The Piscicolidae are a family of jawless leeches in the order Rhynchobdellida that are parasitic on fish. 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. Some of the leeches in this family have external gills, outgrowths of the body wall projecting laterally, the only group of leeches to exchange gases in this way. Worldwide, there are around sixty genera and one hundredspecies of leech in this family, all parasitic on the blood of marine, estuarine and freshwater fishes. These leeches are less common in the tropics and are more abundant in temperate and polar waters.
Taxonomy
Historically, Piscicolidae has been divided into three subfamilies; Pontobdellinae, characterised by two pairs of pulsatile vesicles on each urosome segment, Pontobdellinae, characterised by a single pair of pulsatile vesicles on each urosome segment and Platybdellinae, with no pulsatile vesicles. However, molecular phylogenetic analyses performed by Williams and Burreson in 2006, does not support these subdivisions. Piscicolidae was confirmed as being monophyletic, but Platybdellinae was shown to be polyphyletic with four distinct clades, Piscicolinae was similarly polyphyletic, again with four distinct clades, and Pontobdellinae was paraphyleticwith respect to the genus Oxytonostoma.
Ecology
The Piscicolidae are parasitic on the blood of fishes. Some species are host specific while others can accept a larger range of hosts. Some, such as the European Piscicola geometra, take a blood meal and then drop offthe host and hide somewhere while they digest the blood; 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 semi-permanently. 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 approximately one week to find a suitable fish host for itself.
