ganglia in each segment.
Leeches have between two and ten pigment spot ocelli, arranged in pairs towards the front of the body. There are also sensory papillae arranged in a lateral row in one annulation of each segment. Each papilla contains many sensory cells. Some rhynchobdellids have the ability to change colour dramatically by moving pigment in chromatophore cells; this process is under the control of the nervous system but its function is unclear as the change in hue seems unrelated to the colour of the surroundings.
Leeches can detect touch, vibration, movement of nearby objects, and chemicals secreted by their hosts; freshwater leeches crawl or swim towards a potential host standing in their pond within a few seconds. Species that feed on warm-blooded hosts move towards warmer objects. Many leeches avoid light, though some blood feeders move towards light when they are ready to feed, presumably increasing the chances of finding a host.
Leeches live in damp surroundings and in general respire through their body wall. The exception to this is in the Piscicolidae, where branching or leaf-like lateral outgrowths from the body wall form gills. Some rhynchobdellid leeches have an extracellular haemoglobin pigment, but this only provides for about half of the leech's oxygen transportation needs.
Leeches move using their longitudinal and circular muscles, which other annelids such as earthworms use for peristalsis, with the addition of the use of their posterior and anterior suckers (one on each end of the body) to enable them to progress by looping or inching along, in the manner of geometer moth caterpillars. The posterior end is attached to the substrate, and the anterior end is projected forward peristaltically by the circular muscles until it touches down, as far as it can reach, and the anterior end is attached. Then the posterior end is released, pulled forward by the longitudinal muscles, and reattached; then the anterior end is released, and the cycle repeats. Leeches explore their environment with head movements and body waving.
Leech bites are generally alarming rather than dangerous, though a small percentage of people have severe allergic or anaphylactic reactions and require urgent medical care. Symptoms of these reactions include red blotches or an itchy rash over the body, swelling around the lips or eyes, a feeling of faintness or dizziness, and difficulty in breathing. An externally attached leech will detach and fall off on its own accord when it is satiated on blood, which may take from twenty minutes to a few hours; bleeding from the wound may continue for some time. Internal attachments, such as inside the nose, are more likely to require medical intervention.
Common, but medically inadvisable, removal techniques are to apply a lit cigarette, salt, soap, or vinegar to the leech. These cause the leech to detach quickly, but also to regurgitate its stomach contents into the wound, with a risk of infection. Leeches normally carry parasites in their digestive tracts, which cannot survive in humans and do not pose a threat. However, bacteria, viruses, and parasites from previous blood sources can survive within a leech for months, but only a few cases of leeches transmitting pathogens to humans have been reported.
Leech saliva is commonly believed to contain anesthetic compounds to numb the bite area, but this has never been proven. Although morphine-like substances have been found in leeches, they have been found in the neural tissues, not the salivary tissues. They are used by the leeches in modulating their own immunocytes and not for anesthetizing bite areas on their hosts. Depending on the species and size, leech bites can be barely noticeable or they can be fairly painful.
The leech appears in the Book of Proverbs as the picture of something that is insatiably greedy: "The horseleach hath two daughters, [crying], Give, give. There are three [things that] are never satisfied, [yea], four [things] say not, [It is] enough:". More widely, a leech is a persistent social parasite or sycophant.
The medicinal leech Hirudo medicinalis, and some other species, have been used for clinical bloodletting for at least 2,500 years: Ayurvedic texts describe their use for bloodletting in ancient India. In ancient Greece, bloodletting was practised according to the theory of humours found in the Hippocratic Corpus of the fifth century BC, which maintained that health depended on a balance of the four humours, blood, phlegm, and black and yellow bile. Bloodletting using leeches enabled physicians to restore balance if blood was present in excess. Pliny the Elder reported in his Natural History that the horse leech could drive elephants mad by climbing up inside their trunks to drink blood. Pliny also noted the medicinal use of leeches in ancient Rome, stating that they were often used for gout, and that patients became addicted to the treatment.
William Wordsworth's 1802 poem Resolution and Independence describes one of the last of the leech-gatherers, people who travelled Britain catching leeches from the wild, and causing a sharp decline in their abundance, though they remain numerous in Romney Marsh. By 1863, British hospitals had switched to imported leeches, some seven million being imported to hospitals in London that year.
The use of leeches in modern medicine made a small-scale comeback in the 1980s after years of decline, with the advent of microsurgery, where venous congestion can arise due to inefficient venous drainage; leeches can reduce swelling in the tissues and promote healing. They are used to help restore circulation after microsurgery to reattach body parts. Other clinical applications include varicose veins, muscle cramps, thrombophlebitis, and osteoarthritis.