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Piscicolidae is a family of aquatic leeches in the order Rhynchobdellida (jawless leeches that feed with a proboscis). This family is often referred to as fish leeches, as most species specialize as fish parasites. Piscicolidae are often found in abundance on their hosts, with most species remaining attached to hosts for long periods of time.

Host Specificity
Many Piscicolidae have strong host specificity, with several lineages specialized in elasmobranchs and others on teleost fish. Subfamily classification has mirrored this host specificity and is supported by several morphological trails, but recent phylogenetic studies based on genetic and morphological data reveal all three subfamilies (Piscicolinae, Platybdellinae, and Piscicolinae) form non-monophyletic groups. Accordingly, use of these subfamily classifications should be abandoned.

Not all Piscicolidae have strong host specificity. Most species in the subfamily Piscicolinae do not have strong host specificity within teleost fish species. Many of these species are important vectors of haematozoa. There are also a few Piscicolidae species that either do not only feed on fish or exclusively feed on other organisms, such as marine isopods, mysid shrimp, and octopi.

Anatomy
Many Piscicolidae leeches are triannulate, yet, interestingly, maturing Piscicolidae continue subdividing annuli (unlike most other leeches). Still, Williams and Burreson found annulation patterns to be consistent enough to use in a morphological matrix for phylogenetic analysis of the family. For Rhynchobdellida leeches, Piscicolidae tend to have short proboscises. This likely relates to their affinity towards parasitizing fish; a fish specializing Glossiphoniidae also shares this trait.

Habitats
Piscicolidae leeches are found in a variety of aquatic habitats. Most species are marine; however, brackish water, estuaries, and freshwater also host members of the family. In marine environments, at least one species was found in abundance at a depth of approximately 600 m.

Locomotion
Despite the aquatic nature of most Piscicolidae, the majority of species do not swim – most move along hard surfaces in a fashion similar to inchworms. Yet, Mysidobdella borealis, a mysid shrimp specialist, is an excellent swimmer, with higher swimming stamina than other leeches.

--Mtessler1 (talk) 00:13, 3 December 2013 (UTC)