First report of Rhabdochona acuminata (Nematoda, Rhabdochonidae) in Astyanax aff. fasciatus (Characiformes, Characidae) from Lake Guaíba, southern Brazil Primeiro registro de Rhabdochona acuminata (Nematoda, Rhabdochonidae) em Astyanax aff. fasciatus (Characiformes, Characidae) no Lago Guaíba, no 

Specimens of Rhabdochona (Rhabdochona) acuminata (Molin, 1860) were collected in Astyanax aff. fasciatus (Cuvier, 1819) from Lake Guaíba, municipalities of Guaíba (30°08.28'S, 51°18.53'W) and Barra do Ribeiro (30°17.11'S, 51°18.01'W), southern Brazil. In the present study, the prevalence of R. (R.) acuminata was low (1.43%) when compared to other studies performed with Astyanax spp. from Brazil. The specimens of R. (R.) acuminata showed some morphological differences (mainly the deirids) that have not yet been reported for the species. This is the first report of R. (R.) acuminata from Lake Guaíba, filling a gap of occurrence of this nematode in South America. Neotropical Biology and Conservation 14(4): 479–488 (2019) doi: 10.3897/neotropical.14.e49025 Copyright Moisés Gallas et al. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. RESEARCH ARTICLE Moisés Gallas et al. 480

The genus Rhabdochona was proposed to accommodate Dispharagus denudatus Dujardin, 1845 (= Rhabdochona denudata (Dujardin, 1845) Railliet, 1916), a parasite of Scardinius erythrophthalmus (Linnaeus, 1758). Until now, 98 to 104 species were described for this genus, presenting a wide geographical distribution (Mejía-Madrid et al. 2007;Moravec et al. 2013). According to Moravec (2010), the species of Rhabdochona originated from a marine ancestral in the Cretaceous period, where regions between seas and rivers alternated incessantly and formed rivers, causing a rapid speciation in host fish and parasites.
In the systematic of the genus Rhabdochona, the following subgenera have been proposed : Filochona Saidov, 1953, Rhabdochona Railliet, 1916and Globochona Moravec, 1972(Moravec 1972). However, currently most studies agree with the proposal of Moravec (1975), which recognized four subgenera: Rhabdochona, Globochona, Globochonoides Moravec, 1975 and Sinonema Moravec, 1975. These subgenera could be differentiated mainly by the number of teeth projection in the prostom, presence or absence of lateral alae, shape of deirids and shape of the tail in the females (Moravec 1975;Moravec 2010).
In the Neotropical Region, 12 species have been reported belonging to the subgenus Rhabdochona (Moravec 1998;Ramalho 2005;Moravec 2010), but only Rhabdochona (Rhabdochona) acuminata (Molin, 1860), Rhabdochona (Rhabdochona) fabianae Ramalho, 2005 andRhabdochona (Rhabdochona) uruyeni Diaz-Ungría, 1968 have been reported from South America. The species of Rhabdochona found in South America possibly originated from species found in North America, because all South American species are similar to congeneric species found in Africa (Moravec 2010). The low specificity of the species of Rhabdochona from South America could be the reflex of absence of cypriniform fishes, which are considered to be the hosts of several species of Rhabdochona, in this geographical region (Mejía-Madrid et al. 2007;Moravec 2010). Besides that, the host distribution explains the morphological differences and the absence of phylogenetic relationships between the South American and African species of Rhabdochona (Moravec 2010).
Rhabdochona (R.) acuminata was redescribed by Cremonte et al. (2002) through the examination of specimens collected in fish in Argentina. The present study provides the first record of R. (R.) acuminata from southern Brazil and reports some differences found on the morphology hitherto unreported for the species.
Measurements are shown in micrometers (µm) unless otherwise stated. The parameters reported are range followed by mean ± standard deviation, and sample size between parenthesis. Photomicrographs were taken with a Zeiss Axiolab microscope with phase contrast. Line drawings were made with a drawing tube mounted on a Nikon E-200 microscope, scanned and prepared using CorelDraw X4 and Adobe's Photoshop CS2. Ecological parameters follow Bush et al. (1997). Voucher helminth specimens were deposited in the 'Coleção Helmintológica do Instituto Oswaldo Cruz' (CHIOC), Rio de Janeiro, RJ, Brazil.

