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New host records of Prosthenhystera obesaMartins et al.Leporinus reinhardti and P. pohli are new hosts forP. obesa, Digenea with Neotropical distribution. Inaddition to these two hosts, P. obesa specimens hadpreviously been found in S. franciscanus (Brasil-Sato, 2002, 2003) and in P. maculatus (Brasil-Sato2003, Brasil-Sato & Pavanelli 2004), both hostsfrom the upper São Francisco River. In S.franciscanus, seven P. obesa specimens werefound in five parasitized fish and only one of themwas parasitized by three P. obesa specimens in thegall bladder. Prevalence was 14% and its meanabundance 0.20 ± 0.52. Three P. obesa specimenswere collected from P. maculatus – one specimenper host. Prevalence was 1.2% and meanabundance 0.01 ± 0.11. The morphometric data ofP. obesa are presented in Table 1.Table 1 lists the measurements of P. obesaspecimens in Characiformes, L. reinhardti (onespecimen), new host record, and S. franciscanus(three specimens) and in Siluriformes, P. pohli(one specimen), new host record, and P. maculatus(three specimens) in the upper São FranciscoRiver.Leporinus reinhardti and P. pohli specimens wereclassified as pregnant and immature, respectively.Seven mature specimens were collected from thegall bladder of S. franciscanus, five were pregnantand the three specimens collected from P.maculatus were immature.DISCUSSIONThrough morphometric studies (Table 1),measurements of adult specimens (body lengthand width and oral sucker) found in L. reinhardtiwere lower than those shown by parasitespecimens of S. brevidens and S. maxillosus (= S.brasiliensis) (Kohn et al., 1997) but were similar tothose of S. brasiliensis (= S. franciscanus) (Brasil-Sato, 2002), and egg measurements were similar tothose of P. obesa eggs from S. maxillosus (= S.brasiliensis) (Kohn et al., 1997). Measurements ofbody length and width, oral sucker, acetabulumand pharynx of P. obesa from P. pohli were inferiorto those of immature specimens found in P.corruscans (Kohn et al., 1997). Measurements ofP. obesa in S. franciscanus were similar relative tothe other Characidae studied by Travassos (1922a)and Kohn et al. (1997). Measurements ofspecimens collected from P. maculatus were largerthan those from Pimelodus spp. recorded by Kohnet al. (1997) and in this study, they were a littlelarger when compared to P. pohli. Besides theparameters of P. obesa in the São Francisco River,according to Isaac et al. (2000), prevalence of P.obesa in S. maxillosus was 14.3% and, according toBrasil-Sato & Pavanelli (2004), it was 3% in P.maculatus from the Paraná River.The parasitic parameters were considered low,even though among the characiform hosts,predators of fish (ichthyophagi) both theprevalence and mean intensity were higher thanthose recorded in the siluriform hosts in the basinsof the São Francisco River and the Paraná River.The highest prevalence (33.3%) of P. obesa wasrecorded by Travassos (1940) in P. fur (oneparasitized host of three examined) and C.bimaculatum (three hosts from nine examined),both fish from Salobra, Mato Grosso do Sul.According to Cribb et al. (2002), most digeneansoccurring in fish are defined by the combination ofspecialized alimentary habits and the adoption ofspecialized colonization sites in their hosts.Campos et al. (2009) attribute the distribution ofhelminths to qualitative and/or quantitativechanges in the diet of the host. Leporinus reinhardtiis a foraging species of omnivorous alimentaryhabits, and analysis of stomach contents in fish inthis study showed that this species is opportunisticmaking use of resources that are available in itshabitat: the remains of vegetation, larvae ofarthropods, crustaceans, insects and molluscs. Thepregnant specimen of P. obesa in the gall bladder ofL. reinhardti demonstrates its definitive host.Pimelodus pohli is omnivore-carnivore and thepresence of arthropod larvae and sediment wereobserved in its stomach content. This species couldalso act as a definitive host, but due to the immaturespecimen found, it is not possible to state that.As the life cycle of Callodistomidae remains36
