They occur in both freshwater and seawater, have cylindrical bodies, and typically have a large, bell-shaped, anterior sucker with which they cling to their host.

[3] Worldwide, around 60 genera and 100 species of leeches are in this family, all parasitic on the blood of marine, estuarine, and freshwater fishes.

Some, such as the European Piscicola geometra, take a blood meal and then drop off the host and hide somewhere while they digest the blood;[8] some, mostly in marine or estuarine habitats with soft substrates, attach themselves temporarily to a crustacean after leaving their fish host; others remain attached to a fish semipermanently.

Leeches are hermaphrodites, and mating may take place on or off the fish host, but in either case, the cocoon, usually containing a single egg, is deposited elsewhere, usually stuck to a stone or piece of vegetation, or even to the carapace of a crustacean.

When the egg hatches, the juvenile leech has about a week to find a suitable fish host for itself.