Rhabdochona (Rhabdochona) acuminata (Molin, 1860) Figs 1, 2
Description based on 10 specimens. Small to medium nematodes with transverse striated cuticle. Oral aperture oval, with four papillae, two ventral and two dorsal. Vestibule elongated, dilated anteriorly to form a funnel-shaped prostom. Prostom anteriorly armed with 14 longitudinal projections similar to teeth in the inner surface. Prostom with basal tooth. Simple deirids, anterior to the nerve ring and the excretory pore. Esophagus divided in muscular and glandular portions, of which the muscular is the smaller portion. Tail of both sexes conical, sharply pointed.

Discussion
The measures of R. (R.) acuminata found in the literature until 2002 were compiled and compared by Cremonte et al. (2002). Posteriorly, measurements of R. (R.) acuminata were taken by Ramalho (2005) in Argentina and, Costa et al. (2011) in Brazil. The specimens examined in the present study showed similar measurements when compared with the tables organized by Cremonte et al. (2002). However, males and females of R. (R.) acuminata found in A. aff. fasciatus from Lake Guaíba presented larger specimens when compared to the specimens reported by Cremonte et al. (2002), Ramalho (2005) and Costa et al. (2011) (Tables 1 , 2). Besides that, the table organized by Cremonte et al. (2002) possibly has a scale error, because the width measurements of males and females (62-300 mm) and other traits are higher than the variation in the measurements of length (4.54-28 mm).
According to Cremonte et al. (2002), the longest length recorded to R. (R.) acuminata was 13 mm for males (Table 1) and 28 mm for females (Table 2). In the present study, males were 16.5 to 18.2 mm (17.6 mm) long and females 35 to 42 mm (38 mm) long. Moreover, the deirids observed in the specimens collected  here were in a higher position (next to the basal tooth in the prostom), in relation to other studies where the deirids were reported below to the basal tooth (Moravec 1972;Cremonte et al. 2002;Ramalho 2005). These differences could be related to the larger specimens examined in the present study or, to a morphological variation hitherto unreported.
The morphology of the distal region of the left spicule in the species of Rhabdochona is complex and may present differences according to the position in which it is examined (Moravec 2010). In the present study, the morphology of the right spicule (with membranous ala on distal end) and the left spicule (with membranous heel and dorsal hook with folded tip) were similar to those described by Travassos et al. (1928), Vaz and Pereira (1934), and more detailed by Cremonte et al. (2002) and Ramalho (2005). In the species grouped in the subgenus Rhabdochona the right spicule possibly is associated with the function of a gubernaculum, which is absent in these species (Moravec 2010 guassú and Luque 2007). Such differences could be associated with characteristics in the life cycle of R. (R.) acuminata and in the environments. The life cycle of the species of Rhabdochona found in South America is still unknown, but may present similarities with the life cycles of the other species of the genus, where aquatic insects (ephemeropterans, trichopterans and plecopterans) are considered to be the intermediate hosts (Moravec 2010). The low prevalence found in A. aff. fasciatus here could be associated with the feeding habit and the diet of this characid in Lake Guaíba, where the availability and/ or the ingestion of the insects considered intermediate hosts is smaller than the characids examined in the Rio de Janeiro State (Paraguassú and Luque 2007). Besides that, the period of host collection, pollution and the water regime may influence the prevalence of R. (R.) acuminata.
In Brazil, there are reports of R. (R.) acuminata in the following species of Astyanax: A. bimaculatus, A. fasciatus and Astyanax schubarti Britski, 1964 from the state of São Paulo (Kloss 1966) and A. bimaculatus and A. fasciatus from the state of Rio de Janeiro (Paraguassú and Luque 2007). This is the first report of R. (R.) acuminata in A. aff. fasciatus from Lake Guaíba, southern Brazil. This study extends the known geographical distribution of R. (R.) acuminata and contributes to the knowledge of the helminth fauna of A. aff. fasciatus.