First record of Trichodina heterodentata (Ciliophora: Trichodinidae) in banded knifefish Gymnotus carapo (Gymnotidae) cultured in Brazil

The aim of this study was to report the first record of Trichodina heterodentata in banded knifefish Gymnotus carapo . Banded knifefish cultivated in the municipality of Jataí, state of Goiás, in the central western region of Brazil, experienced fish mortality rates of 3-4% per tank. Macroscopic analysis found that fish had lesions on their skin and gills. Smears of the integument and gills were confectioned and air-dried at room temperature and impregnated with silver nitrate to identify the Trichodinidae species causing the lesions. The trichodinid were identified as T. heterodentata, and their characteristics were compared to those from T. heterodentata recorded from other species of host fish. It was observed that the presence of T. heterodentata was associated with inadequate fish management, low water quality, high rates of stocking density, and inadequate nutritional management.


Introduction
The banded knifefish Gymnotus carapo Linnaeus, 1758, is a species in the order Gymnotiformes and the family Gymnotidae. Banded knifefish have electrical organs that are mainly used for electrocommunication (Rotta, 2004). Banded knifefish are distributed in Central and South America, from south-eastern Mexico to Paraguay, including Trinidad and Tobago (Froese & Pauly, 2019). In their natural habitat, banded knifefish live in places with aquatic plants with dense roots that retain organic matter, and feed on small organisms, such as insects and microcrustaceans (Rotta, 2004;Resende et al., 2006;Froese & Pauly, 2019). This fish species is farmed in several places, including in central Brazil, for its use as live bait for sport fishing in rivers, lagoons, and even in fish-pays (Rotta, 2004). Hence, the use of banded knifefish in sport fishing stimulates the cultivation of this species, and the development of enterprises for its production. However, there is no available information about the parasites that infect cultivated banded knifefish. The present study expands our knowledge on this topic by describing the first record of Trichodina heterodentata Duncan 1977 from this fish species.

Abstract
The aim of this study was to report the first record of Trichodina heterodentata in banded knifefish Gymnotus carapo. Banded knifefish cultivated in the municipality of Jataí, state of Goiás, in the central western region of Brazil, experienced fish mortality rates of 3-4% per tank. Macroscopic analysis found that fish had lesions on their skin and gills. Smears of the integument and gills were confectioned and air-dried at room temperature and impregnated with silver nitrate to identify the Trichodinidae species causing the lesions. The trichodinid were identified as T. heterodentata, and their characteristics were compared to those from T. heterodentata recorded from other species of host fish. It was observed that the presence of T. heterodentata was associated with inadequate fish management, low water quality, high rates of stocking density, and inadequate nutritional management.

First record of Trichodina heterodentata (Ciliophora: Trichodinidae) in banded knifefish Gymnotus carapo (Gymnotidae) cultured in Brazil 2. Material and Methods
Banded knifefish are cultivated in the municipality of Jataí, state of Goiás, Brazil, for the purpose of producing baits for commercial use in sport fishing. The fish included in this study were kept at a local fish farm, 17º52'27.6"S and 51º37'49.3"W, in 500 L water tanks at densities of 90-110 fish per tank. There was neither renewal nor circulation of water, and fish were fed once a day with commercial feed (0.8 mm) containing 45% crude protein.
The reservoirs were cleaned every two days, by siphoning and removing organic matter from the bottom of the tanks, and replacing approximately 5% of the water. At the fish farm included in this study, 3-4 fish per tank died, and the dead fish had small dark skin lesions. Six fish were sent to the Veterinary Pathology and Parasitology Laboratory at the Federal University of Goiás, in Jataí, for parasitological analysis. Smears of body mucus and gills were analyzed under an optical microscope (Nikkon Eclipse E200, Japan) for the presence of Trichodinidae Claus, 1874. The smears were air dried at room temperature and impregnated with silver nitrate for visualization of the adhesive discs of trichodinids (Lom, 1958).

Results
Trichodinid were identified as T. heterodentata (Figure 1), and compared with literature descriptions for T. heterodentata collected from other species of host fish (Table 1). We compared the measurements of the specimens in this study to those reported in other studies, and found that their characteristics were similar to those of T. heterodentata described by Duncan (1977)

Discussion
Trichodina heterodendata has been described since 1977 from a variety of freshwater fish (Jager & Basson, 2019), mainly in cichlid species (Basson & Van As, 2006). In Brazil, T. heterodentata have been reported from small-scaled pacu Piaractus mesopotamicus Holmberg, 1887 (Pádua et al., 2012), channel catfish Ictalurus punctatus Rafinesque, 1818 (Martins et al., 2010), Nile tilapia Oreochromis niloticus Linnaeus, 1758 (Valladão et al., 2016), and in a hybrid toad species, Rhinella pombali Baldissera, Caramaschi & Haddad, 2004(Dias et al., 2009Fernandes et al., 2011). However, T. heterodentata has not been previously reported in banded knifefish until present moment. Jager & Basson (2019) questioned the validity of the species T. heterodentata, based on the variety of reported morphometric data. Also, these authors pointed   ( Denticle L 8.4 ± 0.4 (8.0-8.4) 9.2 (7.5-11.0) 10.3 ± 1.2 (7.0-13.0) 7.55 ± 0.62 (6.31-8.6) 7.7 ± 0.7 (6.1-9.1) Blade L 3.5 ± 0.2 (3.3-3.7) 5.7 (4.7-7.1) 6.2 ± 0.8 (4.0-8.0) 4.46 ± 0.59 (3.9-5.5) 4.6 ± 0.4 (3.7-5.5) Central part W 2.0 ± 0.0 2.6 (1.4-3.4) 3.8 ± 0.7 (2.0-6.0) 2.35 ± 0.28 (1.5-3.1) 1.9 ± 0.4 (1. In fact, several species, not only of parasites, present different morphometric data, especially when they are cosmopolitan. Likewise, species with same morphology, but different taxonomy are re-arranged in a single taxon. This is the scientific dynamic that occurs in studies of species description and distribution. Surely, this study will not end the discussion on trichodinids and T. heterodentata. This study only provides data, so that within the evolution of this scientific knowledge, it will possibly be rearranged as the techniques evolve, and when the analyzing of specimens from various parts of the world becomes possible. There was an outbreak of T. heterodentata in the examined banded knifefish due to the low water quality in the tanks and the excess of organic matter, which were visible. Ciliated trichodinids (Mobilida) are among the main etiological agents of mortality in farmed fish. Trichodinids are of great importance in fish cultivation, especially when the environment favors their reproduction, in which case severe infestations can damage the gills and skin of the fish, making them susceptible to respiratory problems and secondary infections (Zanolo & Yamamura, 2006;Valladão et al., 2016;Rodrigues et al., 2019). In addition, the banded knifefish included in this study were farmed at a high stocking density (90-110 fish/500 L tank), which was well above the recommended stocking density for this species (approximately 100 fish for every 1,000 L of water, Ishikawa et al., 2017). High stocking densities are stressful for fish, making them susceptible to ectoparasite infections, including trichodinid infections.
In summary, the occurrence of T. heterodentata in banded knifefish was associated with poor fish management conditions, indicating the need for adequate management and sanitary conditions.