Distribution of invasive bivalves in Paraguay: filling the gaps in the heart of South America

: La Plata Basin is the second largest hydrographic system in South America but the most impacted by invasive species of molluscs in the continent. Paraguay is a land-surrounded South American country with scarce information about malacological fauna. To fill some of these gaps, a large field expedition was conducted to analyze the distribution of invasive freshwater bivalves in Paraguay. The field survey encompassed 41 sampling stations in seven Departments of Paraguay. The mussels were searched using bare hands and feet. All stations were georeferenced and some environmental parameters were measured. Results indicated the presence of three invasive species of freshwater bivalves in Paraguay: Corbicula fluminea , C. largillierti , and Limnoperna fortunei . The Asiatic clam Corbicula fluminea was the most widespread invasive species, covering main rivers and small tributaries in the four freshwater ecoregions of Paraguay. The two remaining invasive species were more restricted. Corbicula largillierti were found at the lower section of the Paraná River, and Limnoperna fortunei inhabits the main river canals of the Paraguay River and the lower section of the Paraná River. The invasive species were found in three protected areas of Paraguay and can represent a threat to the mostly unknown fauna of native mussels. The invasive bivalves are widespread in Paraguay. Mitigation measurements must be taken to avoid the spread of these species and more drawbacks over native fauna.

Acta Limnologica Brasiliensia, 2024, vol. 36, e2 in the country, most of them from areas shared with Argentina or Brazil (Peso et al., 2011;Oliveira et al., 2015;Hermes-Silva et al., 2021).A comprehensive revision of the golden mussel invasion in La Plata Basin was done recently by Oliveira et al. (2015) and Ludwig et al. (2021), however, with scarce records from Paraguay and several occurrences upstream (Brazil) and downstream (Brazil, Argentina and Uruguay).Thus, there are records upstream and downstream to Paraguay, but any in Paraguayan area.Moreover, the understanding of the invasion process of bivalves in the La Plata Basin was probably biased due this lack of information.
The main goal of this study was to present an updated distribution of invasive bivalves in Paraguay, relying on new records and museum data, relating this to freshwater and terrestrial ecoregions, as also presenting physical and environmental variables (pH, conductivity, TDS).The occurrence of invasive species in protected areas are discussed, and comments on the possible impacts of invasive on native species.
The records of invasive bivalves came from a recent field expedition conducted in Paraguay (September 2021).The field surveys encompassed 41 collecting stations, in seven Departments (Concepción, Guairá, Misiones, Ñeembucú, Paraguari, Presidente Hayes and San Pedro) and the Capital District (Asunción), where bivalves were searched using bare hands and feet.Almost all surveyed sites were never sampled before for freshwater mussels.At all collecting stations, georeferenced coordinates were obtained using a GPS Garmin 64s; and conductivity, pH and TDS (Total Dissolved Solids) using a Hanna HI98129 Paraguay is a landlocked country with a central position in South America, and two main rivers of the La Plata Basin run through the country: Paraná and Paraguay rivers.This is the secondlargest basin in South America and harbors an important molluscan fauna (Strong et al., 2008;Pereira et al., 2014;Cuezzo et al., 2020).A total of 16 freshwater species of molluscs were reported as non-native from South America by Darrigran et al. (2020).These authors reported the presence of seven non-native species in the La Plata Basin, from which were highlighted Corbicula fluminea (Müller, 1774), the Asiatic clam; and Limnoperna fortunei (Dunker, 1857), the golden mussel.Corbicula species arrived in South America in the 1970s and Limnoperna fortunei some years later (around the 1990s), both are widespread now (Darrigran et al., 2020;Miyahira et al., 2020;Hermes-Silva et al., 2021).Invasive species can promote modification of environmental and biological aspects of the invaded habitat (e.g., Darrigran & Damborenea, 2011;Sousa et al., 2014;Boltovskoy & Correa, 2015), affecting not only native molluscs, but also other taxonomic groups.Understanding the distribution of invasive species is the first step to proposing strategies to mitigate the negative impacts.
Multiparameter.Following Brasil (1997), the Paraguay River can be divided into three parts, Upper Paraguay from the headwaters to Jauru River; Middle Paraguay from Cáceres (state of Mato Grosso, Brazil) to Apa River (border of Brazil and Paraguay); and Lower Paraguay from Apa River to the mouth at Paraná River (border of Paraguay and Argentina).All collecting stations at Paraguay River Basin were carried out at the lower part of the basin but covered an important portion of Paraguay, including tributaries from the western and eastern regions of the country.There is scarce information about native freshwater mussels of Paraguay, but based in this piece of information, the sampled area can be pointed as a high diversity area (Schade, 1965;Quintana, 1982).The data from field surveys were complemented with a review from the collection of the Museo Nacional de Historia Natural del Paraguay.
The study was carried out with the authorization of the Ministerio del Ambiente y Desarrollo Sostenible of Paraguay (Permission number 028/2021) and all the specimens obtained were deposited at the Invertebrates Collection of the Museo Nacional de Historia Natural del Paraguay under field trip code PY-21-01 to PY-21-40.
