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Soil & Water Conservation Society of Metro Halifax (SWCSMH)
UpdAted: October 09, 2013
The class Hirudinea in the phylum Annelida (segmented worms) comprises the leeches, the most highly specialized of the major annelid groups.
Leeches are typically dorsoventrally flattened annelids with suckers at both ends and 34 body segments (designated I-XXXIV) which are externally divided into a number of annuli. Most species in North America are found in fresh and marine waters, but many terrestrial species occur in tropical regions. As predators, parasites of animals, vectors of parasites, and as food for semiaquatic and aquatic animals, leeches are important components of food webs. In northeastern North America, there are about 42 species. Leeches are hermaphroditic but do not self-fertilize.
The class Hirudinea comprises two orders: Arhynchobdellida and Rhynchobdellida:
- The arhynchobdellids are divided into three families: Haemopidae, Hirudinidae, and Erpobdellidae. The haemopids and hirudinids have relatively large mouths, occupying the entire cavity of the oral sucker. The haemopids are chiefly aquatic or amphibious, are good swimmers, and are considered blood-sucking or predaceous leeches. The hirudinids are aquatic, are also good swimmers, and are considered truly sanguivorous leeches. The erpobdellids have medium-sized mouths that occupy the entire cavity of the oral sucker. The erpobdellids are strictly aquatic, are good swimmers, and prey on small invertebrates.
- The rhynchobdellids are strictly aquatic leeches that have small, porelike mouths in the oral sucker. The families Glossiphoniidae and Piscicolidae belong to this order. The glossiphoniids have three annuli on each body segment. Many are ectoparasites on both invertebrates and vertebrates, and some forms are predaceous on invertebrates. The piscicolids usually have more than three annuli on each body segment and are parasites of many fishes and rarely of crustaceans.
Leeches are most common in warm, protected shallows where there is little disturbance from currents. Free-living leeches avoid light and generally hide and are active or inactive under stones or other inanimate objects, among aquatic plants, or in detr itus. Some species are most active at night.
Silted substrates are unsuitable for leeches because they cannot attach.
Leeches are usually rare in calcium-poor waters. Some species can tolerate mild pollution.
Leeches are most common in warm, protected shallows where there is little disturbance from currents. Free-living leeches avoid light and generally hide and are active or inactive under stones or other inanimate objects, among aquatic plants, or in detritus. Some species are most active at night. Very rarely are leeches which attach to humans encountered in fast moving water or riffle areas. Many are scavengers or feed on other invertebrates. They are carnivorous, feeding mostly on insects, molluscs and oligochaetes, or scavengers, feeding on dead animal matter.
- Silted substrates are unsuitable for leeches because they cannot attach. Leeches are usually rare in calcium-poor waters. The cannot tolerate high turbidity loading as well. Some species can tolerate mild pollution (Kellogg, 1994; and Mackie, 2001).
- The suckers located at both ends are used for attachment, feeding and locomotion.
- Leech abundance is highly variable, but generally increases in more productive fresh waters.
- Most species are found in waters with pH>7.0 and a total alkalinity>60 mg CaCO3/L. Only the highly tolerant indicator species, such as H. stagnalis and C. complanata, are found in waters with pH<6.0.
- Indeed, the tolerance of leeches to many chemicals makes it difficult to discourage their presence by bathers (Mackie, 2001).
Micrograph of a leech:
References and web URLs:
- Kellogg, L.L. 1994. Save Our Streams. Monitor's Guide to Aquatic Macroinvertebrates. Second Ed. Izaak Walton League of America. 60p.
- Mackie, G.L. 2001. Applied Aquatic Ecosystem Concepts. Kendall/Hunt Publishing Company. xxv, 744 pp. ISBN 0-7872-7490-9
- Pennak, Robert W. 1978. Fresh-Water Invertebrates of the United States. Second Edition. John Wiley & Sons. ISBN: 0-471-04249-8. xviii, 803p.
- Peckarsky, B.L., P.R. Fraissinet, M.A. Penton, and D.J. Conklin, Jr. 1990. Freshwater Macroinvertebrates of Northeastern North America. Cornell Univ. Press. xii, 442pp.
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