Ecological parameters of the endohelminths in relation to size and sex of Prochilodus argenteus ( Actinopterygii : Prochilodontidae ) from the Upper São Francisco River , Minas Gerais , Brazil

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 (18°12’32”S, 45°15’41”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 (rs = -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.

Prochilodus argenteus Spix & Agassiz, 1829, popularly known as "curimatá-pacu" and Prochilodus costatus Valenciennes, 1850, known as "curimatá-pioa", are the only species of Prochilodus Agassiz, 1829 found in the São Francisco River and are both endemic to this basin.The curimatá-pacu, a detritivorous species (ALVIM & PERET 2004), is the largest of the family, reaching up to 15 kg and is of great economic importance in the region of the Três Marias Reservoir, State of Minas Gerais, Brazil, accounting for roughly 50% of the total fish catch (SATO et al. 1996).It is a long-distance migratory fish and reproduces between November and January (SATO et al. 2005).

MATERIAL AND METHODS
A total of 150 specimens of P. argenteus were collected in July, 2003 and January, 2004 from the Upper São Francisco River, downstream from the Três Marias Dam (18°12'32"S, 45°15' 41"W), State of Minas Gerais, by fishermen from the Estação de Hidrobiologia e Piscicultura da Companhia de Desenvolvimento dos Vales do São Francisco e do Parnaíba (EPT/CODEVASF).Fiftythree specimens were males varying from 24.8 to 40.2 cm total length (28.73 ± 4.37), and 97 specimens were females with total length varying from 23.3 to 40.8 cm (29.9 ± 7.7).
The host was identified following BRITSKI et al. (1988) and the valid name of each host species follows CASTRO & VARI (2003).Author's name, valid scientific names and in some cases the citations for the respective synonyms follow the www.fishbase.org(FROESE & PAULY 2009).
The organs of the digestive system and the coelom of each fish were separated and examined for parasites.The Endohelminths collected were fixed and processed following AMATO et al. (1991) andEIRAS et al. (2000).(1994), following the classification cited by SPASSKY (1995).Metacestodes were found encysted in large numbers along the viscera, and it was not possible to count them.Nematodes were identified and classified according to TRAVASSOS et al. (1928) and MORAVEC (1998), respectively, and acanthocephalans were identified following NICKOL & THATCHER (1971) and classified according to AMIN (1987).Ecological terms, prevalence, mean intensity, mean abundance and site of infection follows BUSH et al. (1997).
The ecological analyses included only helminths with prevalence higher than 10%, in accordance with BUSH et al. (1990).The Student's t-test (t) was used to compare total host length between males and females.Spearman's rank correlation coefficient (r s ) was used to calculate possible correlations between total host length and parasitic intensity and abundance.Pearson's coefficient correlation (r) was used to indicate the relationship between total host length and prevalence of helminths, with arcsene transformation of data (arc sen ).The Chi-square test ( 2 ) with Yate's correction and 2x2 contingency table was used to evaluate the prevalence of the parasites in relation to host sex.The Mann-Whitney test (U) was used to evaluate the parasitic intensity and abundance in relation to host sex.The possible covariation among the prevalence and abundance of parasite species was analyzed using Chi-square ( 2 ) test and Spearman's rank correlation coefficient (r s ), respectively.
The statistical analysis followed ZAR (1996) and the statistical significance was p р 0.05.The dominance frequency, the frequency of shared dominance and the relative dominance of each parasite species were calculated according to ROHDE et al. (1995)
Of the total of 150 fish examined, 98 (65.3%) were parasitized, 63 (42.0%) of them with only one species of parasite, 24 (16.0%) with two, six (4.0%) with three, and five (3.3%) with four species.The total length of the female fish was significantly higher than that of the corresponding male (t = 2.02; p = 0.044).
Saccocoelioides nanii was the dominant species in the parasite community of P. argenteus, followed by S. asperus and the metacestodes (Tab.II).The parasitic parameters were not influenced by the sex of the host (Tab.III).Saccocoelioides nanii was significantly more abundant in smaller hosts (r s = -0.21,p = 0.01).It was true for prevalence of the metacestodes found in the intestine (r = -0.91,p = 0.001) (Tab.IV).There was covariation in the prevalence ( 2 = 11.103,p = 0.0009) and abundance (r s = 0.30, p < 0.0001) between S. nanii and S. asperus.

