Two new species of monogenoidean parasites (Platyhelminthes: Neodermata) of ornamental fish of Loricariidae (Siluriformes) from the Xingu River, Brazilian Amazon

Two new monogenoidean species of Unilatus Mizelle & Kritsky, 1967 found in the gills of loricariids in the Lower Xingu-Iriri rivers are described: Unilatus humboldtii sp. nov. from Baryancistrus niveatus (Castelnau, 1855), Panaque armbrusteri Lujan, Hidalgo & Stewart, 2010, Pseudacanthicus sp. (type-host), and Scobinancistrus aureatus Burgess, 1994; and Unilatus luciarappae sp. nov. from P. armbrusteri. Unilatus humboldtii sp. nov. is distinguished from other congeneric species due to its anterior anchor with well-developed superficial root with depressed or truncated distal portion, and inconspicuous or reduced deep root; anterior bar with posteromedial projection; hooks of pair 1 with dilated shaft comprising 2/3 of the hook length; spiraled male copulatory organ with approximately 16–18 counterclockwise loops. Unilatus luciarappae sp. nov. is characterized by having anterior anchor with well-developed superficial root and reduced deep root, slightly curved shaft, elongated, slightly curved, and tapered point; spiraled MCO with approximately 18–19 counterclockwise loops, with median distal bulbous portion and remaining portion tapered and elongated, comprising 1/4 of the body length; and accessory piece comprising approximately 2/3 of the MCO length. More than fifty years after the description of the type species of Unilatus from an artificial environment (U.S. aquaria), this study represents the first formal record and description of Unilatus species from hosts collected in natural habitats in the Xingu River. Considering the impact of the Belo Monte dam on the formerly pristine conditions of the study region and the importance of loricariids for the ornamental fish trade, we recommend monitoring monogenoidean diversity, parasite-host interaction, as well as the dispersion patterns and pathogenicity of these parasites.


