A very fair question. Why, when given a choice, would you want to write a term paper on leeches, some of the most loathed and degraded invertebrates around? First of all, there are very few lower invertebrates that do not bring a cry of disgust when discovered by the neophyte invertebrate zoologist or the casually inquisitive layman, so why not study leeches? Leeches are disgusting, in a way. The grotesque probing and searching of a leech head coming out of a fingerbowl is like a scene from some disturbing horror movie. The motion of a leech in the lab can be nightmarish. But leeches in the field are genuinely beautiful. Those used to finding leeches attached to their legs after wading in standing pools tend to not notice the amazing grace of a leech in the water. Leeches swim like gorgeous flying ribbons, brightly colored, in all different patterns. And their bizarreness tends to grow on you after just a short time. Besides, if you're going to be host to some sort of parasite, leeches are a very good choice. They won't kill you, and only hang around for a twenty minute feeding. In addition, they don't hurt because of their potent anaesthetic, their wounds bleed freely to cleanse themselves and heal quickly after they stop. Finally, a leech wound will never become infected because of antibiotics in leech saliva. Seriously, leeches are widely distributed, being found in all aquatic environments, both freshwater and marine, and even on land. Most importantly, at least to the pragmatists in the crowd, leeches have a myriad of medical uses, from powerful antibiotic leech extracts to their well known bloodsucking ability. This paper will focus on the life history of leeches, with sections on ecological and medical importance and leech distribution in Michigan. Most of the information in this paper is taken from a three volume leech biology review by Roy T. Sawyer (1986). He goes into outstanding detail on every facet of the class Hirudinea. The work of K. H. Mann (1962) is an excellent choice for the more casual leech reader.

• Subclass: Acanthrobdellida
• Branchiobdellida
• Euhirudinea
• Order: Rhynchobdellae
• Glossiphoniidae
• Piscicolidae
• Order: Gnathobdellae
• Hirudidae
• Haemadipsidae
• Order: Pharyngobdellae
• Erpobdellae
• Trematobdellidae
• Semiscolecidae

While individual leech species are easily differentiated, higher leech taxonomy has gone through several major revisions in the past hundred years. The chart above is an amalgamation of the order system presented in Mann (1962) and the new subclasses described in Sawyer (1986). A division of the leeches by philogeny is currently being formulated by Jessie Light here at the Biological Station. Her philogenic trees group all of the freshwater leeches into four families: Glossiphonidae, Piscicolidae, Erpobdellidae and Hirudinidae. This method may turn out to be more correct.

The class Hirudinea is comprised of three subclasses: Acanthrobdellida, consisting of only Acanthobdella peledina, a small fish parasite in isolated arctic regions; Branchiobdellida, small ectocommensals mostly on freshwater crayfish in the Northern Hemisphere; and Euhirudinea, true leeches with both anterior and posterior suckers. The Euhirudinia can be further divided into the Rhynchobdellae, leeches with a proboscis for piercing the skin of their hosts, and the Arhynchobdellids which lack the proboscis. The rhynchobdellids are divided into families by body morphology. The anterior sucker of the Glossiphonidae is continuous with the rest of the body, while the anterior sucker of the Piscicolidae is bell shaped and is clearly marked off from the rest of the body. In addition, the Glossiphonidae are all freshwater species and most of the Piscicolidae are mostly marine.

Arhychobdellids are divided into two orders, Gnathobdellae and Pharyngobdellae. Pharyngobdellae lack the ability to suck blood, and are completely carnivorous. Almost all of these leeches fall into the family Erpobdellidae, which is characterized by lack of jaws and teeth. Sometimes called worm leeches, they swallow their prey whole. The Trematobdellidae are very similar to the Erpobdellidae, but have a gastrophore, a duct from the midgut to the body wall, possibly used for respiration. The final family, Semiscolecidae, have a single jaw rudiment and might be an intermediate between the Pharyngobdellae and the Gnathobdellae, which have tripartite jaws. In this order, the Hirudidae are aquatic and the Haemadipsidae are adapted to terrestrial life. The Hirudidae are commonly represented by the European and American medicinal leeches, Hirudo medicinalis and Macrobdella decora. The Haemadipsidae include the infamous South Asian land leech.