Neotrop. Helminthol., 6(1), 2012unknown (Bray, 2002; Cribb et al., 2002), theevaluation of parasitism by P. obesa in freshwaterfish hosts may be an attempt to help elucidate thepossible cycle of these Digenea. Thus, taking intoconsideration the measurements of specimenspreviously reported in the literature (Kohn et al.,1997; Brasil-Sato & Santos, 2005; and in thepresent study), it can be stated that Characiformesconstitute the hosts in which Digenea haveevolved, matured and invariably shown pregnantuteri. With the exception of P. roosevelti(Pimelodidae), all the hosts with pregnant P. obesaare located in either Characidae or Anostomidae.In the records of Siluriformes, the collectedspecimens from pimelodid fishes often weresmaller in size and had not reached maturity,although they were always located in the gallbladder. From this information, the possibility thateach collected specimen of P. obesa had a shorttime of parasitism in these hosts cannot be rejected.The condition of a wide variety of intermediatehosts in biological cycles among Xiphidiata doesnot permit inference as to which are theintermediate hosts in the cycle of P. obesa, butallows us to state that among freshwater fish,Characiformes are definitive hosts of P. obesa. Inthis case, sexual maturity is reached by eggproduction and consequently the reproductivesuccess of these Digenea is possible, perhaps as aproduct of an older evolutionary history more incommon with freshwater Characiformes than withSiluriformes.Prosthenhystera obesa specimens in fish showedgreat variability in size (Pavanelli et al., 1992;Kohn et al., 1997). Travassos (1940) reported thatin addition to P. obesa specimens collected in P.clarias by Travassos et al. (1928), that collected inP. fur was immature and those in C. bimaculatumwere adults, both from Salobra, State of MatoGrosso do Sul. From this observation, it can beconfirmed that for a long time it has been observedthat not only the size but the maturation of theseDigenea is affected among hosts of differentfamilies. In some records, P. obesa specimenscollected in characids were smaller and immature(and probably had less time to produce eggs in thehost), because according to the literature (op. cit.) itwas from this host family that the largestspecimens were collected and in addition, theywere usually pregnant. In anostomids, Kohn et al.(1997) observed smaller specimens, but despitethis, they were pregnant, indicating a possibility ofreproductive success with lower bodydevelopment, although in our study, the specimen,which was also pregnant, was similar in size tothose reported in characid hosts. In Siluriformes,although some P. obesa specimens were larger thanthose found in anostomids, they were immature.Besides the level of development in relation to thetype of host, the fact that the specimens were foundsolitary in the gall bladder of most hosts wasbiologically relevant. If they remain solitary duringthe parasitism without producing eggs (in the caseof Siluriformes), there must be a mechanism fordispersal of parasite specimens among hosts withno possibility of biological cycle continuity in theaquatic system. Thus, there would be a powerspending investment with no possibility ofproducing embryonic eggs by cross-fertilizationbetween P. obesa specimens. On the other hand,single and pregnant specimens were found in thegall bladder of Characiformes hosts. In thissituation, if they were solitary during all theirdevelopment, when reaching maturity in the gallbladder they would initiate the production of eggsregardless of cross-fertilization (an inherentcondition of hermaphroditism) maximizing the useof the parasitized organ space by the pregnantspecimen.Although the cycle is unknown, the fact thatcharaciform fish are definitive hosts andSiluriformes potential hosts, the details of thisparasitism may be important to unveil the mainevolutionary path of the species in South Americanlacustrine ecosystems. Research on whether thispath would be linked to be used by carnivorouscharaciform hosts as a requirement to reachmaturity, or whether egg production by digeneanswould be dependent on sexual reproduction and thenumber of developing specimens in the gallbladder, affecting the reproductive process of thesecallodistomids, needs to be researched.37
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