Leeches (Euhirudinea) occur worldwide, with the exception of Antarctica, in a wide range of ecosystems including marine, estuarine, moist terrestrial (particularly in Australia and Southeast Asia) and freshwater systems, and are common in both lentic (ponds and lakes) and lotic (stream and river) ecosystems where they are an integral component of the benthic (bottom dwelling) and occasionally pelagic (water column) communities. Predacious leeches are often cryptically coloured (camouflaged) and are often found attached to stones, aquatic vegetation or other submerged substrates. Very few predacious species are pelagic; most are benthic sit-and-wait predators feeding on a variety of invertebrates such as chironomids (gnats, mosquitos), oligochaetes (blood worms, 'earthworms'), amphipods (side swimmers), and mollusks (snails, etc.). However, some species in the Glossiphoniidae and Erpobdellidae (families), particularly in the erpobdellid genus Motobdella, actively seek prey using chemoreception (smell and taste) or mechanoreception (vibration sensation, "hearing") (Mann 1962, Sawyer 1986a,b, Blinn et al. 1987, 1988, 1990, Blinn and Davies 1989, 1990, Davies and Everett 1975, Davies et al. 1981, 1982, 1988, Davies and Kasserra 1989, Davies 1991, Simon and Barnes 1996). Other leeches such as some glossiphoniids, piscicolid, hirudinid and haemadipsid species are temporary ectoparasites feeding on the blood (sanguivory) of vertebrates such as fish, reptiles (turtles and crocodiles), amphibians, waterfowl, and mammals. In addition, leeches are an important component of the diet of predacious invertebrates (dragonfly and damselfly nymphs) and vertebrates (fish, amphibians and waterfowl) (Cywinska and Davies 1989, Dahl 1998, Greenberg and Dahl 1998).
Sanguivorous (bloodsucking) leeches have been recognised for their medicinal properties for many centuries. They have been used as a component of traditional medicines and have recently made a medical comeback. Leeches are being used to treat a variety of circulatory diseases and for reconstructive plastic surgery (Sawyer 1986a,b, Mortenson et al. 1998, Oumeish 1998, Zefirova and Maltseva 1998, Lozano et al. 1999, Mamatova et al. 1999, Ulvik 1999). In addition, salivary extracts such as hyaluronidase (spreading factor), hirudin, hementin, hemenerin and destabilase (anticoagulants) (Table 1) have been extracted and synthesised for medicinal purposes (Sawyer 1986a,b, Blackshear and Ebener 1994, Huang et al. 1998, Arocha-Pinango et al. 1999, Hirsh and Weitz 1999).
|antistasin||prothrombinase inhibitor||Haementeria officinalis||Central America|
|calin||platelet aggregation inhibitor||Hirudo medicinalis||Europe|
|decorsin||platelet aggregation inhibitor||Macrobdella decora||North America|
|destabilase||factor XIII degradation||Hirudo medicinalis||Europe|
|ghilanten||prothrombinase inhibitor||Haementeria ghilianii||South America|
|hementerin||fibrinolytic||Haementeria depressa||South America|
|hementin||fibrinolytic||Haementeria officinalis||Central America|
Historically leeches (Hirudinea) have been divided into five major groups or orders including: the Rhynchobdellida (leeches with a proboscis); Gnathobdellida (jawed leeches); Pharyngobdellida (leeches that lack both a proboscis and jaws); Acanthobdellida (leech-like annelids that have retained chaetae (small bristles) for locomotion) (Clark and Panchen 1971); and the Branchiobdellida. The Branchiobdellida, while leech-like in appearance, are quite different in anatomical structure from both the Acanthobdellida and the Euhirudinea and consequentially, the phylogenetic position of this group has varied greatly. The evolutionary relationships within the Euhirudinea has been the subject of much debate with some researchers recognizing only two orders, the Rhynchobdellida (proboscis leeches), and the Arhynchobdellida (jawed and jawless leeches) (Sawyer 1986b, Davies 1991). This division is primarily based on the presence or absence of the proboscis and the mode of feeding of the included species. The positions of the Acanthobdellida and Branchiobdellida have also been the subject of much debate with some researchers placing these groups within the Hirudinea and others placing them into separate oligochaete groups (Klemm 1985, Klemm 1990, Sawyer 1986b, Epshtein 1987, Davies 1991, Purschke et al. 1993).