By the current standard taxonomic system, Hirudinea is a class of the subphylum Clitellata, in the phylum Annelida. The class Hirudinea has been called an intermediate between the annelids and the arthropods. True leeches are segmented worms, but the only external evidence of this segmentation is the positioning and spacing of sensory organs on the body. The true segmentation is internal, delineated by the nervous system, as developed by Castle (1900) and Moore (1900). These segments correspond to embryological development segments that are lost in the adult leech. Arthropods sometimes show superficial segmentation, especially in larval and pupal forms.

Oligochaetes, and all other more primitive annelids, have a closed circulatory system, with dorsal and ventral longitudinal blood vessels. Arthropods are characterized by a body cavity that is a haemocoel, filled with sinus fluid that circulates with haemoglobin. The subclasses of Hirudinea show a progression from the closed circulatory system of lower annelids to the open system of arthropods. Rhynchobdella have longitudinal vessels, exactly like earthworms. The vessels begin to be obliterated in the Acanthrobdella and Branchiobdella, and are completely eliminated in Arhynchobdellids. The character of the coelom body cavity also changes as leeches become more evolved. The coeloms of oligochaetes are spacious and fluid filled, serving as a hydrostatic skeleton. In leeches, the coelom begins to be filled with spongy mesenchymatous packing tissue. This allows the extensive compression and elongation observed in any leech. Haemocoelic channels appear in the coelom as blood vessels disappear. Arhynchobdellid leeches have a true haemocoel (Sawyer 1986), as do arthropods.

Leeches have straight digestive tubes, like both annelids and arthropods. But all leeches have a clittelum, a structure never found in arthropods. The clitellum is a modified part of the body wall which secretes a cocoon around newly produced offspring. The presence of the clittelum keeps leeches in the annelid phylum, despite their many arthropod characteristics. Leech clitellums are generally not as evident as the clittelum of the common earthworm, but the important gland structures and functions are the same. All leeches are hermaphroditic, and while some annelids have separate sexes, no arthropods are hermaphroditic.

Leeches differ in several ways from oligochaetes, such as earthworms, the other order in the class clitelata. First of all, earthworms eat dirt; there is only one known carnivorous oligochaete. All species of Hirudinea are carnivorous, while not all simply suck blood. The most striking difference between the classes is the presence of suckers. All subclasses of Hirudinea have at least a posterior sucker, and the true leeches, Euhirudinea, have an anterior sucker as well. Sawyer (1986) put it succinctly: "Leeches are carnivorous clitellates with a characteristic sucker mode of locomotion."

Oligochaetes possess setae which aid in locomotion. Only the subclass Acanthobdellae has setae, and only on five anterior segments. In addition, Acanthobdellae have no anterior sucker and possess an oligochaete like coelom. The Acanthobdellid coelom is spacious and segmented by septa in the front half of the animal. A haemocoel is present at the posterior, where the sinus fluid comes in contact with the outside of the intestine for nutrient absorption.

The gradual loss of the blood vessels present in oligochaetes has already been described. Other differences between the two classes of clittelata are present because of the leech predatory lifestyle. All clitellates have photoreceptors, chemoreceptors and tactile receptors, but hirudinea have increasingly more complicated sensory organs. Oligochaetes lack eyes, except for a few aquatic species that have simple eyespots. All leeches have eyespots, or ocelli, and visual organs become more developed as the leeches become more advanced. The hirudidae have very well developed eyes consisting of 30-100 photoreceptive cells in a deep pigment cup. The cup collects light, and the eye is very near the epidermis so light is not scattered by other tissues. The hirudidae eye can give a good directional response to light, assisting in prey capture. Earthworms do not need advanced senses to crawl through the earth. Finally, leeches cannot regenerate, whereas oligochaetes are infamous for their regeneration of lost segments.

Most families of leech have a three part life cycle: egg, larva and adult. The larvae are not free swimming, they stay in the albuminous fluid of the cocoon. Larvae have protonephridia, a temporary larval pharynx and a ciliated larval ectoderm, taking in albumen with ciliary action. These cryptolarvae lack an anus and resemble eggs with mouths. The larval organs are lost and development is finished at metamorphosis, which occurs when all the albuminous fluid is used up. All families of hirudinea except Glossiphoniidae go through the larval stage. Glossiphonid eggs have a relatively large amount of yolk to complete their development without absorbing nutrients from the cocoon.

