Photo: Newborn brood of aquatic leaf leeches. (Photo by Animals Animals ©K. Atkinson, OSF. Reproduced by permission.)
Evolution and systematics
Leeches are completely soft-bodied animals and could not be expected to leave a marked fossil record. Although there are two putative fossils from Bavarian deposits dating from the Upper Jurassic period (about 145 million years ago), Epi-trachys rugosus (Ehlers, 1869) and Palaeohirudo eichstaettensis (Kozur, 1970), neither has both the definitive caudal sucker and the ring-shaped subdivisions of the body that would define them as leeches. The evolutionary relationships of leeches demonstrate that the ancestral hirudinid was a blood feeder in a freshwater environment. This finding suggests that leeches are no older than vertebrates and probably no older than amphibians.
The phylogenetic relationships of leeches have been the subject of several analyses based on morphological (structural) characters and DNA sequence data. Taken together, these analyses demonstrate beyond question that leeches, branchiobdellidans (crayfish worms) and acanthobdellidans form a monophyletic (descended from a common taxon) group of oligochaetes that is closely related to the lumbriculid oligochaetes, or earthworms.
Leeches are subdivided first into suborders based on anatomical adaptations for feeding. The Rhynchobdellida, as their name implies, have a muscular proboscis that allows them to feed on blood drawn from tissues beneath an organism's skin that are well supplied by blood vessels. The large worm-like Arhynchobdellida, of which Hirudo medicinalis is typical, have three muscular jaws, each of which may be armed with a row of teeth creating a serrated cutting edge.
The two principal families of proboscis-bearing Rhyn-chobdellida have pairs of medial cephalic eyespots that can detect two-dimensional movement. The Piscicolidae family comprises small elongate, mostly marine species that feed seasonally on fishes. The Glossiphoniidae tend to be strongly dorsoventrally flattened freshwater species typically preferring anuran (frogs and toads) or chelonian (turtles and tortoises) hosts, though a few are specific to fishes and others will feed on mammals.
The blood-feeding arhynchobdellids include the large aquatic Hirudinidae (medicinal leeches) and the smaller terrestrial Haemadipsidae (jungle leeches). The jungle leeches are more common in the humid forests of Southern Asia, India, Madagascar, Australia, and Indonesia than their aquatic cousins. Both of these groups are equipped with a parabolic arc of 10 eyespots that detect movement in three dimensions. Terrestrial leeches have the additional adaptation of respiratory auricles near their caudal sucker that permit gas exchange without excessive loss of fluid. These leeches also have well-developed sensory systems for detecting vibrations, carbon dioxide, and heat. In addition, there are several predatory arhynchobdellids like the slender Erpobdelliformes (families Erpobdellidae, Salifidae, and Americobdellidae); the larger amphibious Haemopidae; and several other poorly studied families.
Leeches have a clitellum, or specialized saddle-shaped glandular segment, that secretes cocoons or egg cases. These animals are simultaneous hermaphrodites. They have a body consisting of 34 body segments (somites) but lack the chaetae (stiff hairs or bristles) of other annelids. Though not all are sanguivorous, many leeches have special adaptations for blood feeding. Principal among these is the muscular caudal sucker made up of the last seven somites of the segmented body. The sucker is critical for attaching to and remaining on a host. It may double as a powerful swimming fluke (anchor) in larger and more active species. The six somites at the front of the leech also are modified into a region with a ventral sucker surrounding a muscular pharynx. Negative pressure applied by this anterior sucker aids in attachment and in encouraging blood to flow from a bite wound. Blood-feeding leeches usually are equipped with large branched gastric caeca, or pouches, that allow them to expand considerably during feeding; some leeches consume up to six times their unfed body weight. The fact that all present-day blood-feeding leeches have these features suggests that the ancestral leech also had them.
Leeches can be found in one form or another in freshwater and terrestrial locations on all continents except Antarctica. Marine leeches are found in all oceans. The major families of leeches have global distributions with the exception of some species in the Haemadipsidae family. These species appear to have originated at the time of the late Gondwanan continental separation, after Africa and South America had parted from the remainder of the original supercontinent about 180-140 million years ago. Some minor families, including the Americobdellidae and Cylicobdelli-dae, are found only in South America. Most oceanic leeches seem to prefer temperate or arctic waters, with only a few species specific to elasmobranchs being found in tropical marine systems.
The frequency with which leeches are encountered depends on geography. Most species that occur in European, African, or North and South American freshwater ecosystems can be found under submerged rocks and debris or along shorelines when they are not feeding on their respective hosts. Terrestrial leeches are found only in such perpetually moist environments as tropical or temperate coastal rainforests, where they remain in leaf litter unless they are seeking a meal. The habits of marine leeches are not well understood, as they are most commonly encountered while they are feeding on fishes or turtles. Few leeches appear to select specific sites on the hosts they feed from.
Many leeches can swim by coordinating the depolarization of nerve cells along their ventral cord, which causes the lon gitudinal muscles to move the leech's body in a curving or wavelike pattern. The posterior sucker serves as an anchor to provide thrust. Leeches move along solid substrates by alternating attachment of their anterior and posterior suckers between periods of body stretching— much like the movement of an inchworm. When at rest during long periods of digestion or while brooding young, leeches lie under objects along the shorelines, often partially out of water. They frequently are observed performing their wavelike swimming movement in place as though to assist in ventilation. Leeches have several anterior eyespots and are able to detect movement from contrasting patterns of light and shadow.
