Sphincterodiplostomum musculosum ( Digenea , Diplostomidae ) infecting Steindachnerina insculpta ( Characiformes , Curimatidae ) in the Chavantes Reservoir , Southeastern Brazil Sphincterodiplostomum musculosum ( Digenea , Diplostomidae ) infectando Steindachnerina insculpta ( Characiformes , Curima

This study aimed to report the infection by Sphincterodiplostomum musculosum metacercariae in Steindachnerina insculpta from the Chavantes Reservoir, medium Paranapanema River, municipality of Ipaussu, State of São Paulo, Brazil. Thirty specimens of S. insculpta were collected and 29 were infected with S. musculosum metacercariae (prevalence = 96.67%) in the eyes and visceral cavity. The mean intensity of infection and abundance were 96.6 ± 29.41 (7-846) and 93.3 ± 28.6 (0-846), respectively. Positive correlation was observed between parasite abundance in the eyes and standard length (rs = 0.5, p = 0.005), total weight (rs = 0.649, p = 0.0001), and condition factor (rs = 0.439, p = 0.0154). The high parasitism rates by S. musculosum metacercariae in S. insculpta can be an indicative that this fish species is highly susceptible to infection by this diplostomid, and even reflect the presence of a large abundance of the intermediate host. Moreover, S. musculosum metacercariae are reported for the first time in S. insculpta.


Introduction
Steindachnerina insculpta Fernández-Yépez, 1948 is a native fish from Brazil and its occurrence is restricted to the Paraná River basin.This fish species belongs to the family Curimatidae and is commonly known in Brazil as saguiru-do-rabo-amarelo (FROESE; PAULY, 2012).
In the aquatic environment, the penetration of pathogens is facilitated and fish can be infected by numerous parasite species (KLEIN et al., 2004).Therefore, a great number of fish species can act as intermediate or definitive hosts of several parasite species (PAVANELLI et al., 2008).
Among fish parasites, the larval stages of digeneans, such as diplostomid metacercariae, are important agents of diseases in these animals, because they can cause severe ocular pathology associated with cataracts and blindness or even death.Diplostomids present a complex three-host life-cycle which involves: a free-living larva (miracidium), which hatches from the egg; two asexual generations (parthenitae) in an aquatic snail; a second free-living larva (cercaria), which is released in huge numbers and is involved in the transmission to the second intermediate fish host, usually penetrating it through the gills; an unencysted larva (metacercaria), usually in the eyes (lens, humour or under the retina) of fishes; and a sexually mature, hermaphroditic adult in the intestine of piscivorous birds (VALTONEN; GIBSON, 1997).

Materials and Methods
Thirty specimens of S. insculpta were collected in January, 2009 in the Chavantes Reservoir (Figure 1), medium Paranapanema River, municipality of Ipaussu, State of São Paulo, Brazil (23° 7' 36" S and 49.59° 23' 10" W).Fish were collected using a nylon monofilament gillnet from 5:00 PM to 7:00 AM, totaling 14 hours of exposure.The fish specimens collected were transported to a laboratory where they were measured (cm), weighed (g) and necropsied.Organs and body cavities of S. insculpta were examined and the metacercariae found were collected, fixed in alcohol-formol-acetic acid solution under cover slip pressure.After that, they were stained with carmine according to Eiras et al. (2006).
Morphometric analysis of the metacercariae was accomplished using a computerized system for image analysis (Qwin Lite 3.1, Leica Microsystems, Wetzlar, Germany).All measurements were presented in micrometers and represented by the mean ± standard deviation (range).The ecological descriptors of parasitism (prevalence, abundance and mean intensity of infection) were calculated according to Bush et al. (1997).Spearman's rank correlation (rs) was used to study the correlations of the abundance of parasites present in the eyes in relation to standard length, total weight and condition factor -K (LE CREN, 1951).Statistical tests were performed using SigmaStat 3.1 (Systat Software Inc., California, USA), adopting a significance level of 5%.

Results
The mean standard length and total weight of the fish analyzed were 12.05 (9-16.5)cm and 39.95 (13.7-74.3)g, respectively.