Leeches are epimorphic. When the eggs hatch or metamorphosis finishes, the juvenile leeches that emerge from the cocoon have an unchanging number of segments. This is in stark contrast to the oligochaetes and other annelids that add segments as they grow. Two different growth patterns are exhibited in leeches. The Piscicolidae and the predacious leeches grow continuously, and they feed often, because of quick digestion. The higher leeches exhibit saltatory growth, they grow in spurts. These leeches have much slower digestion, and go for periods of months between meals. During a rest period they can grow to four to six times their body weight before feeding.

The lower and higher leeches described above attain sexual maturity at different points in their adult lives. The lower leeches begin to sexually reproduce when they hit a critical body weight, then tend to deposit cocoons continually. The higher leeches hit sexual maturity after a specific number of feedings. In some species, the separate sex organs will mature at different stages. Placobdella ghilianii, of the family Hirudinidae, is a sexually mature male after three feedings and a sexually mature female after four feedings. Other changes in behavior also occur at different stages in many cases.

Leeches are uniformly hermaphroditic, like most other members of the class clitellata, and always reproduce sexually and cross fertilize. Courtship procedures are generally unknown, except for certain land leeches, which are somewhat easier to observe than aquatics (Leslie 1951). Mating is probably usually unilateral, though reciprocal fertilization is possible, and is performed by close intertwining of the two leeches.

The fertilization process of hirudinia is downright bizarre, and the true mechanism is still unknown. One leech will fill a spermatophore with its sperm, then attach the spermatophore to the body of the other leech. The spermatophore is usually attached somewhere near the clitellar region, however, sometimes they miss. This does not have an adverse effect on the process. While the method has not been decided on, the sperm exit the spermatophore, then somehow burrow through the skin into the coelom, and then travel on their own to the ovaries. Brumpt (1900) favored a mechanical, fluid pressure action on the part of the spermatophore. Some proteolytic properties of the spermatophore lead Mann (1962) to believe there is a chemical parting of the epidermal tissues. The Piscicolidae have a copulatory area, with special tissues to conduct the sperm to the ovaries.

There is usually a delay between copulation and cocoon deposition. In most species, this is simply a result of sperm travel time through the coelom to the ovaries. However, some species may store sperm and fertilize their eggs under ideal conditions. Hirudo medicinalis, the European medical leech of the Hirudidae, has a delay of up to nine months before cocoon formation, and eggs are found in the cocoon at the same stage of development no matter how long the delay.

Cocoon production and structure in hirudinea is very similar to that in oligochaetes. Glands in the clitellum secrete the cocoon around the body of the leech, and fill it with albuminous fluid. The cocoon then slides off the head of the leech, picking up the fertilized eggs on its way. Most leeches affix their cocoons to substrate, first preparing the substrate with a secretion from the anterior sucker. They lay on the substrate with the clitellum on the prepared area, then secrete the cocoon on the substrate. This substrate can be anything from a rock to wood, to the exoskeleton of a host crayfish in the case of the Branchiobdellids. The marine leech Notostomum cyclostoma is a fish parasite, but deposits its cocoons on the backs of crabs (Sloan et al. 1984). The leech wriggles out after cocoon deposition, sealing each end of the cocoon with secretions from the anterior sucker.

Further treatment of the cocoon by the parent leech depends on the type of leech. Some families of leeches flatten the cocoon against the substrate, and some leave the cocoon alone. In both cases the cocoon darkens and hardens after a few days. The families Hirudidae and Haemadipsidae deposit their cocoons on land, and the surface of the cocoons differentiate into a spongy outer layer to prevent desiccation. Many leeches exhibit surprising levels of parental care, especially for evil bloodsucking parasites. Some arhynchobdellid leeches cover their cocoons with their bodies until they harden. The rhynchobdellid leeches have thin walled cocoons, and so require a great amount of brooding. The subfamily Glossiphoniinae of the family Glossiphonidae hovers over its cocoon until the eggs hatch, then the baby leeches attach to the parent leech. The babies ride around until they come to a suitable host organism, then they all desert and attack. After their first meal they are ready to survive on their own. The subfamily Haementeriinae of the same family is basically the same, except it deposits its cocoon on its own ventral body surface.

