Introduction

In the Amazon River, distributed by a complex hydrographic network lives a great amount of fish, molluscs and crustaceans. However, the ichthyological fauna is important considering that it represents a resource of great economic value for the region, and for this reason, for some years have been studying some groups of host parasites of the aquatic fauna of the Amazon basin (^{MATOS et al., 2004}).

Considering the great number of fish species in the Amazon region, can be affirmed that only a small percentage of the helminthfauna is known so it is necessary to expand the studies to a larger number of host species. Due this some expeditions were developed in Manaus, state of Amazonas (Brazil), with the purpose to study the helminth parasites of Acestrorhynchus falcirostris Cuvier, 1819. Acestrorhynchus Eigenmann & Kennedy, 1903, is a genus of the family Acestrorhynchidae that currently comprises fourteen valid species of Neotropical fish, based on ^{Froese & Pauly (2017)}. These species are phylogenetically related and endemic to South America (^{TOLEDO-PIZA, 2007}). Acestrorhynchus falcirostris is a freshwater fish that is distributed in South America: Amazon and Orinoco river basins and rivers of Guyana (^{FROESE & PAULY, 2017}). This species is pelagic, diurnal habit, sedentary and inhabits streams and lakes of white and black waters. It is piscivorous, feeding almost exclusively on fish, but shrimps are also part of its diet. In addition to fish, young individuals ingest invertebrates. Females begin the process of sexual maturation at 14cm in length (^{SOARES et al., 2011}).

Until now, two species of Digenea had been reported in A. falcirostris in Brazil: Metacercariae of Austrodiplostomum compactum Lutz, 1928 in state of Amazonas (^{VITAL et al., 2016}) and Clinostomum marginatum Rudolphi, 1819 in the state of Amapá (^{HOSHINO et al., 2016}).

The aim of this study is to contribute to the increase of the knowledge and expansion of the geographic distribution of Digenea parasites of fish of the Brazilian Amazon region, thus parasites belonging to Digenea in state of Amazonas (Brazil) were collected.

Materials and Methods

The capture and examine of fish for the parasitological analyzes were carried out in partnership with the research group of the ichthyofauna of the “Projeto Inteligência Estratégica da Indústria do Petróleo e Gás na Amazônia” (PIATAM).

In order to catch the fish, four excursions were carried out in the months of March, June, September and December of 2013. In all the excursions, the fishing in the lakes was standardized for the sampled area (banks, open water, pauses and floating vegetation in the lakes) and mesh size of nets. The nets measuring 20 m in length by 2 m in height and the mesh sizes 30, 40, 50, 60, 70, 80, 90 and 100 mm between adjacent nodes. The time the nets stayed in the water was approximately 10 hours per lake, daytime period, with two expenditures during this time.

Eighty-two specimens (34 males and 48 females) of A. falcirostris were collected from five floodplain lakes of the Solimões River (Ananá, Aracá, Baixio, Maracá and Preto) and from one lake of the Purus River (São Tomé), between the cities of Manaus and Coari in the state of Amazonas (Figure 1).

Figure 1 Localities of capture of Acestrorhynchus falcirostris. Floodplain lakes of the Solimões River (Ananá, Aracá, Baixio, Maracá and Preto) and from one lake of the Purus River (São Tomé), between the cities of Manaus and Coari in the state of Amazonas. 

The fishes measured 22.5±4.4 cm in total length and 117.6±39.0 g in weight. The specimens of Digenea were fixed in AFA (alcohol, formalin, and acetic acid), without compression, and they were stained using Langeron’s alcoholic acid carmine, dehydrated in an ethyl alcohol series, cleared in beechwood creosote, and mounted in Canada balsam as permanent slides, according to ^{Eiras et al. (2006)}. Only the main measurements of each species are presented, considering that all the species studied here are well described. The measurements are in micrometers, unless otherwise stated, with means in parentheses, followed by the number of specimens measured in brackets, where applicable. Light micrographs were made with the use of a Zeiss® Axioscope 2 microscope. The morphological identification of species were done according to ^{Gibson et al. (2002)} and ^{Bray et al. (2008)}. The specimens studied were deposited in the Helminthological Collection of the “Instituto Oswaldo Cruz” (CHIOC), Rio de Janeiro and in the Invertebrate Collection, Platyhelminthes, “Instituto Nacional de Pesquisas da Amazônia” (INPA), Manaus, Brazil. For comparative analysis, the type material of the genus Bellumcorpus, deposited in the CHIOC were examined.

