Introduction

Mesonauta festivus (Heckel, 1840) (Perciformes: Cichlidae) is a freshwater benthopelagic fish commonly named “acará”. It belongs to the family Cichlidae, which is the most species-rich non-ostariophysan freshwater fish family worldwide (^{Kullander, 1998}). It is a small fish that can reach up to 10 cm in length. It feeds primarily on filamentous algae that commonly occur with floating vegetation on the water surface (^{Froese & Pauly, 2016}; ^{Kullander, 2003}; ^{Santos et al., 2006}). The geographical distribution of this fish species includes freshwaters of South America: Paraná river basin, in the portion drained by the Paraguay river in Brazil and Paraguay; and Amazon river basin, in the portions drained by the following rivers: Guaporé (Brazil and Bolivia), Madre de Dios (Peru), Mamoré (Bolivia) and Jamari and Tapajós (Brazil) (^{Froese & Pauly, 2016}). So far, the only report of helminths in M. festivus comes from the original description of Ichthyouris ro by ^{Inglis (1962)}, from Guyana (Amazon region).

To date, the genus Ichthyouris^{Inglis, 1962}, includes 6 species, all from South American freshwater fish. The first report of this genus in the Brazilian Amazon region was the species Ichthyouris ovifilamentosa ^{Moravec and Thatcher, 2001} parasitizing Cichlasoma sp. (^{Moravec & Thatcher, 2001}).

The aims of this paper were to present new morphological data on I. ro, considering that the original description by ^{Inglis (1962)} lacked detail; and to report M. festivus as a new host for Ichthyouris bursata ^{Moravec and Prouza, 1995}.

Material and methods

During July 2009, 87 specimens of M. festivus (Heckel, 1840) (Perciformes: Cichlidae) from 5 localities along of 400-km stretch of the Solimões river, between the cities of Manaus and Coari, state of Amazonas, Brazil, were examined for helminths. The nematodes collected were washed in physiological saline (0.9% NaCl solution) and fixed in hot AFA (2% glacial acetic acid, 3% formaldehyde and 95% ethanol at 70 ºGL).

For light microscopical examination (LM), the nematodes were cleared in phenol. Observations were made using a Zeiss Axioscope 2 light microscope equipped with a camera lucida. Photomicrographs were taken using the same microscope. All measurements were made in millimeters, and the range is presented followed by the mean in parentheses. For scanning electron microscopy (SEM), specimens of I. ro were fixed in hot AFA, washed in 0.1 M cacodylate buffer (pH 7.2) and postfixed in a solution containing 1% osmium tetroxide and 0.8% potassium ferricyanide (pH 7.2) for 1 hour and subsequently dehydrated through a graded ethanol series, critical-point dried and sputter-coated with gold. They were examined at the “Plataforma de Microscopia Eletrônica Rudolf Barth” of the “Instituto Oswaldo Cruz” (Fiocruz, Rio de Janeiro), using a JEOL JSM-6390 LV scanning electron microscope at an accelerating voltage of 30 kV. The fish names follow FishBase (^{Froese & Pauly, 2016}).

The prevalence and intensity of infection were estimated for our focal species in relation to the entire sample of M. festivus. Calculations for the infection parameters of prevalence and intensity of infection were based on ^{Bush et al. (1997)}.

Results

Family Pharyngodonidae Travassos, 1919

Ichthyouris ro^{Inglis, 1962}

Redescription (Figs. 1-5)

Figure 1 Ichthyouris ro ^{Inglis, 1962}. A, Female, general view; B, anterior end of female, lateral view; C, cephalic end of female, apical view; D, egg; E, tail of female, lateral view; F, tail of female, ventral view. Scale-bars: A= 1.5 mm; B, E, F= 0.5 mm; C= 0.01 mm; D= 0.05 mm. 

Figure 2 Ichthyouris ro ^{Inglis, 1962}. A, Anterior end of male, lateral view; B, caudal end of male, ventral view; C, caudal end of male, lateral view. Scale-bars: A, B, C= 0.05 mm. 