The body of the Euhirudinea consists of two preoral, non-metameric segments and 32 postoral somites (metameres) labelled I through XXXIV, while their close relatives the Acanthobdellida have 29 postoral somites (I through XXIX) (Sawyer 1986a,b, Davies 1991, Epshtein 1987, Purschke et al. 1993). In the Euhirudinea each somite is externally divided into 2-16 annuli with complete somites having the full number of annuli in the middle portions of the animal. The number of annuli and how the annuli are subdivided can be diagnostic for a genus or species (Soos 1966a,b, Soos 1967a, Soos 1969a,b,c, Richardson 1971, Sawyer 1986b, Davies 1991). It is assumed that the primitive number of annuli is three (Moore 1898, Moore 1900a,b, Sawyer 1986a,b, Davies 1991) so that the annuli are numbered based on the three primary annuli a1, a2, and a3 from the anterior, with the neural annulus (containing the nerve cord ganglion and externally delineated by a transverse row of papillae or sensilla) labelled as a2. The loss or repeated bisection of the primary annuli into additional annuli gives the more complex annulation patterns of some genera and species (Moore 1898, Moore 1900a,b, Sawyer 1986a, Davies 1991.) Like the Euhirudinea, the Acanthobdellida have each midbody somite externally subdivided, however, their somites are only divided into four annuli with very little variation (Epshtein 1987).
The Euhirudinea and Acanthobdellida typically have a reduced coelom not subdivided by intersegmental septa in adults, except in the anterior portions of the acanthobdellids and as remnants in some glossiphoniids. This is in contrast to most Annelida, that have a large coelom subdivided by intersegmental septa. In the acanthobdellids, septa are retained in the anterior somites, however, they have been lost in the posterior somites which are similar to those in the glossiphoniids and piscicolids. The coelom of the glossiphoniids, piscicolids, and acanthobdellids has been modified to enclose a true blood vascular system with the ventral lacuna enclosing the ventral blood vessel and nerve cord, and the dorsal lacuna enclosing the dorsal blood vessel. The ventral, dorsal and lateral lacunae are connected via transverse communicating lacunae and in some piscicolids there are modified coelomic chambers (pulsatile vesicles) connected via additional lacunae. In the hirudinids and erpobdellids, the true blood vascular system has been completely lost and blood circulates through a haemocoel formed from a reduced coelomic (channel) system. The blood in the haemocoel is circulated through a complex system of muscularised coelomic lateral channels (heart tubes) allowing blood to be pumped throughout the body of the leech. The heart tubes have become modified with valves and sphincters preventing blood from flowing back through the system maintaining a unidirectional flow of blood throughout the haemocoelomic system (Sawyer 1986a,b, Epshtein 1987, Davies 1991, Purschke et al. 1993).
Typically the bodies of the Euhirudinea and Acanthobdellida, with the exception of most members of the family Piscicolidae, are not divisible into distinct body regions (Klemm 1985, Sawyer 1986a, Davies 1991). Members of the family Piscicolidae (except for the genera Myzobdella Leidy, 1851 and Piscicolaria Whitman, 1889) have the body divided into a narrow anterior trachelosome ('neck') and a longer and wider posterior urosome ('body'). Some piscicolid genera have paired pulsatile vesicles on the neural (a3) annuli of the urosome. Lateral projections or 'gills' can be found in some marine and freshwater piscicolid and ozobranchid genera. The pulsatile vesicles and gills are connected to the coelomic and circulatory systems via coelomic passages and increase the rate of gas exchange and surface area available for gas exchange (respiration) (Sawyer 1986a,b, Davies 1991).
The Euhirudinea have a number of sensory structures including eyes and oculiform spots, papillae and sensilla. The position and arrangement of these structures can be important in identifying genera and species. The eyes are typically arranged in the first few somites and can be found either around the margins of the 'head' or near the midline. In some genera (particularly in the glossiphoniids) the eyes are fused into lobed composite structures although their lobed nature usually indicates the original number of eyes. Eyespots or oculiform spots can also occur on the lateral margins of the body and on the posterior sucker in some piscicolids. Papillae (small protrusible sense organs) and tubercles (large fleshy protrusions that include some of the dermal tissues and muscles) may be present and can be scattered or arranged in rows on the dorsal and ventral surfaces (Soos 1966a,b, Soos 1967, Soos 1969a,b, Sawyer 1986b, Davies 1991, Govedich and Davies 1998). Acanthobdellida peledina lacks eyes, however, Acanthobdellida livanowi has three pairs of eyes with the first close together in somite IV and the second and third pairs on the margins of the middle annuli of somite V and VI respectively (Epshtein 1987).