Results

Were examined 82 specimens of A. falcirostris and 51 were parasitized by Digenea belonging to eight species: Metacercariae of Autrodiplostomum compactum (Diplostomidae) and Clinostomum marginatum (Clinostomidae) and mature specimens of Bellumcorpus majus, Prosorhynchoides carvajali, Rhipidocotyle santanaensis (Bucephalidae); Prosthenhystera obesa (Callodistomidae); Genarchella genarchella (Derogenidae) and Phyllodistomum spatula (Gorgoderidae). Considering that A. falcirostris represents a new host record for six species, in this study the main measurements of each species are presented, to compare possible morphometric variations according to different hosts.

Family Bucephalidae Poche, 1907

Bellumcorpus majus Kohn, 1962 (Figures 2a-f; 3a-d)

Figures 2 Specimens of Bellumcorpus majus from Acestrorhynchus falcirostris collected from floodplain lakes Ananá, Maracá and São Tomé, state of Amazonas, showing the morphological variation of testes from rounded and smooth to lobed (arrow). Bars= 1 mm. 

Figures 3 Total of type specimens of Bellumcorpus majus recovered from Salminus hilarii Valenciennes, 1850 and Bellumcorpus schubarti recovered from Salminus brasiliensis, both species collected from Mogi-Guaçu River, state of São Paulo, showing the similarity of the testes in both species (arrow). (a) Type of B. majus, CHIOC nº 28.746a; (b) Paratype of B. majus, CHIOC nº 28.746b; (c) Paratype of B. majus, CHIOC nº 28.746e; (d) Paratype of B. schubarti, CHIOC nº 30.497. Bars= 1 mm. 

Localities: Ananá (03°50'38.24” S; 61°39'46.8”W), Maracá (03°50'32.8”S; 62°34'32.4”W), and São Tomé (03°49' 39.0” S; 61°25' 24.6” W).

Prevalence and intensity: 9.7% (8 out of 82 fish examined); 3-10 digeneans per fish.

Sites of infection: Intestine and gonads.

Material deposited: Voucher specimens CHIOC n^o: 38.486 a-ab (ab: metacercaria).

Main measurements based on 14 specimens: Body 6.05-10.47 mm (8.54) long by 2.05-4.35mm (3.18) wide; rhynchus 800-2,200 (1,251) [14] by 525-1,100 (831) [13]; pharynx 300-525 (427) by 325-875 (487) [12]; caecum 1,550-3,375 (2,583) by 375-1,075 (737); cirrus sac 1,750-3,200 (2,583) by 175-775 (573); anterior testis 700-1,600 (1,061) by 350-1,000 (725); posterior testis 620-1,525 (1,107) by 320-950 (682) [13]; ovary 350-675 (539) by 250-650 (481); eggs 15-27 (21) by 12-20 (14) [67].

Prosorhynchoides carvajali Muñoz & Bott, 2011 (Figure 4a)

Figures 4 Digeneans from Acestrorhynchus falcirostris collected from floodplain lakes Ananá, Aracá, Baixio, Maracá, Preto and São Tomé, state of Amazonas. (a) Prosorhynchoides carvajali Muñoz & Bott, 2011. Bar= 0.1 mm; (b) Rhipidocotyle santanaensis Lunaschi, 2004. Bar= 0.2 mm; (c) Prosthenhystera obesa (Diesing, 1850). Bar= 0.5 mm; (d) Genarchella genarchella Travassos, Artigas & Pereira. Bar= 0.3 mm; (e) Phyllodistomum spatula Odhner, 1902. Bar= 0.5 mm; (f) Austrodiplostomum compactum (Lutz, 1928). Bar= 0.3 mm; (g) Clinostomum marginatum (Rudolphi, 1819). Bar= 0.5 mm. 