Leech prey is as diverse and varied as the many species of leeches themselves. Leeches have adapted to every type of food; all in all, the parasitic species will suck anything with blood in it, and the predacious species will eat every type of food they can fit in their gut.

The acanthrobdellid fish parasite, of course, feeds exclusively on fish. It takes cold water salmonid fish almost exclusively as hosts, and is semipermanent. Acanthrobdellids usually prefer to attach to the base of the dorsal fin with their posterior sucker. The leech will suck one spot of the fishes skin for long periods of time. It sucks blood and will ingest tissue, frequently leaving wounds that can become infected.

The Branchiobdellida live on freshwater crustaceans, mostly on crayfish, and can be commensal or parasitic. These leeches are generalists and omnivores. They graze the micro organisms living on the crayfish body, and eat particles of food that are dispersed when the crayfish tears apart its prey. This grazing, and to some extent the opportunistic feeding as well, might help to keep the crayfish shell clean (Sawyer 1986), being beneficial to the crayfish. The parasitic branchiobdellids frequently live in the gill chambers of their hosts. Here they feed on gill filaments and blood, and can also filter the water for algae and zooplankton. Cambarincola aliena, the one species of branchiobdellida that lives on a cave isopod, eats the eggs of its host.

Rhynchobdellid leeches usually feed on soft bodied prey because of their relatively weak proboscis. Their prey can include aquatic oligochaetes and insect larvae, molluscs, and even some fish. The jawed leeches, Arhychobdellida, commonly suck the blood of vertebrates. Both groups include parasitic and predaceous species, but rhynchobdellids tend to be more predacious. It seems they can't help it. Glossiphonia complanata will commonly drain its host snail of body fluids, then suck up the soft snail tissue through its proboscis. This is known as liquidosomatophagous feeding, and is most common among the family Glossiphoniidae. The Piscicolids also infrequently exhibit this feeding on marine isopods and octopus.

Predacious jawed leeches tend to show macrophagous predation, simple swallowing. These leeches wander, attacking prey when they find it. There are two groups of macrophagous leeches, differentiated by the location of the mouth. Strepsilaematous species have their mouth on the underside of their dorso-ventrally flattened bodies, and so must twist around the anterior portion of their bodies to grasp prey. The mouths of euthylaematous leeches are on the anterior tips of their bodies. They tend to swallow all prey whole, while strepsilaematous leeches suck and tear, breaking off portions and sucking the juices of prey too large for them. Strepsilaematous leeches are usually more inefficient predators, and must rely on an element of chance, because of their clumsy twisting motion. Euthylaematous leeches seem to be more voracious and scary, because of the mode of ingestion. The European horse leech Haemopis sanguisuga forages at night, slightly above the shoreline, and eats everything from earthworms to small fish. It will frequently grab the end of a worm in its mouth, then suck it down like a strand of spaghetti. The earthworms can survive the process, and may stay alive in the leech gut for up to two days. In addition, H. sanguisuga will swallow small snails shells and all, egesting the shell after digestion, and will eat carrion.

Anyone who has ever worn waders is familiar with the vertebrate parasite leeches. Leech species tend to be specific, attacking either fish, amphibians, reptiles, birds or mammals, with little overlap. Fish leeches are generally not host specific. They usually position themselves on the substrate or in underwater vegetation strategically, in fish traffic areas. Rhynchobdellids frequently parasitize fish, because their proboscis can easily penetrate between scales or in gills. Different rhyncobdellids attack amphibians, also because of their easily penetrated epidermis. Amphibian parasites are somewhat less specific than other vertebrate parasites. Young fish leeches can attack tadpoles, and amphibian leeches may supplement their diets with snails. Marsupiobdella africana is host specific to the clawed toad Xenopu laevus ; juveniles are attracted to that species and stay attached until they hit sexual maturity, and brooding parents will release their offspring near the toads as well.

The three main species of reptile specific leeches feed primarily on turtles, but will also feed on amphibians, birds and the occasional human when turtles are rare. These leeches are all of the genus Placobdella, and are arhynchobdellids. Bird specific leeches are rhynchobdellids and feed in the nasal passages of water fowl. These leeches wait on vegetation at the surface of ponds and slow streams frequented by waterfowl, and attack the birds when they come to drink. Leeches can find their way to the head from anywhere on the body of the bird, and feed and detach from the nares or eyes in a relatively short time. While these leeches are always bird specific, they sometimes make mistakes, as in the case of a Theromyzon tessulatum feeding from a human eye (Pawlowski 1938).

