Leeches are segmented worms with suction cups at each end. Their bodies are flattened, much wider than they are thick. They are usually dark colored, often brown or sometimes black or dark green. Some species have no markings, others have spots and stripes. The smallest leeches grow no more than 5 mm, but some big species may get to be more than 25 cm long. Many leech species have one or more pairs of eyes visible on the top of their front end. Leech species that suck blood have sharp teeth. Predatory species may have teeth, or may have only crushing jaws.
Leeches are found all over the world, and there are hundreds of species. In Michigan there are at least 40 species, and probably more.
Most leech species are found in shallow, slow-moving freshwater, but some live in the oceans, and a few live in moist soil on land.
Leeches lay eggs in cocoons, and the babies that hatch out look and behave like little adults. They don't change much as they grow, they just get bigger. Leeches that live in habitats that freeze or dry out during part of the year bury themselves in mud and stay dormant until the habitat improves. Leech growth rate is strongly affected by temperature and food supply. Most species can mature in a few weeks or months if conditions are good.
Some leeches complete their life cycle in a few months, but many can live for several years.
All leeches can crawl, and some are good swimmers. They search for prey by following the scent or touch of the animals they want to eat. When they first detect food, they extend their bodies and hold very still, probably to carefully sense their prey.
Leeches have very poor vision (often they can only tell the strength of light), but are very sensitive to touch. They also have a strong sense of taste. They cannot hear, but are sometimes very sensitive to vibrations. They communicate with other leeches chemically, and by touch.
Leeches are famous as blood-suckers. The species that feed on blood have special chemicals in their saliva that prevents blood-clotting. Many blood-feeding leeches attack only fish, a few attack any vertebrate (including people), and a few are specialists on another group of animals, like turtles or waterbirds. There are also lots of leech species that don't suck blood. They are predators, eating worms, snails, aquatic insects, and other invertebrates.
Most leeches hide while resting, staying in thick plant growth or hiding in mud. Many leech species are nocturnal, this helps them avoid predators and locate resting prey. If attacked, some species swim away as fast as they can, others go limp and "play dead," others curl into a ball and sink to the bottom. When parasitic leeches attach to their host, they sometimes select places that are hard for their host to reach
Leeches are sometimes important members of aquatic food webs. They are mid-level consumers, eating smaller animals and in turn being eaten by larger predators.
Many leech species are parasites feeding on blood. Some will bite humans. Others may attack domesticated animals that are valuable to humans, such as ducks or fish.
Leeches are used in some medical procedures to remove excess blood from body parts that have been reattached after injury. Also, chemicals that leeches use to prevent blood-clotting have proved to be valuable medicines.
Most species of leeches are not in danger of extinction. Only one species, the medicinal leech Hirudo medicinalis, has been given legal protection. This species was found throughout Europe and western Asia, but has been so heavily collected for medical use and research that it is extinct in many parts of its range. It is listed in Appendix II of the CITES Treaty, and is listed as "Lower Risk" by the IUCN.