| Summary: | The parasitic copepod family Ergasilidae currently comprises 26 genera and more than 180 species, the great majority of which utilises marine, brackish and freshwater fishes as hosts. Thirty-three species of Ergasilidae were obtained from examination of the gills of more than 3000 grey mullet preserved in the collections of the Natural History Museum. These species represent nine current genera of the family Ergasilidae: Acusicola, Dermoergasilus, Diergasilus, Ergasilus, Paraergasilus, Nipergasilus, Paeonodes, Mugilicola and Therodamas. Complete descriptions of twenty-two new species and redescriptions of five existing species are given in the taxonomic part. The cosmopolitan species E. lizae has been erroneously identified by many authors, therefore the type material was redescribed giving new details that have significant taxonomic value and have been overlooked in previous descriptions. Subsequently a group of closely related species, including four new species was recognized and is referred to as the E. lizae-complex. The antenna of Ergasilidae is modified as an attachment organ securing the parasite to its host. The examination and description of antennae of 26 species representing virtually all of the 26 genera included in the family, provided information on the functional morphology and the homology of antennal segments. The results revealed that the antenna of Ergasilidae is 4-segmented plus a curved claw, and that the third endopodal segment was previously overlooked by most authors. The different attachment mechanisms are discussed in relation to the antennal structure in the Ergasilidae. The phylogenetic relationships between the members of Ergasilidae were analysed using cladistic techniques. A character matrix initially comprising 96 characters was constructed from the literature, for most of the species. The results of this analysis suggest that the existing system of four subfamilies is untenable and several of the existing genera are recognized as terminal apomorphies in long lineages, or are paraphyletic. The biogeography of grey mullet hosts and the distribution patterns of the recorded ergasilids were analysed. The host specificity of each recorded species was also examined and it was concluded that many ergasilids exhibit a relatively low level of host specificity, with species tending to occur on a variety of hosts found in particular habitats. The co-evolutionary history of the Ergasilidae and the Mugilidae was examined in an attempt to examine the relative importance of co-evolution and colonization as processes influencing host specificity. A minimum of fifteen colonization events by ergasilids of mugilids as hosts was documented. This suggests that any basic coevolutionary pattern will have been greatly modified by colonization events (shifts in host groups).
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