Mammal specific leeches are probably of the most interest to humans, and they make up a majority of the bloodsuckers. The European medicinal leech, Hirudo medicinalis, is the best known leech in the world, and has been involved in medicinal research since 140 AD (Sawyer 1986). Unfortunately, and somewhat surprisingly, the medicinal leech is now endangered or extinct over much of its original range (Sawyer 1984). The American medicinal leech, Macrobdella decora, is still going strong, and can be found in standing pools all around the Biological Station. These leeches feed on breeding frogs and toads in the spring, and on mammals in the fall. Amphibian consumption is very important to leech presence, because the jaws of young arhynchobdellids are not strong enough to cut mammal skin. Cattle, horses and humans are the mammals most often preyed upon. The leeches lay in the silt at the bottom of the pool until they sense a disturbance, then they swim out to investigate. Their chemoreceptors are very sensitive to blood and sweat, and they detect heat very well. Kneeling in a pool for any extended period of time, even in waders, will produce a flock of inquisitive leeches.

The salivary and crop glands of bloodsucking leeches produce several chemicals that are essential to their survival: anaesthetic, spreading factor, vasodilator, anticoagulants and antibiotics. The anaesthetic is secreted first as the leech is attached to the host's skin by the anterior sucker. The host does not feel the bite of the leech or the pierce of its proboscis, except in the case of the stinging leech of Borneo, Haemadipsa picta. The spreading factor, now called Orgelase hyaluronidase, increases the permeability of mammalian skin, letting leech chemicals spread through the bite location. It also has some antibiotic properties, dissolving the protective polysaccharide capsule of streptococcal and other difficult and virulent bacteria. This intriguing enzyme has been patented and trademarked by a British pharmaceutical company...

Vasodilators are injected into the host to open the capillaries around the bite area. This is the cause of the prolonged bleeding after the removal of a leech. Anticoagulents are used more to prevent the blood from coagulating inside the leech, not to speed blood flow. Blood will coagulate normally ten minutes after removal of a leech, but the blood will continue to flow for over a half hour. Several different anticoagulents have been discovered and isolated from leeches. The most famous proteolytic inhibitor is hirudin, isolated from the European medicinal leech Hirudo medicinalis, a protein that blocks thrombin. Hirudin is being intensively tested for use in medicine as we speak; it appears to be effective on several blood disorders with no acute side effects. Leeches also contain a family of proteinase inhibitors called bdellins, and protease inhibitors called eglins. Hirudin, bdellins and eglins were all isolated from H. medicinalis, and are present together in most leeches.

Two species of leech have unique anticoagulants, and they are both of the same genus. Hemetin has been isolated from Haementeria ghilianii, and is an enzyme that directly degrades the fibrinogen and fibrin in plasma that form blood clots. This prevents clots from forming, and dissolves clots already formed. The medical uses of hemetin are evident, especially since hemetin is insensitive to the inhibitors of proteolytic enzymes that are found in human plasma. A relatively unstudied plasminogen activator found in Haementeria depressa also acts as an anticoagulant. Endosymbiotic bacteria in the gut of leeches produce antibiotics. They were probably evolved to prevent the putrefaction of blood stored in the gut for long periods, but have been reported to effect such bacterial pathogens as anthrax, tetanus, meningitis, strep and staph. The productive bacterium in the medical leech is Aeromonas hydrophila, and it kills tuberculosis, dysentery and diphtheria when cultured in vitro.

They don't digest it. Leeches produce very few of their own digestive enzymes; they instead rely on enzymes produced by endosymbiotic microflora. Leeches possess bacterium in their intestine that produce necessary enzymes, except endogenous enzymes which have been found in all studied leeches. The symbiotic bacteria also produce specialized blood digestion enzymes. These bacterium are species specific. Bloodsucking leeches have one species of bacteria in their intestine whereas predacious leeches have multiple symbionts.