Sanguivorous (blood-feeding) leeches are capable of living on blood because of a number of bioactive chemicals in their saliva. Vertebrate blood, including human blood, is equipped with coagulation (clotting) factors. Most leeches require 30 minutes or more for feeding; however, vertebrate blood can clot in much less time. If the blood that the leech ingests were to clot inside its digestive tract, it could not mate, avoid its predators, or seek another meal. Leeches have circumvented the endpoints of the mammalian coagulation cascade, which is the medical term for the process of blood clotting. The coagulation cascade includes the clumping of platelets, the production of a fibrin matrix (network), and the cross-linking of that matrix into a firm clot. Leeches can interfere with this clotting system at a minimum of seven different points. Hirudin, a potent thrombin inhibitor, was the first anticoagulant (blood-thinning) compound to be isolated from leech saliva. Most other leech-derived anticoagulants are also protease inhibitors.
Some sanguivorous species are very host-specific; Placobdel-loidesjaegerskioeldi feeds exclusively on Hippopotamus amphibius, for example, while other leeches are less discriminating. Of the marine species, those that feed on cartilaginous fishes show no interest in teleosts (bony fishes), and Ozobranchus species feed only on turtles. In freshwater environments, the large and notorious hirudinid medicinal leeches more often acquire their nourishment from amphibians or fishes than from swimming humans.
Many leeches do not feed on blood at all. Glossiphoniids, like species of Helobdella and Glossiphonia, feed on aquatic oligochaetes and snails. The jawless Erpobdellidae feed on chironomid (midge) larvae, and the jawed haemopids consume whole earthworms, shredding them with jaws bearing two rows of large teeth. In addition, there are rarely encountered families like the South American Americobdellidae and Cyli-cobdellidae. The species in these families are terrestrial earthworm hunters and of uncertain phylogenetic affinities. Researchers have typically assumed that nonbloodfeeding varieties of leeches are more primitive than those with the "advanced" behavior of blood feeding. Recent phylogenetic work points to the existence of a blood feeding ancestor, however, and at least six transformations to predation in the history of leeches.
Leeches are hermaphroditic animals with separate male and female reproductive systems. Male and female systems open independently to the exterior, each by means of a small ventral gonopore situated on the midventral line in the clitel-lar somites (normally somites XI through XIII). The most common form of sexual intercourse in leeches is traumatic insemination (direct injection into the female's body cavity). Male leeches randomly implant their spermatophores (small packets containing sperm) in the cuticle or outer covering of a recipient mate. The larger hirudinids and terrestrial haemadipsids have anatomical adaptations for internal fertilization, including a protrusible penis and a receptive vaginal sac permitting direct contact between male and female gonopores.
Reproducing individuals are usually distinguishable by the swelling of a certain number of annuli in the region that includes their gonopores. This swollen area constitutes the clitellum, a glandular segment that secretes the cocoons or egg cases. Unlike the homologous structure in terrestrial earthworms, the degree of prominence of the clitellum varies considerably in leeches.
The fish leeches, or Piscicolidae, exhibit an adaptation that helps their offspring to find an early blood meal. Rather than abandoning a secreted cocoon as the oligochaetes and many leeches do, the piscicolids cement their egg cases to the surface of crustaceans. When a fish eats the crustacean, the young leeches readily attach to the fish host's buccal (cheek) surfaces and migrate to its gills. Such Glossiphoniidae as Haementeria ghilianii are broad and flattened, and normally found feeding on turtles or amphibians. Species in this family secrete a membranous bag that holds their eggs on their underside in a brooding position underneath rocks and other debris. When the brood hatches, the young will turn and attach themselves to their parent's venter, or belly. When the parent finds its next blood meal, the young are carried to their first blood meal.
The European medicinal leech, Hirudo medicinalis, has been overexploited. In addition, its natural habitat has become fragmented and highly restricted. This species is listed as Lower Risk/Near Threatened by the IUCN, and is also listed in
Use of the medicinal leech (Hirudo medicinalis) in the treatment of haemotoma, an accumulation of blood within the tissues that clots to form a solid swelling. The leech attaches its sucker near the injury, makes an incision and deposits an anticoagulant called hirudin, mixed with saliva, into the wound, breaking apart clots that are there, and preventing further clots from forming. (Photo by St. Bartholomew's Hospital/Science Photo Library/Photo Researchers, Inc. Reproduced by permission.)
CITES Appendix II. Many terrestrial rainforest species are as threatened as their specific habitats; examples include Mesob-della gemmata in the Valdivian coastal forests of Chile and Haemadipsa sumatrana.
Leeches were well known for their medicinal applications in the eighteenth and nineteenth centuries, when they were used for such purposes as alleviating headaches and treating obesity. It is unlikely that any of these therapies were successful. Leeches are now, however, experiencing a renaissance of interest in medicine and pharmacology: they are the tools of choice for treating postoperative hematomas (localized collections of clotted blood) in microsurgery. In addition, their powerful salivary anticoagulants are being studied as possible treatments for heart disease and even cancer. Leeches are also potential indicator species for certain measurements of water quality, including heavy metal contamination and dissolved oxygen content.
1. Tiger leech (Haemadipsa picta); 2. North American medicinal leech (Macrobdella decora); 3. Giant Amazonian leech (Haementeria ghilianii); 4. Hippo leech (Placobdelloides jaegerskioeldi); 5. Euopean medicinal leech (Hirudo medicinalis). (Illustration by Bruce Worden)
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