First record of Rhipidocotyle santanaensis ( Digenea ) parasitizing Acestrorhynchus lacustris from Batalha River , Brazil

This paper describes the occurrence of digenetic Rhipidocotyle santanaensis Lunaschi, 2004 in Brazil. This parasite was found in the liver, heart, gonads, intestine, pyloric caeca, stomach, swim bladder and cavity of Acestrorhynchus lacustris (Lütken, 1875) from the Batalha River, São Paulo State, with higher incidence in gonads and digestive tract. The parasite specimens found in this study showed morphological characteristics very similar to specimens from Argentina, with differences only in relation to the ejaculatory duct and seminal vesicle. However, Brazilian specimens presented great morphometrical differences between specimens described in Argentina, with much higher measures. This is the first record of this metazoan parasite species in Brazil and in this host fish.


Introduction
Parasites can be effective tools for comparative studies in ecology and biogeography.Because they are host-dependent regarding the essential features of survival (nutrient acquisition and reside on or within the host's body for long periods of time), its distribution is invariably linked to the presence of that particular host on the environment.In case generalists parasitic species (low host specificity), the geographical range will be much larger in relation to specialists, since it will have the ability to explore different hosts, which in turn present migration patterns that will influence the dispersal of these parasites in the environment (Poulin et al., 2011).
Rhipidocotyle santanaensis was described by Lunaschi (2004) parasitizing the pyloric caeca of Acestrorhynchus pantaneiro (Menezes, 1992) on Paraná River in Santa Ana, Corrientes province, Argentina.This species differs from the congenerics by presenting a rhynchus with two lateral projections, the testes arranged in diagonal and the vitelline follicles forming an arch in the pre-oral region (Lunaschi, 2004).Until the present time, only R. gibsoni (Kohn and Fernandes, 1994) was reported to A. lacustris (Lütken, 1875).
Acestrorhynchus lacustris is a fish belonging to Acestrorhynchidae and it is often found in the basins of the Tietê-Paraná and São Francisco River, mainly in lentic environments.It is a carnivorous species and its predominant feed items are forage fishes such as Astyanax altiparanae Garutti andBritski, 2000, Moenkhausia intermedia (Eigenmann, 1908) and Steindachnerina insculpta (Fernández-Yépez, 1948) (Hahn et al., 2000).According to Agostinho et al. (2004), it is considered a non-migratory or short-distance migratory species with external fertilization and no parental care.In this paper, we provide new information about morphometrical and morphological characteristics of adult specimens of R. santanaensis in this freshwater fish from São Paulo State, Brazil.
The fishes were collected by using gillnets with different meshes.In collect, fishes were packed in individual plastic bags and transported in a refrigerated cooler box to the Laboratório de Ictioparasitologia in the Central de Laboratórios de Ciência e Tecnologia Ambiental at USC (Universidade do Sagrado Coração) in Bauru, where they were kept in a freezer until necropsy.All internal organs were analyzed individually in stereomicroscope.
The digeneans found were counted and stored in alcohol 70° GL.The specimens were stained with Mayer's Carmalúmen and mounted in Canada balsam according to Eiras et al. (2006).The ecological parasitism descriptors were obtained by Bush et al. (1997).Trinocular microscopy (Nikon E200) was employed for the morphologic examination.The measured samples were randomly selected among the various hosts organs where they were found.Measurements were obtained by using a computerized image analysis system (Motic, Moticam 5.0MP).Measures are given in micrometers and presented as the mean followed by the minimum and maximum values in parentheses.Specimens were deposited in Invertebrates Collection of Instituto Nacional de Pesquisas da Amazônia (INPA), Manaus, AM, Brazil, with number 642.
The specimens registered in this paper showed much higher measures to samples recorded by Lunaschi (2004), only the acetabulum of the organisms in current study were much smaller compared to specimens from Argentina and the eggs morphometry of both are similar (Table 1).We observed morphometrical variation among collected samples within the same organ, and this variation was similar on all different infection sites.
Proportionally, the parasites found in this study showed morphologic characteristics similar to organisms of Argentina described by Lunaschi (2004), with differences only in relation to the ejaculatory duct and seminal vesicles.In this study, the vesicle showed tighter when compared to Lunaschi (2004) specimens, which presented vesicle with oval shape.The samples reported in this paper presented an ejaculatory duct occupying a larger space within the cirrus sac, whereas the Argentina specimens showed a lower proportion (Figure 2d).Lunaschi (2004) does not comment anything on the prevalence, abundance and intensity of the R. santanaensis in A. pantaneiro, which hinders any kind of comparison with present study data.Furthermore, in description of R. santanaensis conducted by Lunaschi (2004), only three specimens were measured collected from pyloric caeca, which reduces variation range of the measures  and moreover, different fixatives were used which could influence the parasites distension.However Carvalho et al. (2003) studying the parasitic fauna of A. lacustris collected in the Paraná river floodplain found R. gibsoni with much lower prevalence and intensity values (prevalence = 17.6% and intensity = 3.6) to those found in this study.