DISCUSSION
According to the results of the present study, the helminth fauna of P. argenteus in the upper São Francisco River bears some similarity with that of P. lineatus from the Paraná River reported by LIZAMA et al. (2005).This resemblance is mostly due to shared species of Saccocoelioides Szidat, 1954, Neoechinorhynchus Hamann, 1892(adult specimens) and Valipora Linston, 1927 (larvae).Adult digeneans stood out quantitatively, with S. nanii being the dominant species in P. argenteus.The significant covariation of the S. nanii -S.asperus pair in P. argenteus from the São Francisco River found in this study and the observation of helminths in Prochilodus spp.(besides digeneans, cestode larvae, nematodes -especially Spinitectus spp., and acanthocephalans -Neoechinorhynchus spp.) in other watersheds (LIZAMA et al. 2005) indicate that the transmission of these parasites to their definitive hosts may be through close intermediate hosts or through hosts that are more abundant in their habitats.
The presence of larvae of Valipora sp. in P. argenteus from the São Francisco River and in P. lineatus from the Paraná River (LIZAMA et al. 2005) is consistent with the hypothesis that the Prochilodontidae act as intermediate hosts in the life cycle of bird helminths, especially Ciconiiformes and Ardeidae (BONA 1994), in Brazil. SCHOLZ et al. (2004) listed a wide spectrum of fish species used as hosts by these parasites, while stressing the absence of any definitive host records in various places where these metacestodes have been reported (e.g., South America).According to the authors, there are very few reports of larval pathogenesis in fish, but other studies will be necessary to clarify the subject.
Host total length as a reflection of age is an important factor in the variation of parasite infracommunities (DOGIEL 1961).In the present work, the abundance of S. nanii was higher in smaller (presumably younger) fish, a fact that may result from peculiarities of the fish immune system, or influenced by the feeding habits (smaller fish can use resources that are not accessible to adults) or the environments frequented by young fish, with higher concentration of metacercariaes in vegeta- Digeneans were identified according to TRAVASSOS et al. (1969) and classified following JONES (2005) and OVERSTREET & CURRAN (2005), and eucestodes were identified to genus according to BONA . LIZAMA et al. (2005) recorded the following endohelminths from P. lineatus: Megacoelium sp., Colocladorchis sp., Lecithobotrioides sp.(junior synonym of Saccocoelioides Szidat, 1954), Saccocoelioides magnorchis Thatcher, 1978, S. nanii, S. saccodontis Thatcher, 1976 (senior synonym of Saccocoelioides leporinodus Thatcher, 1978), Saccocoelioides sp., Unicoelium prochilodorum Thatcher & Dossman, 1975, and two indeterminate digeneans, plerocercoids of Proteocephalidea and Valipora campylancristota (Wedl, 1855), the nematode Raphidascaris sp., and juvenile acanthocephalans of N. curemai and Quadrigyrus sp.In Prochilodus reticulatus Valenciennes, 1850 the digeneans Lecithobotrioides medicanoensis
. Voucher specimens of P. argenteus were deposited in the Museum of Zoologia of the Universidade de São Paulo (USP), State of São Paulo, Brazil (MZUSP 95167).Voucher specimens of helminths were deposited in the Coleção Helmintológica do Instituto Oswaldo Cruz (CHIOC), State of Rio de Janeiro, Brazil (see respective deposit numbers below).

Table I .
Helminths and their parameters of infection of P. argenteus from the Upper São Francisco River, Minas Gerais.(SD) Standard deviation.

Table II .
Dominance analyses of helminths of P. argenteus from the Upper São Francisco River, Minas Gerais.(SD) Standard deviation.

Table III .
Analysis of the influence of the sex of P. argenteus from the Upper São Francisco River, Minas Gerais, on the parasitic prevalence ( 2 ), intensity (U) and abundance (U).(2)Chi-square with Yate's correction, (U) Mann-Whitney test.

Table IV .
Analysis of the influence of total length of P. argenteus from the Upper São Francisco River, Minas Gerais, on the parasitic prevalence (r), intensity (rs) and abundance (rs).(r) Pearson correlation, (rs) Spearman's rank correlation coefficient.