We analysed the association between a monogenean (Udonella cf. caligorum Johnston 1835) and its copepod host (Caligus sp.) living on a wild population of Arius herzbergii Bloch, 1794 in a north-eastern coastal lagoon from Venezuela. This study characterized infestation levels and analysed the effects of monogeneans on the fecundity and hatching success of the copepod host, as well as damage to its egg capsules and genital complex. A total of 218 Caligus specimens were analysed (94 males, 110 females and 14 immature stages) in which a total of 1017 monogeneans were found. These included 311 mature stages and 706 egg capsules. Monogenean stages were found attached to the cephalothorax, abdomen, genital complex and egg capsules of the copepods. No significant differences were found in fecundity and egg hatching when infested and non-infested ovigerous females were compared. No damage was observed on egg capsules or genital areas of infested ovigerous females. Our results suggest that this association, at the level of prevalence and intensity observed, is closer to commensalism than parasitism. The importance of considering that the nature of interaction is dynamic and changing with environmental conditions and time scale is highlighted.