Figure 3 Ichthyouris ro ^{Inglis, 1962}. LM micrographs (differential interference contrast) of female. A, Anterior region, lateral view (arrow: excretory pore); B, detail of anterior region, lateral view, showing the teeth-like structure (thin arrow); C, anterior region in ventral view (asterisk: lateral alae); D, uterus bearing many eggs (e); E, egg leaving the uterus through vulva (arrow-head); F, posterior region (lateral view) showing anus (arrow). Scale-bars: A, B, C= 0.5 mm; D= 0.085 mm; E= 0.154 mm; F= 0.5 mm. 

Figure 4 Ichthyouris ro ^{Inglis, 1962}. LM micrographs (differential interference contrast) of male. A, Anterior region, ventral view; B, detail of anterior region showing the lateral alae (asterisk); C, D, posterior region of male, ventral view (arrowhead: lateral alae; thin arrow: spicule); E, posterior region of male, lateral view. Scale-bars: A= 0.05 mm; B= 0.32 mm; C= 0.03 mm; D= 0.27 mm; E= 0.028 mm. 

Figure 5 Ichthyouris ro ^{Inglis, 1962}. SEM micrographs of female. A, Anterior end of body, showing hexagonal oral aperture (asterisk), lateral alae (la) and cuticular striations (arrow); B, cephalic end provided with 6 well-developed oral lamellae (l), cephalic papillae (p) and 1 pair of papilla-like amphid (a). Scale-bars: A, B= 10 μm. 

Small nematodes with cuticle densely transversely striated. Narrow lateral alae initiating anteriorly near anterior extremity and extending posteriorly to caudal alae in males and at level of anus in females. Oral aperture hexagonal, provided with 6 well-developed oral lamellae (2 dorsal, 2 ventral and 1 lateral on either side), arising from its inner surface. Four rather large spherical cephalic papillae and 1 pair of smaller lateral papilla-like amphids. Oral apparatus differs between sexes: simpler in male than in female. Esophagus formed by long, almost cylindrical corpus with slightly outlined “pharynx”, a very short isthmus and well-developed bulb. Nerve ring situated in anterior part of corpus. Excretory pore at some distance posteriorly to esophagus. Tail very long, narrowing posteriorly.

Female (10 specimens). Length of body including caudal spike 2.330-3.425 (2.792), maximum width 0.190-0.275 (0.223). Cuticular striations at middle part of body 0.005-0.007 (0.006) long. Lateral alae very narrow. Anterior end of esophagus forming short, slightly outlined “pharynx”, which anterior end is modified as cuticular edges, like a teeth in lateral view. Buccal cavity 0.015-0.032 (0.028) long and 0.015-0.020 (0.017) wide. Entire esophagus 0.470-0.570 (0.510) long; corpus including “pharynx” 0.380-0.630 (0.436) long and 0.050-0.062 (0.053) wide, “pharynx” 0.017-0.030 (0.023) long; isthmus 0.005-0.010 (0.006) long and 0.017-0.027 (0.026) wide; bulb 0.100-0.125 (0.109) long and 0.110-0.150 (0.125) wide. Nerve ring and excretory pore 0.125-0.142 (0.133) and 0.500-0.740 (0.603), respectively, from anterior extremity. Vulva slightly anterior half of the body, 0.780-1.135 (1.047) from anterior end. Uterus prodelphic. Muscular vagina directed anteriorly from vulva. One ovary anterior to level of vulva; other one posterior to it. Uterus containing numerous oval eggs, 0.077-0.092 (0.087) × 0.025-0.032 (0.028). Each egg pole provided with 1 very long filament. Entire tail 0.670-0.940 (0.797) long; length of slender caudal spike 0.520-0.730 (0.628).

