Leeches are annelids or segmented worms, and although closely related to the earthworms, are anatomically and behaviourally more specialised.
Leeches are segmented worms in the Subclass Hirudinea that are usually ectoparasitic. They belong to the Class Clitellata (along with earthworms, Subclass Oligochaeta) because of the presence of a clitellum, which is a swelling towards the head of the animal, where the gonads are located. However, unlike the oligochaetes, leeches do not show the clitellum all year. Instead, it only becomes visible during the breeding season.
Leeches are bilaterally symmetrical, with thick muscular bodies. Usually they are dorso-ventrally (front to back) flattened and segmented, though the segments are not often seen. Some leeches are long and worm-like, others pear-shaped and broad. Most can vary considerably in shape both between the elongated and contracted state and between the starved and full condition.
The body tapers towards the head and has a small oral sucker surrounding the mouth and a larger caudal (tail) sucker at the rear end, except the marine fish parasites, Pisciolidae, which have a larger oral sucker. The anus is on the dorsal surface (top) just in front of the rear sucker.
Euhirudinea ('true' leeches) have 32 internal segments when mature and Acanthobdellida (a small group of fish leeches) have 29, but counting is difficult because four to six segments are included in an front sucker and seven in a rear sucker, while the remaining segments are secondarily annulated (ringed) to give two to five apparent segments per internal septum (internal membrane).
Unlike other annelids, leeches do not have parapodia ('feet') or chaetae (bristles) (except for Acanthobdellida).
Leeches usually have three jaws and make a Y-shaped incision. The Australian land leech has only two jaws and makes a V-shaped incision.
Most leeches are freshwater animals, but many terrestrial and marine species occur.
Land leeches are common on the ground or in low foliage in wet rain forests. In drier forests they may be found on the ground in seepage moistened places. Most do not enter water and cannot swim, but can survive periods of immersion.
In dry weather, some species burrow in the soil where they can survive for many months even in a total lack of environmental water. In these conditions the body is contracted dry and rigid, the suckers not distinguishable, and the skin completely dry. Within ten minutes of sprinkling with a few drops of water, these leeches emerge, fully active.
Freshwater leeches prefer to live in still or slowly flowing waters, but specimens have been collected from fast flowing streams.
Some species are considered amphibious as they have been observed in both terrestrial and aquatic habitats.
There are around 500 species of leeches world wide. These are divided into two major infraclasses
- Euhirudinea: the 'true' leeches – marine, freshwater and terrestrial – which have suckers at both ends and lack chaetae (bristles)
- Acanthobdellida: a small northern hemisphere infraclass ectoparasitic on salmoniid fish, which lack an anterior sucker and retain chaetae.
The Euhirudinea is further divided into two orders:
- Rhynchobdellida: jawless marine and freshwater leeches with a protrusible proboscis and true vascular system
- Arynchobdellida: jawed and jawless freshwater and terrestrial leeches with a non-protrusible muscular pharynx and a haemo-coelomic system.
Leeches can be found almost anywhere in Australia where there are suitable damp areas and watercourses although they are absent from the permanently arid areas. There are even marine leeches, but these feed on the blood of fishes (including the Electric Ray with its fearsome electric shocking abilities) and other marine life – not humans.
Feeding and diet
Most leeches are sanguivorous, that is they feed as blood sucking parasites on preferred hosts. If the preferred food is not available most leeches will feed on other classes of host. Some feed on the blood of humans and other mammals, while others parasitise fish, frogs, turtles or birds. Some leeches will even take a meal from other sanguivorous leeches which may die after the attack.
Sanguivorous leeches can ingest several times their own weight in blood at one meal. After feeding the leech retires to a dark spot to digest its meal. Digestion is slow and this enables the leech to survive during very long fasting periods (up to several months).
Leeches are grouped according to the different ways they feed:
- One group (the jawed leeches or Gnatbobdellida) have jaws armed with teeth with which they bite the host. The blood is prevented from clotting by production of a non-enzymatic secretion called hirudin. The land leech commonly encountered by bushwalkers is included in this group.
- A second group (the jawless leeches or Rhyncobdellida) insert a needle-like protrusion called a proboscis into the body of the host and secrete an enzyme, hemetin which dissolves clots once they have formed. Leeches which live on body fluids of worms and small freshwater snails possess such an apparatus.
- A third group, (the worm leeches or Pharyngobdellida) have no jaws or teeth and swallow the prey whole. Its food consists of small invertebrates.
