Checklist of parasites in fish from the upper Paraná River floodplain: An update.

This study provides an updated list of parasite species and their respective hosts in the upper Paraná River floodplain. The list of parasites is structured by phylum, class, order and family, followed by a record of each host species. A total of 315 taxa of parasites were reported, of which 201 were identified at the species level. These 201 species comprise 3 Flagellata, 3 Myxozoa, 50 Monogenea, 43 Digenea, 40 Cestoda, 41 Nematoda, 8 Acanthocephala, 6 Copepoda, 5 Branchiura and 2 Pentastomida, arranged in 84 host fish species. This work carried out in the floodplain of the upper Paraná River contributes to the listing of parasite species and host interactions of the local ichthyofauna. Little is known about these communities and its riches are underestimated due to the high density of fish found in this region, highlighting the importance of conducting studies on the local fauna.


Introduction
The Paraná River is the tenth longest river in the world in water discharge and it is located in south-central South America, running through Brazil, Paraguay and Argentina. The upper Paraná River floodplain is the only dam-free stretch remaining, located between the Porto Primavera and Itaipu dams (Agostinho et al., 2001). In this stretch, the river has a vast portion of wide-braided channels from numerous lagoons, whether permanent or temporary, which are distributed throughout the plain and are fed by groundwater or by the flooding process (Junk et al., 1989;Agostinho & Júlio, 1999;Thomaz et al., 2004).

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Parasites in fish from the Paraná River The recurrent floods experienced by this ecosystem are an important selection pressure that favors certain adaptations in the organisms inhabiting this area, resulting in communities with unique structures and functions (Junk et al., 1989;Thomaz et al., 2007). One of the main characteristics of floodplain river systems in tropical and subtropical regions is the high diversity of species, particularly of fish (Horne & Goldman, 1994;Toussaint et al., 2016;Vitule et al., 2017). In this context, the high diversity of the environment can alter the population dynamics of the ichthyofauna, along with their ecological, physiological and biological conditions. This, in turn, influences the structure and composition of parasitic fauna in fish (Pavanelli et al., 1997;Yamada et al., 2008;Luque & Poulin, 2008;Locke et al., 2014), including the trophic category and the intermediate hosts of parasites (Landsberg et al., 1998).
The parasite fauna of fish from the upper Paraná River floodplain were previously detailed by Pavanelli et al. (1997), Takemoto et al. (2009) and further studies that reported taxonomic descriptions of new species, such as Ferrari-Hoeinghaus et al. (2010), Tavernari et al. (2010), Karling et al. (2011Karling et al. ( , 2014. Over the last decade, studies recorded in this area have indicated an increase in the diversity of fish parasites, so the purpose of this study was to provide an updated list of parasites and their respective fish hosts in the upper Paraná River floodplain.

Material and Methods
This study was conducted with data from published articles, dissertations, thesis, and research project reports (consult supplementary material) on parasites from fish carried only at the Laboratório de Ictioparasitologia of Universidade Estadual de Maringá between 1988 and 2019, in the region of the upper Paraná River floodplain ( Figure 1).
The list of parasite species presented in this study is clustered by phylum and classified in class, order, and family. For Myxozoa and Flagellata, the classification is in accordance with Brusca et al. (2018a,b); for Monogenea,  Cohen et al. (2013); for Digenea, Thatcher (1993) and Kohn et al. (2007); for Cestoda, Khalil et al. (1994) and Vidal-Martínez et al. (2000); for Nematoda, Moravec (1998); for Acanthocephala, Yamaguti (1959) and Thatcher (2006); for Crustacea, Eiras et al. (2010), Ahyong et al. (2011) and Luque et al. (2013); and for Hirudinea and Acari, Brusca et al. (2018c,d). Some informative changes in recent literature, such as the World Register of Marine Species, have also been considered. When the species are unknown, the parasites were cataloged in class, family or genus, while the identified species are presented in alphabetical order, followed by the author and year of publication. For the hosts, the identification and registration of occurrence in the floodplain were checked and confirmed according to Ota et al. (2018), and these are listed opposite their respective parasites. Information about the parasite's life stage and the infection site was also added.
For the quantification of interactions, all association formed between a given parasite and its host was considered. For parasites with more than one host, the number of interactions was added, taking into account all their respective hosts. We consider the most specific taxonomic level of the parasite, since all hosts are at the species level.

