Leeches are segmented worms belonging to the Class Hirudinea, which is a subclass within the Phylum Annelida, the same group that includes earthworms. These organisms are found across the globe on every continent except Antarctica. While the most familiar species are freshwater dwellers, a smaller number exist in marine, estuarine, and moist terrestrial environments. North America north of Mexico is home to a significant diversity of these invertebrates, with at least 81 nominal species cataloged across eight families.
Preferred Aquatic Environments
Leeches are most commonly encountered in freshwater habitats that offer still or slow-moving conditions, such as ponds, marshes, ditches, and the sheltered coves of larger lakes. These environments typically have minimal water current, allowing the leeches to maintain their position and conserve energy. They tend to congregate in the shallow, protected areas where water temperatures are warmer and light penetration is low.
The presence of a stable physical substrate is necessary for leeches, as they use their anterior and posterior suckers to anchor themselves and move across surfaces. They are often found clinging to the underside of stones, submerged logs, boards, or masses of aquatic vegetation and organic debris. Conversely, leeches generally avoid areas with soft, shifting bottoms like pure silt, because they cannot effectively attach themselves. When water bodies dry up during a drought, some species burrow into the sediment and enter a dormant state, surviving until the water returns.
Regional Concentrations in the United States
The distribution of leeches across the United States is not uniform, with concentrations correlating strongly to the availability of ideal aquatic environments. The eastern half of the country, particularly the Midwest and Great Lakes region, supports some of the densest and most diverse leech populations. This area features expansive wetlands, numerous shallow lakes, and slow-moving river systems that provide continuous, favorable habitat.
For example, the North American medicinal leech, Macrobdella decora, is widely distributed across North America east of the Rocky Mountains, centered around the Great Lakes and extending into the northern US. Moving south, the extensive swamps and wetlands of the Southeast, including Florida and the Gulf Coast, also maintain high leech populations, with species adapted to these warmer, often brackish waters. The common North American turtle leech, Placobdella parasitica, is notably abundant throughout the eastern and north-central states.
In contrast, the leech populations in the Western states are generally less concentrated and diverse. The Rocky Mountains serve as a natural barrier to the westward expansion of many common eastern species. However, leeches do exist in western aquatic systems, including species found in the Colorado River basin and the San Francisco Bay watershed. These populations are often found in isolated water bodies or are the result of species introduced via game fish stocking from eastern regions.
Identifying Common US Species
Leeches found in the United States display a variety of forms and feeding habits, grouped into two main categories: predatory and parasitic. Many species are predators or scavengers, such as those in the Erpobdellidae family. These types consume small invertebrates like insect larvae and worms whole and lack the specialized jaws used for piercing skin.
The parasitic species, which feed on blood or tissue fluids, utilize specialized mouthparts for attachment to a host. Species like Macrobdella decora are characterized by three internal jaws used to make an incision and are often medium-sized, exhibiting a greenish-brown back with spots. Placobdella species are typically dorso-ventrally flattened and are commonly ectoparasites of aquatic vertebrates like turtles.
Regardless of their feeding strategy, all leeches in the US are segmented, muscular, and possess suckers at both the anterior (head) and posterior ends. While many are small, ranging from half an inch to two-and-a-half inches, some US species can reach up to ten or even twenty inches when fully extended. Their coloration is highly variable, often featuring black, brown, or olive hues, with some displaying speckled or striped patterns.