Male (10 specimens). Length of body including caudal spike 0.900-1.450 (1.212), maximum width 0.070-0.100 (0.075). Striae at middle of body 0.002 long in all specimens studied. Anterior end of esophagus forming short, slightly outlined “pharynx”, without structures like a teeth in lateral view. Entire esophagus 0.245-0.300 (0.280) long; corpus including “pharynx” 0.177-0.222 (0.206) long and 0.020-0.025 (0.022) wide; isthmus 0.070-0.012 (0.010) long and 0.015-0.020 (0.016) wide; bulb 0.055-0.070 (0.063) long and 0.055-0.072 (0.061) wide. Nerve ring and excretory pore 0.087-0.135 (0.108) and 0.350-0.460 (0.393), respectively, from anterior extremity. Genital cone rudimentary. Tail provided with relatively short, wide membranous lateral caudal alae. Preanal papillae: 1 pair of large, elongate, ventral posteriorly oriented papillae just anterior to level of cloacal opening. Postanal papillae: 2 pairs of small ventral papillae oriented anteriorly; 1 pair of subventral elongated papillae oriented posteriorly and 1 pair of small sessile papillae lying just posterior to more posterior pair of long papillae. Caudal alae present from anterior preanal papillae to posteriorly sessile papillae. Single spicule, well sclerotized 0.040-0.050 (0.046) long and 0.002-0.005 (0.003) wide, its distal tip pointed. Entire tail 0.265-0.300 (0.290) long; its broader anterior part 0.050-0.100 (0.067) long, length of slender caudal spike 0.200-0.225 (0.215). Tail forming 21- 29% (24%) of entire length of body.

Discussion

To date, the genus Ichthyouris comprises 6 species. I. ro, the type species of the genus, was originally described from the intestine of M. festivus (= Cichlasoma festivum) from Guyana (formerly British Guiana). Four species have been described from Brazil: Ichthyouris brasiliensis^{Moravec, Kohn and Fernandes, 1992}, from the catfish Megalancistrus aculeatus (Peters, 1881); Ichthyouris laterifilamenta Moravec, Kohn and Fernandes, 1992, from the catfish Trachydoras paraguayensis (Eigenmann and Ward, 1907); Ichthyouris voltagrandensis^{Martins, Yoshitoshi and Umekita, 2001}, from Myleus tiete (Eigenmann and Norris, 1900); and Ichthyouris ovifilamentosa^{Moravec and Thatcher, 2001}, from Cichlasoma sp. Ichthyouris bursata^{Moravec and Prouza, 1995}, was described from the aquarium-reared discus Symphysodon discus Heckel, 1840, in Europe (host apparently originally introduced together with fish from South America) (^{Inglis, 1962}; ^{Martins et al., 2001}; ^{Moravec et al., 1992a}, ^b; ^{Moravec & Prouza, 1995}; ^{Moravec & Thatcher, 2001}).

The type material of I. ro was described in South America from the intestine of M. festivus (= Cichlasoma festivum) from Guyana. Subsequently, it was reported in Peru parasitizing Geophagus steindachneri Eigenmann and Hildebrand, 1922 (^{Tantaleán et al., 1985}). The present study provides the first report of this species in Brazil parasitizing the type host.

In the original description, ^{Inglis (1962)} described the anterior region of the female as having 2 structures similar to teeth, in dorsal view. In the specimens studied here, only 1 tooth was visible in lateral view. Inglis presented measurements of 0.036 × 0.011 mm for a single egg, and no filament was mentioned; while in our material the eggs are larger than in the type material (0.087 × 0.028) and present 1 long filament at each pole. In addition, we also described other important morphological features that were not previously included in the original description, such as buccal cavity and details of the esophagus and pharynx.

After the original description of I. bursata, it was redescribed by ^{Moravec and Laoprasert (2008)} from Symphysodon spp. and hybrids, and from S. discus in Thailand. In the same paper, these authors reported I. bursata parasitizing S. aequifasciatus from an unknown locality in Brazil. In the present paper, we present the first record of I. bursata in M. festivus, which represents a new host for this nematode. The specimens described here are in agreement with the original description by ^{Moravec and Prouza (1995)}.

The present study contributes with new morphological data, and it increases knowledge of biodiversity besides expanding the geographic distribution of nematode parasites of fish.