ABSTRACTThe abundance of six species of trematodes: Aphanurus stossichi, Bacciger israelensis, Diphterostomum israelense, Plagioporus idoneus, Lepocreadium album and L. pegorchis, parasitic in the digestive tract of marine teleostei (Sparidae) collected near Jounieh (east Mediterranean), was analysed as a function of the host-size. In two parasite/host systems, infections were observed from the lowest size classes of the sample, with a clear tendency to an increase of abundance in older fish. In four others, parasites appear only above a rather high threshold class, young individuals never being infected. In the last three parasite/host systems, host invasion may occur early or late, but infection decreases above a well defined size class, old fishes rarely or never being infected. A given trematode species. when parasitizing several host species, shows similar abundance/host size relationships, e.g. P. idoneus in Diplodus vulgaris and Oblada melanura. When more than one species of trematode infects a single host species, curves can be markedly distinct; for instance, L. pegorchis was collected from Pagellus erythrinus below 15 cm. whereas D. israelense parasitized the same fish approximately above the same size. There is no evidence that such a replacement of one trematode by another in the course of host growth is a result of interspecific competition.
