Abstract Ten endoparasite species found in Pygocentrus piraya, which is a piranha species native to São Francisco river, were collected from 108 fish caught in Três Marias reservoir in 2004 and 2005, namely: Digenea - Austrodiplostomum sp. (metacercariae); Eucestoda – Proteocephalidae gen. sp. (plerocercoids); and Nematoda - Procamallanus (Spirocamallanus) inopinatus, Cystidicoloides fischeri and Capillostrongyloides sentinosa (adults); Spinitectus rodolphiheringi (juvenile); Hysterothylacium sp., Contracaecum sp. Type 1, Spiroxys sp. and Goezia sp. (larvae). In helminth fauna, P. (S.) inopinatus has shown higher prevalence and dominance. Fish sex has influenced the prevalence of Hysterothylacium sp., which was higher in female specimens. Longer total length of fish has positively influenced the abundance of C. sentinosa and Hysterothylacium sp.. The rainy season has favored parasitic indices, mainly P. (S.) inopinatus abundance, as well C. sentinosa and Hysterothylacium sp. prevalence and abundance. There were co-occurrences between adult and some larval nematodes. Endemic piranhas, as well as final hosts, are important intermediate and/or paratenic hosts, given the relevant number of larvae (proteocephalideans and nematodes) of heteroxene cycle found in them. Proteocephalidae gen. sp., C. sentinosa, Hysterothylacium sp., Contracaecum sp. Type 1, Goezia sp. and S. rodolphiheringi are new records parasitizing P. piraya in São Francisco basin.

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.

Ergasilus chelangulatus sp. nov. é descrita do peixe de água doce, Pimelodus maculatus Lacépède, 1803 (Siluriformes: Pimelodidae) do alto rio São Francisco, Minas Gerais, Brasil. A nova espécie está baseada em espécimes fêmeas e o macho é desconhecido. Esta espécie tem uma seta serrilhada no primeiro exopodito e tem o primeiro endopodito bi-segmentado, como a maioria das espécies amazônicas deste gênero. O corpo é alongado e projetado anteriormente. As antenas são alongadas com sensila proeminente no segundo e no terceiro segmentos. Ergasilus chelangulatus sp. nov. é distinta de todas as outras espécies conhecidas de Ergasilus por ter o quarto segmento antenal (garra) dobrado em ângulo quase reto.

Ergasilus chelangulatus sp. nov. is described from the freshwater fish, Pimelodus maculatus Lacépède, 1803 (Siluriformes: Pimelodidae) of the upper São Francisco River, Minas Gerais State, Brazil. The new species is based on female specimens and the male is unknown. This species has a serrate seta on exopod one and a two-segmented first endopod, as do most Amazonian species of this genus. The body is elongate and produced anteriorly. The antennae are elongate and have prominent sensilla on segments two and three. Ergasilus chelangulatus sp. nov. can be distinguished from all known species of Ergasilus by the fourth antennal segment (claw) which is bent at nearly a right angle.

Ergasilus salmini sp. nov. um parasito das brânquias do "dourado", Salminus franciscanus Lima & Britski, 2007, do alto rio São Francisco, Minas Gerais, Brasil, é descrita baseada em espécimes fêmeas. A nova espécie tem uma seta curva, serrilhada, no primeiro exopodito como a maioria das espécies conhecidas deste gênero. A maioria dos ergasilídeos tem a cabeça e os dois primeiros segmentos torácicos completamente fusionados formando o cefalotórax. Na nova espécie, a cabeça é incompletamente fusionada, o primeiro segmento torácico é fusionado, mas o segundo segmento torácico é livre. Ergasilus hydrolycus Thatcher, Boeger & Robertson, 1984, também possui o segundo segmento torácico livre mas nessa espécie a cabeça é completamente fusionada e o terceiro segmento abdominal é o mais comprido. Na nova espécie, o terceiro segmento é o mais curto dos três. A nova espécie também é menor medindo 691 x 207 ìm comparado com 784 x 278 ìm em E. hydrolycus. Essa espécie tem o quarto endopodito bi-segmentado enquanto na nova espécie essa estrutura é tri-segmentada. Adicionalmente, a nova espécie é de um hospedeiro diferente e de um sistema hídrico distinto.

Ergasilus salmini sp. nov. a branchial parasite of the "dourado", Salminus franciscanus Lima & Britski, 2007, from the upper São Francisco river, Minas Gerais State, Brazil, is described based on female specimens. The new species has a serrate, curved seta on the first exopod as do most of the known South American species of the genus. Other than that, it does not closely resemble any known species of this genus. Many of the known ergasilids have head, and first two thoracic segments completely fused to form a cephalothorax. In the new species, the head is incompletely fused, the first thoracic segment is fused, but thoracic segment two is free. Ergasilus hydrolycus Thatcher, Boeger & Robertson, 1984, also has a free second thoracic segment but in that species the head is completely fused and the third abdominal segment is the longest. In the new species, the third segment is the shortest of the three. Also, the new species is smaller averaging 691 x 207 µ m compared to 784 x 278 µ m for E. hydrolycus. The latter species has a two-segmented fourth endopod whereas in the new species this structure is three-segmented. In addition, the new species is from a different host and a separate river system.