Monogeneans (Dactylogyridae) parasitizing gills of Salminus hilarii from a Neotropical reservoir, Brazil Monogenéticos (Dactylogyridae) parasitando brânquias de Salminus hilarii de uma represa Neotropical, Brasil

With the aim of creating an inventory of the metazoan gill parasites of Salminus hilarii in the Taquari River, state of São Paulo, Brazil, five species of monogeneans (Anacanthorus contortus, A. bicuspidatus, Annulotrematoides parisellei, Jainus iocensis and Tereancistrum arcuatus) are reported the first time for this host. A total of 28 fish were sampled quarterly between April 2011 and January 2012, with 10 hosts in a lentic ecosystem and 18 in a lotic ecosystem. Quantitative ecological descriptors (prevalence, intensity of infestation and abundance) were calculated for the purpose to comparing the two ecosystems sampled (lentic and lotic ecosystems). However, no quantitative difference between the lentic and lotic ecosystems was observed. The present study has made available a checklist for species of the genus Anacanthorus and their hosts and geographical distribution in the Neotropical region up to the present time.


Introduction
Monogeneans included in Dactylogyridae are primarily parasites of the gills of marine and freshwater fish. The class Monogenea is the most diversified group and contains the largest number of species parasitizing Neotropical fish (BOEGER; VIANNA, 2006). Specific monogeneans are common helminths parasitizing bony fish worldwide (AKOLL et al., 2012), which is a factor justifying the importance of taxonomic knowledge of species of this class. Eiras et al. (2011) highlighted the increasing numbers of studies on monogeneans. However, the high number of monogenean species does not necessarily imply that this group has more species than other groups. This may simply have occurred because monogeneans have been more intensively studied than other groups.
The present study underscores and broadens the importance of taxonomic knowledge of species of this parasite group. This is the first study to record occurrences of monogeneans at species level, parasitizing the gills of S. hilarii. Kohn et al. (1985) identified one nematode, two digenean, and two monogenean parasites of S. hilarii, but the last ones were only identified at superfamily level (Dactylogyridae).
The Taquari River, located in the state of São Paulo, is a tributary of the Paranapanema River, which is influenced by the Jurumirim reservoir. From studies on gill parasites of S. hilarii in this context, occurrences of five monogenean species were reported and their quantitative parameters were compared between lentic and lotic ecosystems.

Study area
The Jurumirim dam (23° 12' 17" S and 49° 13' 19" W) is the first of a cascade of dams on the Paranapanema River. The reservoir behind this dam is operated such that it regulates others further downstream. The dam was built in the late 1950s and operations began in 1962 (HENRY; NOGUEIRA, 1999). The Taquari River (23° 15' 12" S and 49° 12' 34" W), located in the state of São Paulo, is the second largest tributary of the Jurumirim reservoir   (Figure 1).

Fish collection and laboratory procedure
Twenty-eight specimens of S. hilarii from the Taquari River were collected between April 2011 and January 2012 in order to study monogenean parasites. The fish were collected using nylon monofilament gillnets with mesh sizes of 3 to 14 cm and with standardized effort. Nets were deployed at 5:00 pm and gathered in at 7:00 am the following day (total exposure time: 14 h). Limnological parameters such as temperature (°C), pH and dissolved oxygen (mg O 2 .L -1 ) were measured with aid of a multi-parameter analyzer. Individuals were measured in terms of standard length (L s , to the nearest 0.1 mm) and weight (to the nearest 0.1 g). The gills were frozen and subsequently removed and placed in vials containing 5% formalin solution. The parasites collected were preserved in alcohol 70%. Some parasite specimens were mounted unstained in Hoyer's medium in order to study the sclerotized structures, while others were stained with Gomori's trichrome to observe internal organs (EIRAS et al., 2006).

Hierarchical levels of study and statistical analysis
In accordance with Bush et al. (1997), the following community descriptors were calculated at the infracommunity level: prevalence (%), mean intensity of infestation and mean abundance for each parasite species. After checking the assumptions of normality (Lilliefors test), Pearson's linear correlation (r) was applied to examine the relationships between parasite abundance and the limnological parameters. The Mann-Whitney U test Figure 1. Jurumirim reservoir on the upper Paranapanema River, Brazil. Sampling areas: *lentic and ** lotic ecosystems in the Taquari River (arrow). Source: satellite image from Google Earth Digital Globe was performed to measure the effects of the lentic and lotic ecosystems on the abundance of each parasite species. Yates' corrected X 2 together with Spearman's rank correlation (rs) were also performed on pairs of co-occurring parasite species to investigate possible interspecific associations (i.e. co-operative or antagonistic relationships) (LUDWIG; REYNOLDS, 1988). The Z-test for proportions was performance to check differences in prevalence between infrapopulations of the lentic and lotic ecosystems.
The Berger-Parker index was applied to appraise the numerical dominance trends among parasite species (MAGURRAN, 1988). The variance-to-mean ratio of parasite abundance (dispersion index) and the discrepancy index, computed using the Quantitative Parasitology 3.0 software (RÓZSA et al., 2000), were used to detect distribution patterns of the infrapopulations (POULIN, 1993). The Shannon index (H') was calculated to compared the biological diversity of communities in the lentic and lotic ecosystems (MAGURRAN, 1988). The hierarchical levels and terminology used in this study followed Bush et al. (1997) and Poulin (2004) and the significance level used was p < 0.05.
The monogenean species were identified as described by Cohen et al. (2012) and Boeger and Vianna (2006). Voucher specimens were deposited in the Helminthological Collection of the Institute of Biosciences (CHIBB), UNESP, Botucatu, São Paulo, Brazil.

