Leeches are a class of segmented worms closely related to aquatic earth worms that live primarily in freshwater, although some marine and terrestrial species exist. Leeches have a somewhat soft, very muscular body that is flattened from top to bottom. They range in length from 4-450 mm. Leeches have two suckers, one on each end of the body. The anterior (“head”) of the leech has multiple eyespots and a mouth located within its sucker. Leeches are hermaphroditic, but they only reproduce sexually. Because leeches undergo no metamorphosis as they age, juveniles can only be distinguished from adults by their smaller relative size.
Many people think of leeches as blood sucking parasites, but in fact most leeches are predatory. These leeches consume the body fluids of their small prey by either consuming it whole or piercing and killing it with specialized mouthparts. Predatory leeches may feed on other invertebrates such as insect larvae, earthworms, snails, and other leeches, while parasitic leeches feed on the blood of fishes, birds, amphibians, and mammals. Leeches are most common in protected, shallow areas of ponds, lakes, marshes, and slow sections of streams. Free-living leeches can usually be found on solid substrates such as cobble and vegetation. Because most leeches require a hard substrate to move using their suckers, they are rarely found in areas of soft substrate, such as silt. Parasitic leeches in search of a host are attracted to areas of disturbance in the water, making them a nuisance to noisy, splashing swimmers.
Like other worms, leeches absorb dissolved oxygen through the surface area of their entire body. They are moderately tolerant of pollution and low oxygen conditions. Although they can naturally be found at high density in optimum habitats, if they make up a large proportion of invertebrates in a water body it may be an indicator of low dissolved oxygen due to organic pollution.
Photo: A leech crawling on hard substrate (Placobdella sp.)
Source: The North American Benthological Society
Voshell, Jr., J. Reese; illustrated by Amy Bartlett Wright. 2002. A Guide to Common Freshwater Invertebrates of North America. Blacksburg (VA): The McDonald & Woodward Publishing Company. 442 p