The Euhirudinea lack chaetae and typically have anterior (oral) and posterior (caudal) suckers sometimes used during locomotion and for attachment to the substrates, prey or hosts. The anterior sucker of the Euhirudinea contains the mouth and is usually smaller than the posterior sucker. Acanthobdella livanowi has both anterior and posterior suckers, however, the mouth is terminal and not within the anterior sucker. Acanthobdella peledina has a posterior sucker but lacks an anterior sucker. Both acanthobdellid species have chaetae used to hold on to the substrate during locomotion (Sawyer 1986a,b, Epshtein 1987, Davies 1991, Purschke et al. 1993).
Many Euhirudinea (particularly terrestrial and some freshwater species), and Acanthobdellida use a looping (inch-worm like) form of locomotion that consists of elongating and shortening the body and alternating between using the anterior (Euhirudinea and Acanthobdella livanowi only) and posterior suckers to attach to substrates. Most aquatic Euhirudinea species are also able to swim by flattening their body and using dorsoventral undulations of the body to propel the leech in an 'eel-like' motion. Erpobdellids and hirudinids are typically very good swimmers with only a few glossiphoniids and piscicolids being good swimmers. Some glossiphoniid species (Placobdella hollensis (Whitman, 1892)) can swim readily as juveniles and as adults, but others (Placobdella ornata (Verrill, 1872)) and Placobdella parasitica (Say, 1824)) can swim only as juveniles losing the ability as they mature (Sawyer 1986a, Davies 1991). The erpobdellid genus Motobdella Govedich, Blinn, Keim and Davies, 1998 is unique in being a pelagic predator feeding almost exclusively on pelagic amphipods by using specialised sensilla and mechanoreception to identify and track its prey in the water column (Davies et al. 1985, Blinn et al. 1987, 1988, 1990, Blinn and Davies 1990, Govedich et al. 1998).
The anterior (oral) sucker of the Euhirudinea varies in structure among the different families. The oral sucker in some groups can be large and prominent with the mouth taking up only a small portion of the sucker (glossiphoniids and piscicolids) or the sucker may be relatively simple consisting only of the edges of the mouth (erpobdellids, hirudinids and haemadipsids). The posterior (caudal) sucker is generally wider than the body at the point of attachment and is usually ventrally directed. Leeches with a small mouth pore (glossiphoniids and piscicolids) typically have a modified pharynx in the form of an eversible muscular proboscis and lack a buccal cavity and jaws. Leeches with a large mouth have a buccal cavity that may or may not contain jaws and a velum (a flap of skin that separates the sucker from the buccal cavity). In jawed leeches (hirudinids and haemadipsids) the buccal cavity contains two or three muscular jaws bearing teeth or a cutting edge arranged in one (monostichodont) or two (distichodont) rows. In jawless forms (erpobdellids) the buccal cavity may contain three muscular ridges and in some genera these ridges may be armed with small tooth-like stylets (Salifa Blanchard, 1897 and Barbronia Johansson, 1918) (Soos 1966a,b, Soos 1967, Soos 1969a,b, Richardson 1968, Richardson 1969, Sawyer 1986a,b, Davies 1991).
Salivary glands open into the pharynx of both sanguivorous and predacious species, however, the salivary secretions of each vary in function. In predators the salivary glands primarily secrete digestive enzymes used to break down the fluids and tissues of their prey. In sanguivorous species the salivary glands secrete additional compounds primarily related to the process of bloodsucking rather than digestion. These compounds perform functions such as: lubrication of the mouth parts (mucus), a spreading factor to increase the permeability of mammalian skin (hyaluronidase), blood vessel dilation (vasodilation - an histamine-like secretion), blood clot prevention or breakdown (anticoagulants) (Table 1), and possibly an anesthetic-like compound (Sawyer 1986a,b, Davies 1991, Arocha-Pinango et al. 1999). The salivary compounds (including anticoagulants and vasodilators) of Australian species have not been studied extensively and most research has focused on the European Hirudo medicinalis Linnaeus, 1758, Asian Hirudo nipponia Whitman, 1886, North American Macrobdella decora (Say, 1824), Central American Haementeria officinalis Filippi, 1849, and the South American Haementeria ghilianii Filippi, 1849 and Haementeria depressa (Blanchard, 1849).