Results
A total of 28 fish were captured and all were parasitized by one or more monogenean species (overall prevalence = 100%). About 35% of the fish examined were parasitized by three parasite species (Figure 2). Calculations on the absolute frequencies of the total parasites collected highlighted Anacanthorus contortus as the most important numerically (53.2%), followed by Annulotrematoides parisellei (20.9%), Tereancistrum arcuatus (15.1%), Jainus iocensis (7.4%) and A. bicuspidatus (3.4%). Salminus hilarii is a new host record for all the monogeneans identified.
The component community was composed of five monogenean species totaling 417 specimens. Among these, 183 were collected in a lentic ecosystem and 234 in a lotic ecosystem, with means of 3.36 ± 7.53 and 2.6 ± 4.21, respectively. Comparing the prevalence and abundance of the component communities of the lotic and lentic ecosystems, no significant difference was observed. Fish from both ecosystems had the same magnitude of species richness and no differences in diversity (H' lentic = 1.19 and H' lotic = 1.27; degrees of freedom = 27; t test = 0.988; p > 0.05).
Comparing the component communities, A. contortus showed high values for quantitative descriptors in both the lentic and the lotic ecosystems, except for the prevalence values. Among these, A. parisellei in the lentic ecosystem stood out with prevalence of 80%. The parasite A. bicuspidatus presented the lowest prevalence and J. iocensis and T. arcuatus presented similar prevalences (Table 1).
Significant covariations of abundance were observed between pairs: A. contortus versus A. bicuspidatus, A. contortus versus T. arcuatus and A. parisellei versus T. arcuatus. Significant associations of prevalence were observed between pairs: A. contortus versus A. bicuspidatus and A. parisellei versus A. bicuspidatus (Table 2).
Anacanthorus contortus had the highest dominance frequency at both sites (0.59 and 0.48 in lentic and lotic ecosystems, respectively). All the parasites had a typically aggregated distribution pattern, and A. bicuspidatus was the monogenean species that showed the highest discrepancy index values (Table 3).
In relation to the limnological variables (Table 4), the Pearson linear correlation (r) only revealed a statistical difference in the abundance of monogeneans in relation to pH: abundance versus temperature (R 2 = 0.81; p = 0.09); abundance versus oxygen (R 2 = 0.15; p = 0.61); and abundance versus pH (R 2 = 0.89; p = 0.05). Considering the months of May to July to be the dry period and October to January to be the rainy period, greater abundance of monogeneans was observed during the rainy period ( Figure 3).  Table 1. Prevalence (%), total abundance (TA), mean intensity ± standard error (MI ± SE) and mean abundance (MA) of monogenean species in Salminus hilarii in the Taquari River, upper Paranapanema River, Brazil.

Monogenean species
Lentic ecosystem (n = 10) Lotic ecosystem ( host species. Furthermore, subgroups within Anacanthorus, based on the general morphology of the copulatory complex, appear to express high host specificity to familial groups within the Characidae. Because of these traits, species of Anacanthorus may provide valuable models for studying biogeography (KRITSKY et al., 1992). These characteristics may explain the higher abundance and mean intensity of infestations relating to A. contortus.
The genus Anacanthorus has large species diversity. To date, 70 species have been described, mostly from the Amazon region (Table 5). According to Van Every and Kritsky (1992), species diversity among Amazonian fish and their parasites may have resulted from lacustrine resource partitioning, similar to that proposed to explain cichlid diversity in some African lakes (LOWE-MCCONNELL, 1987) and Pliocene diversity in lake Idaho (SMITH, 1975 (KRITSKY et al., 1979;KRITSKY et al., 1992;COHEN, 1998;FISCHER et al., 2003;MALTA, 2006;MONTEIRO et al., 2010;COHEN et al., 2012).
During this study, we observed differences in correlations between the abundance of monogeneans and pH, which possibly influenced the presence or absence of these organisms. However, Table 4. Mean values and standard deviations of monthly limnological parameters in the Taquari River, between April 2011 and January 2012.

Discussion
The present study reports occurrences of five monogenean species parasitizing S. hilarii in the Taquari River, Brazil. Individual species of Anacanthorus show varying ability to infest closely related   studies on the physiology of the host are needed in order to confirm the degree of influence of the pH of these organisms. Regarding parasite dispersion, in most cases, the parasites are almost universally aggregated between their hosts (KRASNOV; POULIN, 2010). That is to say, most of the hosts have few if any parasites, while a small number of hosts are infected with many parasites (POULIN, 1993). This pattern is expected in most animals in nature, as observed regarding monogeneans parasitizing the gills of S. hilarii in the present study, except for A. parisellei in the lentic gradient. This type of distribution is possibly related to variations in the characteristics of the environment or the behavior of living beings that try to group (NERING; ZUBEN, 2010).
According Thomas et al. (2005), considerable progress has been made in understanding the functional value of parasites in ecosystems. Numerous theoretical and empirical studies have shown that parasites, in spite of their small size, are biologically and ecologically important in ecosystems. The influence of monogeneans on fish behavior in nature is difficult to measure, but we considered them within the life history of the host in order to gain better understanding of both organisms within their ecological niches.
Knowledge of the geographical and systematic distribution of monogenean species is also of considerable importance, since these organisms can be used as tools to aid in understanding, conserving and preserving aquatic ecosystems.
The present study provides the first record of occurrences of the monogeneans A. contortus, A. bicuspidatus, A. parisellei, J. iocensis and T. arcuatus parasitizing the gills of the host S. hilarii in the Taquari River in the state of São Paulo, Brazil. This study contributes towards the knowledge of occurrences of these species of parasites in fish of the family Characidae and their geographical distribution, as well as listing the host species and geographical distribution of the genus Anacanthorus in South America.