A hungry leech is very responsive to light and mechanical stimuli. It tends to change position frequently, and explore by head movement and body waving. It also assumes an alert posture, extending to full length and remaining motionless. This is thought to maximise the function of the sensory structures in the skin.
In response to disturbances by an approaching host, the leech will begin 'inchworm crawling', continuing in a trial and error way until the anterior sucker touches the host and attaches. Aquatic leeches are more likely to display this 'pursuit' behaviour, while common land leeches often accidentally attach to a host.
Other behaviours and adaptations
Respiration takes place through the body wall, and a slow undulating movement observed in some leeches is said to assist gaseous exchange. Aquatic leeches tend to move to the surface when they find themselves in water of low oxygen content. As a fall in atmospheric pressure results in a small decrease in dissolved oxygen concentrations, rising leeches in a jar of water provided nineteenth century weather forecasters with a simple way of predicting bad weather.
Sensory organs on the head and body surface enable a leech to detect changes in light intensity, temperature, and vibration. Chemical receptors on the head provide a sense of smell and there may be one or more pairs of eyes. The number of eyes and their arrangement can be of some use in Identification, however to properly identify a leech, dissection is required.
The Rhyncobdellids are capable of dramatic colour changes but this is apparently not an attempt at camouflage, and the significance of this behaviour is unknown.
Leeches move by either an undulating swimming motion (eel-like) or by an 'inch-worm' like crawling motion using the anterior and posterior suckers. The posterior sucker is attached to a substrate and the leech stretches out and attaches to the substrate with the anterior sucker, the posterior sucker is then detached and pulled up to the anterior sucker.
As hermaphrodites, leeches have both male and female sex organs. Like the earthworms they also have a clitellum, a region of thickened skin which is only obvious during the reproductive period. Mating involves the intertwining of bodies where each deposits sperm in the others' clitellar area. Rhyncobdellids have no penis but produce sharp packages of sperm which are forced through the body wall.
The sperm then make their way to the ovaries where fertilisation takes place. The clitellum secretes a tough gelatinous cocoon which contains nutrients, and it is in this that the eggs are deposited.
The leech shrugs itself free of the cocoon, sealing it as it passes over the head.
The cocoon is either buried or attached to a rock, log or leaf and dries to a foamy crust. After several weeks or months, the young emerge as miniature adults. Studies show that the cocoons are capable of surviving the digestive system of a duck.
Leeches die after one or two bouts of reproduction.
Medicinal use of leeches
For over 2000 years, leeches were needlessly applied for many ailments as an adjunct to blood letting. Their use in Europe peaked between 1830 and 1850, but subsequent shortages led to a decline in their use. Today there is a real clinical application in that they are of great value to plastic surgeons when venous congestion of skin and muscle flaps is a problem.
Leeches are treated in the same way as blood products and are reused only on the same patient.
Medical use of leeches also includes treatment of black eyes, and hirudin is used in the treatment of inflammation of the middle ear. Hirudin is also being developed for experimental use as a systemic anticoagulant, and may prove useful in invitro blood sampling.
Leeches in Sydney suburbs
The Sydney suburban sprawl is resulting in houses extending into areas such as wet valleys that leeches normally prefer. It is therefore less distance for these leeches to travel in wet weather before they end up in backyards and can get a meal from domestic animals or humans living there. Similarly the domestic animals and humans themselves unwittingly bring many leeches home with them from bush walks, holidays, and other travels. If these leeches are adult they will invariably find a suitable damp spot in the garden to lay their eggs and suddenly you have a colony of leeches in your backyard.
Fish, birds and other invertebrates are the main predators on leeches.
The most common enquiry regarding leeches concerns repellents. It is unknown whether a specific preparation is commercially available but there is a plethora of tried and tested, but unproven leech-protection ideas. These include a lather of bath soap smeared on exposed parts and left to dry, applications of eucalyptus oil, tropical strength insect repellent, lemon juice and impenetrable barriers of socks and pantyhose.
Danger to humans
The presence of hirudin in the wound following a leech bite may cause oozing to continue for several hours. Although inconvenient, blood loss is not significant.
Gut bacteria can cause wound infection. In the post-operative use of leeches this is closely monitored and dealt with by use of the appropriate antibiotic.
There may also be a delayed irritation and itching after a bite. There appears to be no support for the theory that mouthparts left behind after forced removal of the leech causes this reaction.
Can leeches transmit disease?
There is no evidence to suggest that they do. The presence of trypanosomes, (malarial parasites), in the gut of jawless leeches has been noted, but jawed leeches do not appear to be hosts.
Allergy to leech bite has been reported. Medical opinion should be sought, depending on the severity of the reaction.