Discussion
The present study reports the infection by S. musculosum metacercariae in S. insculpta from the Chavantes Reservoir, medium Paranapanema River, municipality of Ipaussu, State of São Paulo, Brazil.This parasite species was reported in a few fish species (CESCHINI et al., 2010a;TAKEMOTO et al., 2009;ABDALLAH et al., 2005) in comparison with other diplostomidae, such as Austrodiplostomum compactum, which was previously reported in approximately 25 fish species (MACHADO et al., 2005;EIRAS et al., 2010;PAES et al., 2010).Furthermore, the studied S. insculpta specimens presented high parasitism rates of S. musculosum, indicating that this fish species is high susceptible to infection by this parasite.
The morphological and morphometric data observed in this study to S. musculosum metacercariae from S. insculpta corroborated the data presented by Ceschini et al. (2010a), who found a high quantity of this diplostomid in the ovaries of S. brevipinna.
The abundance of S. musculosum in the eyes of S. insculpta was positively correlated to the standard length and total weight.Similar results were found by Ceschini et al. (2010b), who observed positive correlation between the abundance of S. musculosum and the standard length of S. brevipinna from the tributaries Guairacá and Corvo, in the low Paranapanema River, State of Paraná, Brazil.According to Ceschini et al. (2010b), these results support the idea of cumulative effect of parasites by increasing the area of infection with the growth of the host, as mentioned by Luque et al. (1996), and with the increase of the standard length with the ontogenetic development of the host, the area of the body surface also increases, as well as the possibility of occurring the process of active penetration of cercariae.This pattern was also observed by Paes et al. (2010) and Machado et al. (2005) in relation to other diplostomidae, such as Austrodiplostomum compactum metacercariae.
Positive correlation was also observed between the condition factor (K) and the abundance of S. musculosum in the eyes of S. insculpta.The condition factor is given by the relationship between the individual's weight and length and it is an important indicator of the fish health, reflecting recent nutritional conditions and the interactions between the fish and the biotic and abiotic factors (LE CREN, 1951;SATAKE et al., 2009).Thus, the results observed in this study suggest that the parasitism by S. musculosum could not affect the physiological status, health or welfare of the studied S. insculpta specimens.
Sphincterodiplostomum musculosum found in this study were in the larval stage, suggesting that this fish species occupies an intermediate position in the food chain and may be part of the diet of piscivorous birds (definitive hosts of this parasite species).In this study, a great number of S. musculosum metacercariae was found parasitizing the eyes of S. insculpta.According to Evans et al. (1976), around 40 metacercariae of diplostomids per eye may be responsible for cataract or blindness in fish (depending on the size of the host).In our study, no histopathological study was performed to analyze whether the S. insculpta specimens were affected by the parasites.However, the fact that the metacercariae were found parasitizing the eyes at high infection rates makes the fish an easier prey for capture by the definitive host (commonly piscivorous birds), so the parasite can complete its life cycle faster (ABDALLAH et al., 2005).
In summary, the high parasitism rates by S. musculosum metacercariae in S. insculpta can be an indicative that this fish species is highly susceptible to infection by this diplostomid, and even reflect the presence of a large abundance of the intermediate host.Moreover, S. musculosum metacercariae are reported for the first time in S. insculpta.

Figure 1 .
Figure 1.Map of Brazil, highlighting the State of São Paulo and the sampling area at the Chavantes Reservoir, Paranapanema River, municipality of Ipaussu, State of São Paulo, Brazil.

Figure 2 .
Figure 2. Correlation between standard length and abundance of Sphincterodiplostomum musculosum from the Chavantes Reservoir, medium Paranapanema River, municipality of Ipaussu, State of São Paulo, Brazil.

Figure 3 .
Figure 3. Correlation between total weight and abundance of Sphincterodiplostomum musculosum from the Chavantes Reservoir, medium Paranapanema River, municipality of Ipaussu, State of São Paulo, Brazil.

Figure 4 .
Figure 4. Correlation between condition factor (K) and abundance of Sphincterodiplostomum musculosum from the Chavantes Reservoir, medium Paranapanema River, municipality of Ipaussu, State of São Paulo, Brazil.

Table 1 .
Comparative morphometric data between Sphincterodiplostomum musculosum metacercariae of the Chavantes Reservoir, Paranapanema River, municipality of Ipaussu, State of São Paulo, Brazil, and Steindachnerina brevipinna from the tributaries Guairacá and Corvo of the low Paranapanema River, State of Paraná, Brazil, described by Ceschini et al. (2010a).