Between May and September 2006, 640 specimens of the rabbitfish Siganus rivulatus Forsskål (Teleostei, Siganidae) were examined for infections with intestinal helminths. These fishes were caught in the Red Sea off the coast of Sharm El-Sheikh, South Sinai, Egypt, examined in a field laboratory and separated into three size groups of regular length intervals. Only three species of helminths were recovered: the trematode Gyliauchen volubilis Nagaty, 1956 (Gyliauchenidae Fukui, 1929), the acanthocephalan Sclerocollum rubrimaris Schmidt et Paperna, 1978 (Cavisomidae Meyer, 1932) and the nematode Procamallanus elatensis Fusco et Overstreet, 1979 (Cucullanidae Cobbold, 1864). The distribution of these helminths along the intestine of S. rivulatus, in all patterns of single and concurrent infections and the corresponding prevalences and intensities of infection in the different size groups of the fish were recorded. In single infections, each parasite species was found distributed in a well-defined fundamental niche along the intestine of Siganus rivulatus, and a distinct partial overlap between the niches of G. volubilis and Sclerocollum rubrimaris was observed. In concurrent infections with these two species, their fundamental niches were significantly reduced, probably being affected by interactive site segregation and individuals of each species were found segregated in a restricted realised niche. In all other patterns of concurrent infections, each parasite species was normally found in its original fundamental niche. The prevalences of these parasites in the fish examined were relatively low and somewhat similar. In all patterns of single and concurrent infections, the intensity of infection was directly related to fish size. In concurrent infection with G. volubilis and S. rubrimaris, a significant decline was observed in the intensities of both species in the different size groups of the fish, but, in all other patterns of concurrent infections, no significant change in intensity was observed. These observations suggest that the interaction between G. volubilis and S. rubrimaris is probably a negative and symmetrical one. The mode of transmission of these parasites to the fish is also discussed.
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