Fish parasite assemblages from different areas and environments were examined in search for significant patterns related to parasite host specificity. Ecological niche models were used to test the relevance of ecological factors in the qualitative determination of the parasite species assemblages on host species. The host/parasite cospeciation paradigm was discussed, with the proposal of a theoretical framework aimed to integrate the role of coevolutionary and ecological factors in the determination of parasite host range. Host specificity was investigated through different statistical techniques. A new protocol to assess relative nestedness within a species assemblage was developed. Relative nestedness of parasite taxa was then proposed as a measure of their intrinsic host specificity. Trophic ecology of fish hosts resulted to be a key factor in determining the qualitative composition of parasite assemblages. The fundamental ecological and evolutionary role played by contactability (the reciprocal potentiality of a host and a parasite to come in contact) provided new insights on the traditional theoretical paradigm stating that host range of a parasite species results from the intersection between the set of compatibile host species and that of encountered ones.

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