Leeches are segmented worms belonging to the subclass Hirudinea, closely related to earthworms. Nearly 700 species are described worldwide, possessing soft, muscular bodies specialized for diverse feeding methods. While many species are hematophagous (blood-sucking), others are active predators, consuming small invertebrates whole. Their habitats span from freshwater bodies and high-humidity forests to the deep sea.
Common Freshwater Habitats
The majority of leech species (approximately 480) reside within freshwater systems. These environments are characterized as lentic (still waters like ponds and lakes) or slow-moving lotic systems (streams and rivers). Leeches seek shallow, warm, and protected areas where the water current is minimal, creating ideal ambush locations.
Leeches thrive in areas with abundant aquatic plants, submerged logs, and debris, which provide shelter and a solid substrate for movement. They use their suckers to move in an inchworm-like looping motion across the bottom or vegetation, avoiding silted substrates where they cannot anchor effectively. This preference for stable environments with high moisture retention explains their abundance in marshes, swamps, and the edges of shallow lakes.
In times of drought, some freshwater species burrow into the sediment and create a mucus-lined cell. Within this protective structure, the leech can enter a dormant state, or aestivate, sometimes surviving for over a year without a meal until the water returns. This physiological tolerance allows them to persist even in temporary bodies of water that periodically dry up.
Land and Sea Habitats
Leeches are not confined to aquatic environments; about 90 species have adapted to terrestrial life, and 100 species inhabit the oceans. Terrestrial leeches, primarily from the family Haemadipsidae, are common in the damp, high-humidity forests of tropical and subtropical zones. They require constant moisture to prevent desiccation, often inhabiting leaf litter, mossy areas, or dense vegetation.
These land-dwelling species employ specialized hunting strategies to locate hosts, using receptors that detect body heat, vibration, and carbon dioxide in breath. Some terrestrial leeches, particularly those found in the Indo-Pacific rainforests, climb trees, waiting on low-hanging leaves to latch onto passing mammals or birds. They use their anterior and posterior suckers to move across the ground in a distinct looping crawl, actively seeking a blood meal.
In the marine environment, saltwater leeches are typically ectoparasites, meaning they live on the outside of their host. They are often found in shallow coastal waters, lagoons, and estuaries. The family Piscicolidae, for example, is primarily parasitic on fish, attaching to the skin or gills. Other marine species target sea turtles, and some have been discovered in extreme environments, such as deep-sea hydrothermal vents, demonstrating adaptability to pressure and temperature extremes.
Global Geographical Distribution
Leeches have a cosmopolitan distribution, meaning they can be found on every continent except Antarctica, which lacks the required terrestrial and freshwater habitats. Their highest concentrations and greatest diversity occur in tropical and subtropical regions. The consistently warm temperatures and high moisture levels in these zones support the large number of both aquatic and terrestrial species.
Temperate zones, particularly in the Northern Hemisphere, also contain a significant number of species, with lakes and ponds being key hotspots for abundance. For example, some species of the aquatic family Hirudinidae have a wide global range that includes these temperate regions. The sheer variety of habitats, from the Amazon’s vast rivers to the humid forests of Southeast Asia and the temperate lakes of North America, demonstrates the ecological versatility of this segmented worm.