Discussion
Parasites provide an ideal model for testing ecological constraints, such as the size of the host, on the evolution of body size in a group of organisms (Harvey and Keymer, 1991;Poulin, 1995).Changes in body size related to evolution towards parasitism follow diverse trends depending on the parasite group (Morand et al., 1996).The intraspecific morphometric and morphologic changes are much common in parasites and some authors comment that these changes may be related to the different times of infection and/or intense competition for space and other host resources (Dobson, 1986;Shostak and Dick, 1987;Szalai and Dick, 1989).Physiological differences found in different hosts may affect the establishment, grow and sexual maturity of the parasites; to specific polymorphism of the parasites; to parasites different stages of development and to environmental variability (Watson and Pike, 1993;Willis, 2002;Francisco et al., 2011;González et al., 2013).
Fish endoparasites adults feed either on the digested contents of the host's intestine or the host's own tissues (including blood).They might also feed by means of osmotic absorption (Markov, 1946).The structure of various digestive tract parts, the histology of each organ and the spatial relationship between the organs are factors that also determine the degree of endohelminth infection (Dogiel et al., 1970).In this work, the infection sites with higher parasites incidence and abundance were the gonads and the digestive tract, that is, sites with high energy content available for the parasitic absorption.Isaac et al. (2004) studying the parasitic fauna of Gymnotus spp.found the digenetics Crocodilicola sp. 1, Crocodilicola sp. 2 and Herpetodiplostomum sp. 1 parasitizing gonads of this host.They comment that the use of gonads as microhabitat can interfere in host reproduction or even lead to parasitic castration and this can be a total or partial castration.In connection with this topic, Paperna (1974) recorded nematode larvae Eustrongylides sp. in gonads of Haplochromis spp.and remarked that a large number of cysts can deform the ovaries, making them increasingly irregular, leading to their degeneration and the formation of a large cystic mass.
Digenetic are parasites commonly found on different sites (microhabitats) within the hosts.It is rare for the initial encounter with a host individual to happen exactly where the parasite's microhabitat will be.The invading parasite must detect signals and move to reach the microhabitat.When a parasite penetrates a host it enters an environment that has numerous highly predictable characteristics.Certain parasites also show intermicrohabitat migrations, and this may be related to the period of feeding, digestion, migration patterns or reproducing of the host for example.To identify the appropriate microhabitat, parasites have surface molecules that recognize host molecules.There are powerful selective pressures for the discovery of the correct habitat, because parasites that localize outside the normal microhabitat would likely not transmit their genes,  (17-21 × 10-15) either they die because the resources are not correct or they survive but cannot correctly disperse their offspring (Stunkard, 1974;Combes, 2001).
Beyond the present study, there is only one record of R. santanaensis parasitizing the congeneric species A. pantaneiro in Argentina.Due to the few records, it is difficult to consider this species as a specialist or generalist.It would be more appropriate to classify it as stenoxenus species (or with stenoxenus tendency), which according to Euzet and Combes (1980), are parasites found in a small group of related host species, usually in the same genus or family.Finally, we would like to comment that the fishes collected by Lunaschi (2004) came from the Parana River, which belongs to the same hydrographic basin of studied fish (Basin of Paraná-Tietê), since the Batalha River is a tributary of Tietê River, thus concluding that such basins have connectivity.This is the first occurrence of R. santanaensis in Brazil and also in A. lacustris.

Figure 1 .
Figure 1.Map of the study area, showing details of Batalha River with your geographic location coordinates and highlighting the Hydrographic Basin of Tietê-Batalha on Sao Paulo State map, where is located the Batalha River.