Localities: Ananá (03°50'38.24” S; 61°39'46.8” W) and São Tomé (03°49'39.0” S; 61°25'24.6” W).

Prevalence and intensity: 4.8% (4 out of 82 fish examined); 1-3 digeneans per fish.

Sites of infection: Intestine, pyloric caeca.

Material deposited: Voucher specimens CHIOC n^o: 38489 a-g.

Main measurements based on seven specimens: Body 0.75-1.35 mm (0.96) long by 0.27-0.47 mm (0.33) wide; rhynchus 70-140 (103) by 75-135 (117); pharynx 55-100 (78) by 55-80 (71) [4]; caecum 105-175 (143) by 55-85 (70) [3]; cirrus sac 330-495 (395) by 80-110 (89); anterior testis 70-175 (108) by 75-155 (118); posterior testis 55-170 (106) by 65-135 (96); ovary 65-115 (86) by 70-120 (88); eggs 15-20 (16) by 10-12 (11) [50].

Rhipidocotyle santanaensis Lunaschi, 2004 (Figure 4b)

Localities: Ananá (03°50' 38.24” S; 61°39'46.8” W), São Tomé (03°49'39.0” S; 61°25'24.6” W) and Baixio (03°17'27.2” S 60°04'29” W).

Prevalence and intensity: 10.9% (9 out of 82 fish examined); 1-5 digeneans per fish.

Sites of infection: Intestine, pyloric caeca.

Material deposited: Voucher specimens CHIOC n^o: 38491 a-f.

Main measurements based on four specimens: Body 0.97-1.10 mm (1.05) long by 0.30-0.47 mm (0.41) wide; rhynchus 115-175 (145) by 125-165 (153); pharynx 50-90 (65) by 32-70 (56) [3]; caecum 195-320 (275) by 55-70 (63); cirrus sac 435-500 (443) by 85-90 (87); anterior testis 120-150 (130) by 75-120 (92); posterior testis 90-120 (107) by 85-125 (102); ovary 85-110 (101) by 85-120 (105); eggs 12-17 (15) by 7-12 (10) [25].

Discussion

The genus Bellumcorpus was erected by ^{Kohn (1962)} with the species B. majus, to allocate specimens recovered from the stomach of Salminus hilarii Valenciennes, 1850 from the Mogi-Guaçu River, state of São Paulo (Brazil). In 1963, Kohn described Paurorhynchus schubarti from the liver of Salminus brasiliensis Cuvier, 1816 (=Salminus maxillosus) from the same locality, based on two metacercariae previously recovered and deposited in CHIOC. ^{Kohn (1970)} described the new species Bellumcorpus schubarti from the coelom of S. brasiliensis from state of Mato Grosso and considered P. schubarti as a synonym of B. schubarti. The genus Bellumcorpus has been reported only in freshwater fishes from the Brazil. Until now, B. majus had been referred in Acestrorhynchus falcatus Bloch, 1794 (^{KOHN et al., 1985}; ^{KOHN & FERNANDES, 1987}), Acestrorhynchus lacustris Lütken, 1875 (^{PEDRO et al., 2016a}), S. brasiliensis (^{BRASIL-SATO, 2002}) and S. hilarii (^{KOHN, 1962}; ^{KOHN & FERNANDES, 1987}). Bellumcorpus schubarti had only been reported from S. brasiliensis (^{KOHN, 1963}, ¹⁹⁷⁰; ^{KOHN & FERNANDES, 1987}). ^{Kohn (1970)} distinguished B. schubarti from B. majus mainly through possessing lobed testes. In this opportunity examining the type material of both species (Figures 3a-d) and comparing with the specimens collected from A. falcirostris of this study, we observed that in the same sample, specimens with either lobed or more lobed testes could be found (Figures 2a-f). We also could verify that examining isolated specimens, we can find differences in the shape and size of testes, but examining the whole sample, we concluded that these are simple specific variations. Considering this and that there is no difference between the measurements of both species, in our opinion, the variation in the shape of the testes, is probably due to intraspecific variations, so in this paper B. schubarti is considered as a synonymy of B. majus. Acestrorhynchus falcirostris is presented as a new host record for this species.

