ABSTRACT
Helminth parasites of fishes comprise several thousands of described species belonging to well-characterized taxonomic entities such as nematodes, acanthocephalans, trematodes (digeneans and aspidogastreans), cestodes and monogeneans. Thus, it has been estimated that a total of more than 30 000 helminth species parasitizing fish have been described (Williams and Jones, 1994). Trematodes, cestodes and monogeneans have traditionally been grouped together as platyhelminths based on their external morphology, the conspicuous slightly dorsoventrally flattened body. Among these flatworms the monogeneans depart due to their outstanding morphology and ecology. These worms are mostly ectoparasites with preferences for fins, body skin, gills, gill chamber, buccal cavity, cornea or nostrils.
