The piscicolids usually have more than three annuli on each body segment and are parasites of many fishes and rarely of crustaceans. Habitat Leeches are most common in warm, protected shallows where there is little disturbance from currents. Free-living leeches avoid light and generally hide and are active or inactive under stones or other inanimate objects, among aquatic plants, or in detr itus. Some species are most active at night. Silted substrates are unsuitable for leeches because they cannot attach.
Leeches are usually rare in calcium-poor waters.
Some species can tolerate mild pollution. References Kellogg, L.L. 1994. Monitor's Guide to Aquatic Macroinvertebrates. Izaak Walton League of America . Peckarsky, B.L., P.R. Fraissinet, M.A. Penton, and D.J. Conklin, Jr. 1990.