Results
In this study, 13 groups of parasites were registered for 84 species of ichthyofauna from the upper Paraná River floodplain, of which eight were groups of ectoparasites and six of endoparasites, with emphasis on the Pentastomida group considered for two types of parasitism. Three hundred and fifteen taxa were cataloged and 201 of these were identified at the species level. These 201 species comprise 3 Flagellata, 3 Myxozoa, 50 Monogenea, 43 Digenea, 40 Cestoda, 41 Nematoda, 8 Acanthocephala, 6 Copepoda, 5 Branchiura, and 2 Pentastomida. The parasites belonging to Isopoda, Hirudinea and Acarina were identified only at the group level, but are added together in the interaction records.
The parasites of the Monogenea class had the largest number of identified species, totaling 50. However, the largest number of different interactions recorded, regardless of the level of identification (order, family, or larval stage, immature form), were the parasites of the phylum Nematoda with 227 interactions, followed by the Digenea subclass with 176 interactions, and then the Monogenea class with 149 interactions. Parasites belonging to the phylum Nematoda and to the subclass Digenea were the groups with the largest number of cataloged interactions, which may be associated with the fact that they exploit different hosts species in their life cycles (Esch et al., 2002;Poulin, 2002;Luque, 2004;Takemoto et al., 2009). In the Cestoda class, it was possible to observe a significant increase in the number of identified species and interactions with several hosts, which is in accordance with studies that classify the order Onchoproteocephalidae, for example. That study covers the largest number of parasites in freshwater fish species (Luque, 2004); however, studies on ecological and pathological aspects of their cycle are lacking .

Rhinoxenus bulbovaginatus
The species of the phylum Acanthocephala were found in low richness in this study. It is important to study these organisms because they are commonly used as bioindicators, due to their high capacity for absorbing trace elements (Taraschewski, 2000;Dos Reis et al., 2017). They have a large bioaccumulation capacity, especially due to the absence of a digestive system, which leads them to absorb the pre-digested nutrients through diffusion directly from the intestinal content of fish. In addition, the location of the parasite in the fish -as well as its developmental stage -can perform an important role in the bioaccumulation process (Nachev & Sures, 2016;Paller et al., 2016;Sures et al., 2017).
The Hirudinea subclass has few records of interactions and it is still debated whether they are ectoparasites or associated organisms (Richardson et al., 2015). However, they play an important role in acting as vectors of flagellated protozoa of the Trypanosomatidae family, which were recorded parasitizing different hosts in our study. Much has been discussed about the specificity of Trypanosoma spp. and future studies with descriptions of new species for new hosts, using molecular biology, can help to understand these important parasites (Marchiori & Martins, 2013).
The Myxozoa parasites recorded just three species parasitizing three different hosts, all native to the Paraná River. However, we can highlight here the importance of studying their hosts (vertebrates and invertebrates) more thoroughly, in association with these parasites, given that more recent studies have showed their life cycles with several hosts (Takemoto et al., 2009;Eiras & Adriano, 2013).
The phylum Arthropoda represents the greatest diversity of parasite groups (Arguloida, Pentastomida, Copepoda, Isopoda and Acarina), with a wide diversity of sizes, shapes and strategies of parasitism. In most of the cases, they are ectoparasites and are found in the gills and external surface of fish (Arguloida, Isopoda and Acarina), and also in some cavities of the body, such as nostrils (Copepoda), while the parasites of the subclass Pentastomida can be found in internal organs such as the gas bladder (Eiras et al., 2010). Although there is a large number of studies with these different groups, there is a lack of ecological studies (Luque et al., 2013), and the data published in this work partly reflect the lack of interest in studies on these groups acting as parasites.
In relation to infected host species, there was an increase of 14% in studies on their parasitic fauna in relation to the previous work, which can be explained by the constant process of invasion of non-native species of fish. The floodplain is located between two large reservoirs that, despite favoring the development of the local and regional economy, have seriously affected the aquatic biota with the emergence of non-native species (Gois et al., 2015;Agostinho et al., 2016). Invasive fish may be hosted by parasites that are not present in the local biota, and when introduced to the site together with their hosts, they can increase competition and affect the parasitic composition of native fish. In addition, it is known that 45% of fish species (Ota et al., 2018) whose parasitic fauna have already been studied are not native to the floodplain.
The biological characteristics of parasites have revealed information about host ecology, biogeography and phylogeny (Gardner & Campbell, 1992;Raga et al., 2008), and have also provided indicators of current and historical ecological relationships (Aznar et al., 2001). Therefore, to understand the role of the parasite community in an ecosystem, it is necessary to have prior knowledge of the species that belong to it (Takemoto et al., 2009). Although the studies conducted in the upper Paraná River floodplain have contributed towards surveying the parasite species, the species richness is still underestimated because of the high diversity of fish and the small number of these studies on this community. This emphasizes the importance of the present survey for deepening knowledge about the local fauna.