INTRODUCTION
Brazil is a regional leader in the global exportation of ornamental fish from inland waters, representing 23% of the South American market (Prang 2007). Much of this trade is comprised of fish species from the Amazon basin, which are favorites among Brazilian and foreign aquarists and represent an important source for international trade (Carvalho Júnior et al. 2009). Total revenues in this market are estimated at USD 68,246,689.00 between 2012 and 2019 (Siscomex 2020), with high profitability driving an active global market (Araújo et al. 2017).
The state of Amazonas, especially the Rio Negro region, is the largest source of ornamental fish in Brazil, representing approximately 60% of the country's total exports, with the state of Pará, accounting for an additional 30% (Lacerda 2005). In Pará, the Xingu region is one of the main centers of ornamen tal fish extraction, with approximately 422 species with trade interests reported for the region (Carvalho Júnior et al. 2009). Among these species, loricariids (Loricariidae), freshwater rays (Potamotrygonidae), and cichlids (Cichlidae) (Camargo 2011) are noteworthy, in part because several species are endemic to the Xingu region, giving them a high added value (Araújo et al. 2017).
In Brazil, the capture, commercialization, and exportation of fish were regulated by Normative Instruction MPA/MMA 001/2012, which was enforced until 2020. This legislation cove red 725 fish species from Brazilian continental waters, of which 125 (~17%) belonged to the Loricariidae. Commercial interest in species of this fish family is considerable in the Xingu River basin since, of the 55 species reported, at least 38 are commer cialized as ornamental species (Camargo et al. 2011). Loricariids, widely known as commonly called "plecos" or "L" by aquarists.
Endemic to the Neotropics, the Loricariidae is one of the largest fish families in South America with approximately 916 valid species, encompassing about 1/4 of the diversity of the order Siluriformes (Armbruster 2011, Eschmeyer and Fong 2016, Nelson et al. 2016. Loricariids are highly valued on the international ornamental fish market. For example, species like Scobinancistrus aureatus Burgess, 1994, commonly known as "Goldie Pleco", can be sold by 1,500% of the Brazilian initial export cost (Araújo et al. 2017). However, loricariid collectors -known as acarizeiros -earn marginal wages. The relatively reduced income of the acarizeiros results in a greater effort of capture. The high individual value of loricariids of commercial interest stimulates intensified selective fishing and may lead to a depletion of natural stocks (Ramos et al. 2015). This is the case of the "Zebra Pleco", Hypancistrus zebra Isbrücker & Nijssen, 1991, a species endemic to the Xingu River basin, whose capture was prohibited in 2004 (Ibama 2004) due to overfishing.
Studies on the pathogens or parasites that affect ornamen tal fishes exported from Brazil (Fujimoto et al. 2013, TavaresDias et al. 2017, Cardoso et al. 2018, Hoshino et al. 2018) and abroad (Mehdizadeh Mood andRassouli 2016, MendozaFranco et al. 2018) are still in their early stages. Recent data on monog enoidean platyhelminths, for example, suggest that only 8% of ornamental fish species regularly sold in Brazil are examined for these parasites (Cohen et al. 2013). For monogenoidean parasites, it is currently estimated that only approximately 4% of this diversity has been revealed (Cohen et al. 2013, Branches and Domingues 2014, Acosta et al. 2017a, 2017b, Franceschini et al. 2018, Vianna and Boeger 2019, Kritsky et al. 2020. Cur rently, 40 species of monogenoids are known from species of Loricariidae, of which 27 species were reported to parasitize the gills and 13 the body surface (Table 1). However, there are no records of monogenoidean parasites from species of Loricariidae in the Xingu basin, despite their importance of the Loricariidae in the regional ichthyofauna (Winemiller et al. 2016).
While investigating the monogenoidean parasite diversity from the gills of Baryancistrus niveatus (Castelnau, 1855), Panaque armbrusteri Lujan, Hidalgo & Stewart, 2010, Pseudacanthicus sp., and S. aureatus of Volta Grande, Xingu River, Pará, two new species of Unilatus Mizelle & Kritsky, 1967 were found. These two new species are described here, representing the first species of the genus formally described from loricariids in the Xingu River basin.
Etymology. The species is named after Alexander von Humboldt in celebration of his 250 th birthday anniversary in 2019.
Remarks. Unilatus humboldtii sp. nov. resembles U. brittani Mizelle, Kritsky & Crane, 1968 due to the morphological similari ties of the anterior bar. However, the new species is distinguished from U. brittani morphologically and morphometrically since it has an anterior anchor with welldeveloped superficial root with depressed or truncated distal portion, inconspicuous or reduced deep root, anterior bar with posteromedial projection; hooks of pair 1 with dilated shaft comprising 2/3 of the hook length, and MCO spiral with approximately 16-18 coils.
Etymology. The species is named in honor of Dra Lucia Rapp of the Instituto Nacional de Pesquisas da Amazônia (INPA), Brazil, in recognition of her contribution to the knowledge of loricariid diversity.
Remarks. Unilatus luciarappae sp. nov. resembles U. brit tani by to the morphology of the anterior anchor/bar complex. However, U. luciarappae sp. nov. is distinguished by having a posterior anchor with shaft and uniformly recurved point, and posterior bar with tapered ends, while U. brittani has a posterior anchor with a slightly curved shaft and long and straight point, as well as a posterior bar with expanded and rounded ends. These species can also be differentiated by the ratio of the distance from the proximal portion of the MCO to the dilation of the MCO and the distance from the dilation of the MCO to the distal end of the MCO (Unilatus luciarappae sp. nov. 1:2; U. brittani 1:1).
Unilatus presently comprises eight species, including the two described in this study. Of these, seven have been reported in loricariids sold on the Brazilian ornamental fish market. Species of the genus have only been recorded in fish exported from Brazil in the cases of U. unilatus and U. britanni. There are no records of these parasites being transferred to other hosts after their introduction, and their pathogenicity is unknown in native and alien environments. Jogunoori et al. (2004) suggest that possible economic losses and environmental impacts may occur due to the intro duction of monogenoidean species into new environments and hosts. However, they note a low potential for monogenoids to transfer between alien and native host species. JiménezGarcia et al. (2001) found that monogenoidean species typically found in African cichlids used worldwide in aquaculture systems had parasitized freshwater cichlid species native to Mexico. Also, GarcíaVásquez et al. (2017) recorded Gyrodactylus cichlidarum Paperna, 1968, a monogenoidean parasite typically reported from the African "tilapia", on poeciliids (Cyprinodontiformes) in Mexico. However, even if there is no evidence of problems these parasites can provoke after becoming established, care should be taken to prevent their establishment in new ecosystems. Adopting prophylactic measures and quarantine procedures to mitigate the transfer of parasites among fish sold on the Brazilian ornamental fish market is important. However, studies on the diversity of parasites that occur in these fish are also needed, as they shed light on the potential of dispersion of these parasites as well as their pathogenicity.
More than 50 years after the description of the type species of Unilatus from an artificial environment (U.S. aquariums), this study is the first formal record and description of Unilatus spe cies from hosts collected in natural habitats in the Xingu River basin. Considering the loss of pristine conditions in the studied region as a result of the construction of the Belo Monte Dam, we recommend monitoring monogenoidean diversity, parasitehost interactions, parasite indices, and parasite dispersion patterns and pathogenicity. Such studies may provide important insights into the impacts of changes to the hydrological regime in the Xingu River caused by the Belo Monte Dam.
Studies of the Belo Monte Dam's impacts on the per sistence of the diversity of the loricariid hosts and the associa ted helminth fauna recently described for the Xingu basin are urgently and continuously required. The changes in ecological dynamics are characterized by the 'lentification' of the aquatic environment once formed by rapids in the region upstream of the dam and by the reduced flow stretch for the 100km stretch of the 'Big Bend'. These changes have caused high mortality of ichthyofauna in the region while decreasing oxygenation and increasing water temperature due to the Xingu River's rocky bed. Thus, the risks for the conservation of the diversity of the recently described helminth fauna and the parasitehost ecolog ical dynamics are increased. Furthermore, all aquatic biota and the livelihood of human populations in the region have been negatively impacted.

ACKNOWLEDGMENTS
We would like to Geusivam Barbosa Soares for assistance during the collecting trips. This work was partially supported by Research grants from the Conselho Nacional de Desenvolvi mento Científico e Tecnológico (CNPq, PROTAX 001/2015 440526/20159), and Fundação Amazônia de Amparo a Estudos e Pesquisas do Pará (FAPESPA) (ICAAF 0172018) to M.V.D. Specimens were collected under the license for collection of bi ological material (43381) granted by the Instituto Chico Mendes de Conservação da Biodiversidade (ICMBio).