Medicinal leech - Hirudo medicinalis Linnaeus, 1758
Classification
Kingdom: AnimaliaPhylum: Annelida
Class: Clitellata
Order: Arhynchobdellida
Family: Hirudinidae
Genus: Hirudo
Species: H. medicinalis
Geographical Distribution
Once abundant in Europe from Ireland to the Ural mountains (western Russia) and from southern Scandinavia to the Mediterranean. It is still present in one or more localities in 24 European countries although it is threatened in at least 12 of these. Its status is uncertain in Portugal, Sicily and Turkey and it has been extinct in Ireland for at least 100 years. It was imported into North America for medicinal purposes but no recent records have shown its presence in the wild. Present in small numbers in Britain particularly in rural areas such as New Forest, the Lake District, South Wales, Anglesey and the west of Scotland. The largest population in Britain is thought to be in Dungeness, Kent where a mark-release-recapture experiment indicated a self-sustaining population of 6,000 - 12,000 leeches in 1985.Ecological and Habitat Requirements
Hirudo medicinalis typical habitats consist of shallow,
eutrophic ponds or lakes with dense stands of macrophytes and high
summer temperatures. High temperature requirements have important
implications for the distribution of the medicinal leech, for example,
it cannot survive in many lakes and most of the tarns in the Lake
District due to the relatively low temperatures. The average
temperatures for 10%, 50% and 90% activity in the medicinal leech are
11.9˚C, 19.0˚C and 22.9˚C respectively. The minimum temperature
threshold for swimming activity is 5-9˚C. In a temperature gradient of
7-43˚C, studies have shown that 21˚C is the preferred temperature. The
optimum temperature ranges for growth and breeding are 22-25˚C and
25.5-27.5˚C respectively with the upper lethal range being 39-43.5˚C.
Food and Feeding
H. medicinalis is the only species known to attack humans.
Like other Gnathobdellid leeches, it has a short muscular pharynx
usually with toothed jaws. It feeds almost exclusively on the blood of
mammals but other hosts include amphibians and small fish. A single
leech can take two to five times its own weight of blood in a meal,
which is digested slowly over several months. For example, a leech
weighing 128mg dry weight took 640mg dry weight of blood in one meal,
took about 200 days to digest this meal and survived for a further 100
days without another meal. Therefore, one meal can sustain a leech for a
year. The bacterium Aeromonas hydrophila is a normal symbiont
in the gut of medicinal leeches which produces enzymes necessary to
digest the blood ingested by the leech. Although it takes only about
half an hour for a leech to become satiated on a mammalian host, the
host can move a considerable distance in this time and therefore it is
thought that in nature, leeches frequently detach from the host long
before they are satiated to ensure they do not become isolated from
their habitat.
Life Cycle Summary
Leeches are protandrous hermaphrodites and tend to cross fertilise by taking up a head-to-tail position with another leech. H. medicinalis has an eversible penis, a single male and a single female pore, a single pair of ovaries and between 10 and 100 pairs of testes. Matings occur in summer and sperm can be stored so there can be a delay of 1-9 months between copulation and cocoon deposition. Segments near the middle of the body are modified in mature worms to form a clitellum that secretes a cocoon for the eggs. Each sclerotized cocoon is about 10mm long, either oval or spherical in shape with the wall consisting of an inner smooth layer and an outer spongy layer and contains between 5 and 15 eggs. They are laid in a damp place just above the water line on the shore or bank of lakes or ponds. Cocoons are normally found in July and August and hatching time varies from 4 to 10 weeks depending on temperature. In laboratory conditions, H. medicinalis lay 1-7 cocoons in which 3-30 eggs develop and each individual can produce 2 broods per year under optimum conditions. Newly hatched leeches weigh approximately 0.12-0.18g live weight and this increases to 0.5-0.6g by end of first year, about 1.4g in second year and 2.4g in third year. It is thought that they take at least 2 years to reach sexual maturity in the field and slow-growing leeches may not breed until 3 or 4 years of age.Life Stages
Life Stage: Adult
Definitive Host: Mammals (including humans), amphibians and fish.
Site Within Host: Ectoparasitic.
Host Habitat: Freshwater ponds and lakes.
Free Living Environment: Freshwater ponds and lakes (also comes onto the shore to lay cocoons containing eggs).
Reproduction Capacity: Sexually mature.
Morphology: Red-yellow longitudinal stripes on the dorsal surface forming part of a fairly ornate pattern, anus small and scarcely visible, each jaw is armed with a single row of numerous sharp teeth capable of piercing human skin.
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