When bloodsucking leeches feed, they fill their crop with blood. The crop is much longer and wider then the intestine where the blood is digested. Blood can remain in the crop for long periods, and it is slowly degraded by bacterial enzymes. Crop antibiotics prevent putrefaction and haemolysis from foreign micro organisms until the leech's own bacteria can break it down. The blood is mixed with mucous and the haemoglobin is released into solution over time, then the contents of the crop are passed into the intestine a little at a time by a sphincter. All the globin of the solution is absorbed, and pure haem is released from the anus as feces.

The most important predators on leeches are fish and birds. Secondary predators are aquatic insects, crayfish, and other leeches. Some Haemopis species are even specialized to feed on leeches. There are few specialized predators on leeches, such as a specialized cichlid in Lake Victoria, East Africa (Witte and Witteman 1981) and the common garter snake.

Leeches have developed many defensive measures to prevent their consumption. The most common defense is hiding. Leeches commonly burrow into muddy substrate until they sense a potential host, avoiding predators. Several species thrive in temporary ponds with low fish populations, another form of predator avoidance. These leeches burrow into the mud when the pond dries up. We found an example in Wilderness State Park in Emmet County, and the shriveled up leech was restored almost instantaneously in a vial of water. Many leeches are nocturnal to lower visibility to predators, and some thrive in turbid waters.

Leeches have also evolved physical reactions and defenses, many of which are displayed to the leech collector. Leeches can exhibit an amazingly strong hold with their posterior sucker when pulled from a substrate. They can also flatten tightly against substrate. These measures can save a leech from the crop of a bird or the vial of a collector. If both suckers are dislodged from the substrate, some species go into defensive postures. The most common one around the Biological Station is balling, where the leech rolls up into a hard ball and stays curled for a period of time. Other postures include knotting, coiling and flaccidity, where the leech goes limp. Leeches also protect their cocoons by brooding, discussed earlier.

The leech is a classic example of a parasite, but there are also parasites that prey on leeches. Leeches are best known as vectors, but they can be primary hosts as well, though few cases are documented. Trematodes are the most common examples; normally ingested by macrophagous leeches, they can also burrow through leech skin. A specialized rotifer in western Europe lays its eggs in the skin of Erpobdella octoculata. Leeches are vectors for several different haematozoa, protistan blood parasites, as well as some platyhelminthes and nematodes. The most common blood parasites are the Trypanosoma species, which are passed between fish and amphibians by bloodsucking leeches. The predacious leeches are not vectors for blood parasites, but act as intermediate or even definitive hosts for digenetic trematodes and nematodes. These worms are usually passed to waterfowl upon consumption of the leech.

Leeches are an amazingly widely distributed taxa. In all types of water and even on land, leeches are present. The land leeches are of the family Haemadipsidae and live in moist tropical jungle. Several genera are amphibious, living on land, usually in soil, for only part of the year. Most aquatic leeches are freshwater; only one fifth of all species live in salt water, and most are of the family Piscicolidae. This section will focus on the habitats and distributions of freshwater leeches which are widespread in Michigan (table 1).

The factors which determine leech distribution in freshwater environments are, in approximate order of significance (Sawyer 1986): availability of food organisms; nature of the substrate; depth of water; presence of water currents; size and nature of the body of water; hardness and pH; temperature of the water; dissolved oxygen; siltation and turbidity; and salinity. The single most important factor in a leech life is the presence of food. Host organisms may also affect the distribution of leech species because of host specificity. Mann (1962) reviewed several studies of leech distribution in Europe and discovered a correlation between distribution and the characteristics of the water, but Sawyer (1986) suggests that the species present are not determined directly by abiotic factors of the environment. Instead, host distribution is determined by the environment, and this affects the presence of leeches.

The substrate is also important because of the suckers of the leech. Leeches need suitable substrates for their processes of locomotion, feeding and reproduction. Leeches tend to prefer submerged vegetation, independent of depth. Muddy or silty unstable substrate will impede the presence of leeches. In larger lakes, leeches are more commonly found in the shallows because of lack of food and lack of suitable substrates at lower depths. Nearly all leeches prefer standing water to running water. Most would probably not be washed away in lotic water, because of their suckers, but only a few species are evolved to keep their cocoons safe under current conditions. Running water may also have an adverse affect on leech hunting.

