Aspects of the ecology of proteocephalid cestodes parasites of Sorubim lima (Pimelodidae) of the upper Paraná River, Brazil: I. Structure and influence of host's size and sex.

Between March 1992 and February 1996, 107 specimens of Sorubim lima were collected in the floodplain of the upper Paraná river. Ninety-five (88.78%) specimens were parasitized with at least one species of proteocephalid cestodes. 7,573 specimens of four different species were collected (average intensity 79.71 parasites/host): Paramonticellia itaipuensis Pavanelli & Rego, 1991; Nupelia portoriquensis Pavanelli & Rego, 1991; Spatulifer maringaensis Pavanelli & Rego, 1989 and Spasskyellina spinulifera Woodland, 1935. S. maringaensis was the most prevalent and abundant. There were three dominant species P. itaipuensis, S. maringaensis and N. portoriquensis) and one co-dominant species (S. spinulifera). The three environments (lotic, semilotic and lentic) in which collection was undertaken showed high similarity with regard to parasitic fauna. A high Simpson index value (0.359) indicates dominance tendency among species of parasites. Concerning S. maringaensis significant positive correlation was observed between parasite intensity and standard length of hosts. No species had negative correlation. With regard to S. maringaensis and N. portoriquensis prevalence was positive and significantly correlated with standard length of hosts. No species had negative correlation. In the case of S. maringaensis and N. portoriquensis influence of host's sex was noted on parasite intensity. There was no sex influence on parasite prevalence in any species.