Prosorhynchoides carvajali was described by ^{Muñoz & Bott (2011)} from the intestine and rectum of Auchenionchus microcirrhis Valenciennes, 1836 (type host), Auchenionchus variolosus Valenciennes, 1836 and Sicyases sanguineus Müller & Troschel, 1843, in Chile. ^{Muñoz et al. (2015)} described the life cycle of this Digenea in the same hosts. ^{Cruces et al. (2015)} reported it parasitizing Labrisomus philippii Steindachner, 1866 in Peru. Until now, P. carvajali have been referred only in marine fishes, this is the first report in a freshwater fish. The material studied in this paper agree with the original description with eggs a little shorter. This is the first report of this species in Brazil and in a new host.

Rhipidocotyle santanaensis was erected by ^{Lunaschi (2004)} for specimens recovered from the pyloric caeca of Acestrorhynchus pantaneiro Menezes, 1992, in Argentina. In South America, this species was also reported in Brazil by ^{Pedro et al. (2016b)}, parasitizing A. lacustris. The material studied herein is longer than the original specimens and smaller than Pedro’s material, with smaller eggs. This species has been reported only in fish belonging to the genus Acestrorhyncus, and in this opportunity, this species is referred in a new host, from the same genus, A. falcirostris.

In South America, Prosthenhystera obesa was previously reported in several fish species in Brazil and Argentina (see ^{KOHN et al., 2007}). Posteriorly it was referred in Brazil by ^{Isaac et al. (2000)} in S. brasiliensis and by ^{Sabas & Brasil-Sato (2014)} in Pimelodus pohli Ribeiro & Lucena, 2006. ^{Martins et al. (2012)} referred it parasitizing Leporinus reinhardti Lütken, 1875 and ^{Vasconcelos et al. (2013)} in Astyanax aff. bimaculatus Linnaeus, 1758. The specimens studied are similar to those reported by ^{Kohn et al. (1997)} and it is reported here for the first time in A. falcirostris.

In South America, Genarchella genarchella was reported in several fish species in Brazil and Argentina (see ^{KOHN et al., 2007}). Subsequently, G. genarchella was also reported in Brazil parasitizing Cichla piquiti Kullander & Ferreira, 2006 by ^{Franceschini et al. (2013)}, Hemibrycon surinamensis Géry, 1962 by ^{Hoshino et al. (2014)}, Ageneiosus ucayalensis Castelnau, 1855 by ^{Ferreira & Tavares-Dias (2017)} and Hoplias malabaricus Bloch, 1794 by ^{Gonçalves et al. (2016)}. The specimen studied is in agreement with those already reported. In this opportunity, A. falcirostris is referred as a new host to G. genarchella.

In South America, Phyllodistomum spatula was previously reported in Argentina parasitizing Pimelodella laticeps Eigenmann, 1917 and Rhamdia sapo Quoy & Gaimard, 1824 by ^{Lunaschi and Martorelli (1990)}. In Brazil, it was reported by ^{Fernandes (1984)} in Colossoma macropomum Cuvier, 1816 and by ^{Costa et al. (2015)} in Hoplias intermedius Günther, 1864 and H. malabaricus. The specimen studied here is in agreement with those already reported having eggs slightly wider. This is the first time that this species is reported in A. falcirostris.

Conclusion

In this paper, B. schubarti is considered synonymy of B. majus. Six digenean species are reported for the first time in this host and one species, P. carvajali is also reported for the first time in Brazil. The new data contribute to expanding the knowledge and geographic distribution of Digenea parasites of fish of the Brazilian Amazon region.