Of the 650 species of leeches that exist in North America, some 500 inhabit fresh water. Most are predacious, feeding on worms, snails, fish eggs, and aquatic insects. Some are scavengers. Others survive by feeding on the blood of freshwater vertebrates, including the turtle in this picture.
Generally speaking, leeches of the genus Placobdella are found on turtles. Bottom-dwelling species such as the common snapping turtle, eastern mud turtle, and musk turtle are especially vulnerable; aerially basking species, such as painted turtles, have fewer of these parasites, probably because basking forces leeches to detach to avoid drying out.
A leech can ingest several times its weight in blood from one host before dropping off and not feeding again for weeks, or even months. Leeches inject hirudin, an anticoagulant, to keep the turtle’s blood from clotting.
Leech predators include birds and fish, and turtles sometimes enlist their help to groom the leeches off. Map turtles allow common grackles to land in basking areas and peck at the leeches that cling to their skin, and minnows have been seen cleaning leeches from wood turtles in the water. There have even been reports of turtles burying themselves in ant mounds to rid themselves of these pesky parasites.