Freshwater leeches live everywhere in the world except Antarctica, and marine leeches live in all the oceans. Most leeches are found in fresh water, such as streams, rivers, ponds and marshes, but some species thrive in salt water. Some live on land in moist, muddy areas.
During the day, leeches most often hide from fish, birds and other predators amid underwater foliage or burrowed in the mud. They tend to be most active at night, when they venture out to find their prey. They can crawl and are good swimmers, so they can be encountered in virtually any body of water.