Mann's study (1962) showed differential species domination according to the calcium concentrations of the water. Helobdella stagnalis dominated pools with greater than 8 mg/l calcium concentrations, and Erpobdella octoculata dominated in all softer bodies of water. This is probably due to the adaptation to low calcium of Erpobdella, which thrives in extremely soft lakes (Young and Ironmonger 1981). Calcium may simply be an indicator of the productivity of the lake, showing food supply.

Leeches are limited by a very low pH, but otherwise the level of alkalinity does not have an effect on leech distribution. Temperature also does not change leech population distributions except at extremes, but temperature does have dramatic effects on leech development and reproduction. Dissolved oxygen concentration, which is usually dependent on water quality and the nature of the body of water, should probably be used as a correlative delineator of these factors instead of a factor in itself. A minimum dissolved oxygen level is, of course, necessary for any aquatic life. Water turbidity and salinity also play rather small roles in leech distribution. The hardy, fun loving leech survives and thrives in most aquatic habitats.

Leech importance to trophic webs has been ignored to a surprising degree, considering their wide distribution and diversity. Leeches have sometimes been included in trophic webs (eg. Holomuzki and Collins 1987), but their specific functions have not been studied. Leech predators are generally secondary consumers, eating herbivores and detritivores, but will swallow anything they can get their jaws around, including smaller carnivores, other leeches and even carrion. Parasitic leeches can even top humans on a trophic pyramid. Various organisms consume leeches as well, but there are very few reports of specialized predators on leeches. Leeches play a much greater role in the transmission of haematozoa to aquatic vertebrates.

The minor specialization of leeches partitions their resources. That is, parasitic leeches on mammals and liquidosomatophagous leeches that eat earthworms have completely different niches. In a hypothetical pond described by Sawyer (1986), ten different species of leeches coexist with little competition. He uses European leeches; North American leeches have more interspecific competition.

All of the few existing studies of the pollution ecology of leeches have been performed on freshwater lotic ecosystems (eg. Richardson 1925, Carlson 1968), despite their high abundance in all types of aquatic habitats. Leeches have a high tolerance for pollution, although they are never classified as pollutional species (Sawyer 1974). Pollutional species are more common in the pollution zone of a river than below it. Leeches are very likely to be subpollutional-unusually tolerant or subpollutional-less tolerant-less common species. This means that leeches are most commonly found in the less polluted sections of streams, but are fully capable of surviving in polluted areas. Leeches are especially resistant to organic enrichment, oil pollution, copper sulfate and most pesticides, including DDT. Leeches can handle small concentrations of dissolved lead, but have no tolerance for dissolved zinc, and can be eliminated by toxic substances dissolved in oil. Leech resistance to desiccation can explain these tolerances. Leeches can passively withstand pollutants and toxins to a point, and then can migrate to less polluted zones or climb out of contaminated water.

Almost all of the leech species associated with polluted water feed on aquatic oligochaetes and dipteran larvae. These prey are classified as pollutional, they have high concentrations in polluted water. The pollution tolerant leeches take advantage of their densities and thrive near polluted zones of rivers. Thus, leeches are not a true pollution indicator, but are a secondary indicator. While only lotic systems have been documented, these same species of leeches commonly inhabit the wave zones of lakes, so might be a useful indicator of lake pollution as well. To extrapolate to other leech species in other environment, we should look at the relative densities of the prey of the leeches. Since leeches can withstand water pollution, they should always indicate increased densities of pollution resistant prey.

Hopefully the leech is now more than a simple "carnivorous clitellate with suckers" after the reading of this paper. There is great depth below the surface image of annoying parasite that stigmatizes the leech. The medical uses of leeches alone should call for increased study of the order. Leeches have already proved their effectiveness in their traditional medical application, bleeding, now used by plastic surgeons to restore circulation in grafts (eg. Pantuck et al. 1996). There are numerous secrets in the chemicals of leeches, possibly even the archetypal cure for cancer (eg. Gasic et al. 1983). Although leeches are shown to be resistant to the negative effects of pollution, we must be extremely careful not to disturb their natural habitats too much. Even the widely known medicinal leech, source of most of our leech medical knowledge, is endangered or extinct over most of its original range. Habitat destruction could harm the interesting and intrinsically beautiful leech beyond repair.