The pharynx leads to a crop that may be either acaecate or may contain lateral caeca. In most erpobdellid leeches the simple tube-like crop is acaecate, however, the North American genus Motobdella has one or two pairs of simple caeca or postcaeca located in the posterior portions of the crop (Davies et al. 1985, Govedich et al. 1998) and the Australian genus Vivabdella Richardson, 1971 has one pair of simple postcaeca (Richardson 1971). The crop in other leech families is adapted for the storage of fluids and usually has 1-11 pairs of lateral caeca; the number and arrangement of which can be genus and species specific. From the crop the intestine leads to the anus and only in the Glossiphoniidae does the anterior portion of the intestine give off four pairs of simple caeca. The anus usually opens on the dorsal surface on or near somite XXVII, just anterior to the posterior sucker. In a few species (eg. Actinobdella peduculata (Hemingway, 1908), Marsupiobdella africana Goddard and Malan, 1912 and Branchellion torpedinis Savigny, 1822), the anus is displaced further anteriorly (Soos 1966a,b, Soos 1967a, Soos 1969a,b, Sawyer 1986b, Davies 1991).
Digestion and absorption of food and nutrients occurs within the intestine of both predacious and sanguivorous species with only the predacious species additionally utilising the crop for these functions. In sanguivorous species symbiotic bacteria aid in the digestion of blood meals by producing enzymes that aid in the breakdown of blood. Several species of bacteria have been found to colonise sanguivorous leeches such as Hirudo medicinalis, especially members of the genus Aeromonas (A. hydrophila and A. veronii biovar sobria) (Sawyer 1986a,b, Davies 1991, Graf 1999). Digestion in sanguivorous leech species may take weeks or even months due to the reliance on endosymbiotic bacteria. Predatory species utilising their own enzymes digest their food within a few days (Davies et al. 1977, 1978, 1981, 1982, Wrona et al. 1979, 1981, Govedich and Davies 1998).
The alimentary canal of the acanthobdellids is similar to that of the piscicolid and glossiphoniid leeches consisting of a small eversible proboscis that leads to a muscular pharynx. The pharynx leads into a short oesophagus that connects to the large tubular acaecate crop that is divided into seven compartments, followed by the intestine with six pairs of lateral caeca. The intestine then leads to the dorsal anus located between somites XXV and XXVI. Digestion is likely to be similar to the sanguivorous leeches and may use endosymbiotic bacteria (Epshtein 1987, Davies 1991).
All Euhirudinea are hermaphrodites and reproductive structures are important characters for delineating families, genera and species. The male and female gonopores are visible in reproductively mature leeches near the midline on the ventral surface of somites XI and XII (respectively), and are separated by an often species-specific number of annuli. In mature specimens the anterior male gonopore is typically larger and more obvious than the posterior female gonopore and may be raised or surrounded by papillae. Euhirudinea do not have true testes or ovaries but have paired testisacs and ovisacs; thin walled sacs derived from specialised coelomic sacs that produce either the spermatozoa or eggs from a germinal epithelial layer lining the sac. Gametes are then stored within the liquid filled sacs, where they develop and mature (Anderson 1973, Sawyer 1986a, Davies 1991). Testisacs are typically located posterior to somite XI and may be discrete, paired intersegmental spherical structures (Hirudinidae, Glossiphoniidae, and Piscicolidae) or multifollicular columns (resembling bunches of grapes) lying on either side of the ventral nerve cord next to the crop (Erpobdellidae) (Singhal and Davies 1985, Davies et al. 1985, Govedich et al. 1998). Short vasa efferentia connect the testisacs to the vasa deferentia on each side of the body, which run anteriorly to form large coiled epidymes or sperm vesicles. Paired ejaculatory ducts run from the epidymes through the atrial cornua and unite to form a medial atrium. The form of the male atrium is also important and can be modified into a protrusible penis. The single pair of ovisacs can be either small, spherical organs confined to somite XII or elongate tubes that may be coiled or recurved back on themselves so that their ends lie close to the oviducts and the female gonopore. A pair of short oviducts connect the ovisacs and converge and form a common oviduct that leads to either the vagina or directly to the female gonopore (Soos 1966a,b, Soos 1967a, Soos 1969a,b, Richardson 1969, Sawyer 1986a,b, Singhal and Davies 1985, Davies et al. 1985, Davies 1991, Govedich et al. 1998).
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