This work examined the possible preference of Saccocoelioides nanii Szidat, 1954 for the three major intestinal portions of Prochilodus argenteus Agassiz, 1829 and statistically evaluated whether the river water level dynamic interfered in the parasite's ecological parameters and in its transmission to the definitive host. One-hundred-fifty specimens of P. argenteus were collected in July, 2003 (dry season) and in January, 2004 (rainy season) in the upper São Francisco River, State of Minas Gerais, Brazil. Of the 150 hosts, 96 (64%) were parasitized by S. nanii. The mean intensity and abundance were 32.6 ± 34.8 and 20.8 ± 31.9, respectively, and the range of infection intensity was from 1 to 177. The abundance of S. nanii was higher in the middle intestine than in either the anterior or posterior portions. This selection could be evidence of niche restriction to facilitate mating. The proportion of non-pregnant specimens of S. nanii was higher in the dry season as well as the intensity and abundance. At least two hypotheses can be considered for elucidated this inference: the presence/absence or change in the quantity of some substance in the host body amending/inhibits the reproduction of the parasite or that non-pregnant specimens corresponds to newly recruited individuals who did not have time to reproduce.

Myxobolus franciscoi sp. nov. (Myxozoa: Myxosporea: Myxobolidae) is described from the "curimatá-pacú" fish, Prochilodus argenteus Spix & Agassiz, 1829 (Actinopterygii: Prochilodontidae) from the Upper São Francisco River, Brazil. This parasite forms whitish plasmodia (about 1 x 1 mm) that develop in the connective tissue of fins. The spores are more or less round in frontal view and ellipsoidal in lateral view, measuring 6.4 (6.0-6.9) µm in length, by 6.0 (5.8-6.4) µm in width and 3.2 µm in thickness. The polar capsules are very small, measuring about 2 µm in length by 1.5 µm in width and ending in a tapered anterior neck. The polar filament makes three turns in a plane at right angles with the longitudinal axis of the spore. Thorough comparisons with the remaining species of Myxobolus Bütschli, 1882 described from South American fish, as well as with almost all species of Myxobolus described so far, are provided. This paper also includes a revision of Myxobolus species from South American fish hosts.

This research aimed to investigate the ecological indexes of the helminths of the digestive system and coelom of Prochilodus argenteus Spix & Agassiz, 1829. A total of 150 specimens (53 males and 97 females) taken from the upper reaches of the São Francisco River in the municipality of Três Marias (18º12'32"S, 45º15'41"W), State of Minas Gerais, Brazil, were examined. The specimens were collected in July, 2003 and January, 2004. Ninety-eight fish (65.3%) were infected by at least one species of helminth. Five helminth species were found: one digenean, Saccocoelioides nanii Szidat, 1954; two Eucestoda, Valipora sp., and one undetermined metacestode; one nematode, Spinitectus asperus Travassos, Artigas & Pereira, 1928; and one acanthocephalan, Neoechinorhynchus prochilodorum Nickol & Thatcher, 1971. The sex of the host did not influence parasite indexes. The total length of the hosts influenced the abundance of S. nanii (r s = -0.21, p = 0.01) and the prevalence of the metacestode (r = -0.91, p = 0.01). Saccocoelioides nanii was the dominant species in the parasite fauna of P. argenteus. Saccocoelioides nanii, Valipora sp., S. asperus and N. prochilodorum are reported here for the first time in P. argenteus and their known distribution is expanded to the São Francisco River.

Cento e treze espécimes de Franciscodoras marmoratus (Reinhardt, 1874) foram coletados no alto São Francisco (18° 12' 32" S, 45° 15' 41" W, Estado de Minas Gerais), entre setembro de 1999 e janeiro de 2004, para a investigação de sua parasitofauna. Desse total 45 (39,8%) estavam parasitados por, pelo menos, uma espécie de parasito. A riqueza parasitária foi constituída por seis espécies representantes de Hirudinea (n = 20), Monogenoidea (n = 25), Eucestoda (n = 55), Nematoda (n = 1, n = 2) e Acanthocephala (n = 41), encontradas nos períodos da seca e da cheia, que totalizaram 144 espécimes. Proteocephalus renaudi Chambrier & Vaucher, 1994 foi a única espécie com prevalência superior a 10%, com típico padrão de distribuição agregado. A prevalência, intensidade e a abundância parasitária de P. renaudi não foram influenciadas pelo comprimento total e sexo dos hospedeiros e nem pelos períodos de coleta, respectivamente. O fator de condição relativo indicou que os hospedeiros de P. renaudi não tiveram higidez significativamente afetada em relação aos peixes não parasitados. Os peixes estocados em tanques, antes das necropsias, foram oportunistamente infestados por Lernaea cyprinacea Yashuv, 1959. Os diversos parasitos encontrados indicam F. marmoratus onívoro e potencial hospedeiro definitivo. As espécies de parasitos, com exceção de Acanthocephala, têm ampliada sua distribuição geográfica conhecida para a bacia do rio São Francisco. A comunidade parasitária foi considerada isolacionista devido à baixa diversidade endoparasitária, infrapopulações com baixa intensidade, interações parasitárias não evidentes e raros sinais de agressões dos parasitos contra seus hospedeiros.

One hundred and thirteen specimens of Franciscodoras marmoratus (Reinhardt, 1874) were collected in the upper São Francisco River (18° 12' 32" S, 45° 15' 41" W, state of Minas Gerais) between September, 1999 and January, 2004 to investigate their parasite fauna. From this total, 45 (39.8%) were afflicted by at least one parasite species. The parasitic richness consisted of six species represented by Hirudinea (n = 20), Monogenoidea (n = 25), Eucestoda (n = 55), Nematoda (n = 1, n = 2) and Acanthocephala (n = 41) found in the dry and wet periods making a total of 144 specimens. Proteocephalus renaudi Chambrier & Vaucher, 1994 was the only species with prevalence higher than 10% and a typical aggregate distribution pattern. The prevalence, intensity and abundance of P. renaudi were not influenced by the total length or sex of the hosts or by the collection periods. The relative condition factor indicated that the health of the P. renaudi hosts was not significantly affected in relation to fish not infected by parasites. The fish stocked in tanks before necropsy were opportunistically infested by Lernaea cyprinacea Yashuv, 1959. The various parasites found indicate that F. marmoratus is omnivorous and a potential definitive host. The parasite species, except for Acanthocephala, have expanded their known geographic distribution to the São Francisco River Basin. The parasite community was considered isolationist because of the low endoparasite diversity, infrapopulations with low intensity, lack of evidence of parasite interactions and sparse signs of parasite aggression against their hosts.