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There are over 500 species of leeches throughout the world.
  • Leeches can be found everywhere except in Antartica.
  • They can be found in marine, estuarine, and freshwaters, and some are also terrestrial.
Leeches can be carnivores, detritivores, but are mostly parasitic.
  • Some parasitic species feed exclusively on invertebrate hosts, such as worms, other leeches, snails, and crustaceans, but most of them parasitize vertebrates (species that have backbones). Such vertebrate hosts can include snails, frogs, fish, turtles, birds, and mammals, including humans.
  • When feeding, the leech anchors to the host using suckers, and presses its mouth against the surface of the host's body. It releases an anesthetic as it makes its incisions in order to desensitize the victim's skin so that it can go unnoticed while it sucks blood from the host.

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  • Predatory leeches eat frequently, but those that feed on blood are more likely to feed at widely spaced, irregular intervals, depending on host availability.
  • Some predatory species can survive for well over a year without feeding. When they do feed, they consume several times their own weight in blood.
Leeches live in fresh water and are mainly surface dwellers; they move overtop the river bottom rather than through it.
  • They are most common in warm, protected shallow areas where currents are minimal and shelter is provided by plants, stones and debris.
  • Free-living leeches tend to avoid light and hide under stones or other objects, among aquatic plants, or in detritus (decomposing orgainc material).
  • Leeches cannot live on silted substrates because they cannot attach themselves to the bottom. 
  • In drought conditions, leeches can burrow into sediment and construct a mucus-lined cell, laying dormant until water returns.
Leeches are hermaphrodites (they have both male and female reproductive organs).
  • Female and male reproductive organs are not active at the same time in the life cycle, so the leeches must find partners with which to mate with. 
  • During mating, sperm is transferred from each individual to the other, and fertilization takes place within the female system of each Leech.
  • The sperm can also be stored for later use. 
  • Between two days and several months after mating, depending on the species, eggs are laid inside a cocoon in the female reproductive system.
  • The eggs hatch while still inside and the juveniles develop while feeding on the nutritive albumen within the cocoon. They are then released into the environment.
Leeches serve as food for some higher predators in the food chain.
  • Predators include fish, birds, snakes, amphibians, and to a lesser extent, insects and snails.
  • Leeches are a highly sought after snack for these predators so leech populations can be easily effected by increased numbers of predators.  
  • Medicinal leeches were used for centuries by doctors to control diseases that were believed to be caused by an excess of blood. Recently, surgeons have become interested in using leeches when trying to reattach severed limbs or digits (like legs or fingers). This is because leeches are able to do a better job of controlling swelling in the reattached limb.
  • Leeches can crawl and swim, but are not able to burrow. To crawl, leeches use their anterior and posterior suckers to alternately anchor themselves to the substrate.
  • Leeches don't need gills because they can absorb oxygen through their body walls.
  • The leech's saliva, which is released into the wound after the leech punctures the skin, acts as a painkiller; the leech has a better chance of having a good meal if it goes unnoticed.

Brusca, R. C, and Brusca, G. J., 2003. Invertebrates, 2nd ed. Sinauer Associates, Inc., Publishers, Massachusetts. Pages 404,409-410, 432-433.
Soil & Water Conservation Society of Metro Halifax (2006). Class Hirudinea.  Available here.
University of Michigan Museum of Zoology (2008). Subclass Hirudinea. Available here
Canada's Aqautic Environments (2002). Hirudinea. Available here.
Waterwatch South Australia (2004). Sponges, Hyrdas and Worms. Available here.