Paraguay is divided into four freshwater ecoregions (Abell et al., 2008) and these were used for analyzing the invasive species distribution.The Paraguay River divides the country on a north-south axis, at the West, one large ecoregion (Chaco) that drain a huge dry area, and in the East, a more humid area with perennial rivers (Paraguay, Lower Paraná and Upper Paraná ecoregions) (Abell et al., 2008;Koerber et al., 2017).We also analyzed the distribution of bivalves in terrestrial ecoregions (Olson et al., 2001), which retain some information about the nearby environment.There are five main terrestrial ecoregions in Paraguay: Cerrado, Dry Chaco, Upper Paraná Atlantic Forest, Pantanal and Humid Chaco.There are also small portions of the Paraná flooded savanna and Southern Cone Mesopotamian savanna that was considered along with the surrounding Humid Chaco in our analysis.The terrestrial ecoregions were also evaluated because add different division of Paraguayan territory that bring environmental information that cannot be perceived at freshwater ecoregions analysis.The georeferenced data of protected areas in Paraguay were obtained from World Database on Protected Areas (WDPA, 2021).The percentage of invasive species occurrences was calculated with a 1km buffer around each occurrence to discount any inaccuracy in the GPS measurements.
Three non-native invasive bivalves species were found in Paraguay, two species of Asiatic clams, Corbicula fluminea and C. largillierti; and the golden mussel, Limnoperna fortunei (Figure 1).The invasive species occurred in 33 localities (21 from field survey and 12 from museum collection), and in 20 localities no invasive bivalves were found (Figure 2A-C).Only C. fluminea was found at the museum survey, the other two species were found at field survey.
Corbicula fluminea was the species with wider distribution, occurring in 33 localities and all four freshwater ecoregions of Paraguay (Chaco, Paraguay, Upper Paraná and Lower Paraná).Corbicula fluminea was more flexible in environmental preferences, occurring in waters with a lower pH, and also with higher conductivity and TDS (Table 2).Previous data only included some scattered records in the rivers shared with nearby countries (mainly Brazil and Argentina), with the contribution of this manuscript the distribution of invasive bivalves in Paraguay is now more detailed, with new records inside the country, including large rivers and small tributaries.Most of the records were to the east of Paraguay River, except by two sites in Confuso River near to the mouth of this river in Paraguay River.The gap in invasive species distribution in Paraguay is not a real absence of these species, but a lack of sampling effort.The works done before in Paraguay were isolated efforts (Schade, 1965;Quintana, 1982) and was never established continuous research in malacology at Paraguay.Therefore, the recent records of invasive species at Paraguay were done at the borders (e.g.Oliveira et al., 2015).Our records indicate that invasive bivalves were also widespread in this part of the La Plata Basin which corresponds to the lower portion of the Paraguay River.
The occurrence of invasive species was concentrated at the east of the Paraguay River.The large freshwater ecoregion of Chaco dominates the area west of the Paraguay River, and despite the sampling effort in our survey was concentrated at south (Humid Chaco), is probably that this ecoregion is really poor in bivalve occurrences.We raise this hypothesis considering that the collecting stations done by us at the south portion of the freshwater ecoregion of Chaco returned no invasive bivalves (Figure 2A-B).The north portion of Chaco (which corresponds to the terrestrial ecoregion of Dry Chaco) was an area not sampled by us, however, data from the literature indicates that the environmental conditions became even harsher (Lemenkova, 2021) and suggests to us that should be unsuitable for most invasive bivalves.Although can be possible to find some species of Sphaeriidae that resist extreme dry climates (e.g., Vila et al., 2020), this is not usual for invasive bivalves (i.e.Corbicula and Limnoperna) and large native freshwater mussels (Unionida).Guareschi & Wood (2020) showed a massive die-off of C. fluminea in water temperatures higher than 25°C.Limnoperna fortunei can survive in higher temperatures than C. fluminea (Oliveira et al., 2015), although extreme water level variation during the year as occurs at Chaco is also a limitation.Annual variation in water level associated with water acidification at Pantanal was suggested as a limitation to the establishment of large populations of golden mussel in this region (Pessotto & Nogueira, 2018).Therefore, the combination of dry climates and extreme water variation makes almost half of Paraguayan territory unsuitable for freshwater bivalves (native and invasive).
The species of Corbicula were widely distributed in South America, and in Paraguay the situation is similar.These species, especially C. fluminea, occur virtually in all basins of South America (e.g., Darrigran et al., 2020;Miyahira et al., 2020).Corbicula fluminea occurred from small to large rivers in Paraguay, suggesting some flexibility in environmental requirements.There is also a wide range of environmental variables (Table 2) in the sites where this species occurs.However, more information is needed to test the relationship of the occurrence of C. fluminea with environmental variables in Paraguay.Corbicula fluminea was the only invasive species to occur in interior basins.Corbicula largillierti presented a restricted distribution in Paraguay, and this species is usually displaced by C. fluminea in invaded sites (Reshaid et al., 2017).(Karatayev et al., 2007;Facetti-Masulli et al., 2009;Ferreira-Rodríguez & Pardo, 2016;Crespo et al., 2017).Those bivalves (invasive and native) probably colonize the Confuso River seasonally from the Paraguay River, during the period of floods, and were extirpated during dry periods of higher influence of the Pilcomayo River.Our collecting survey was conducted during a period of low level of the Paraguay River waters.As we considered that these species do not truly occur under these conditions, these values were not considered in Table 2. Darrigran et al. (2011) already proposed the Pilcomayo River as a salinity barrier for the dispersion of L. fortunei to the Andean tributaries of La Plata Basin.
In Paraguay, the golden mussel Limnoperna fortunei was restricted to the main channel of large rivers (Paraguay and Paraná) and nearby areas.However, not only on the borders with Brazil and Argentina but also on the Paraguay River inside the country.Limnoperna fortunei is usually restricted to large rivers and rarely attains small tributaries (Barbosa et al., 2016).The faster dispersion of the golden mussel throughout the La Plata Basin is usually linked to commercial navigation (Pessotto & Nogueira, 2018).This can also explain the distribution observed in Paraguay (i.e. in large rivers with more ships), although a dispersion in large tributaries of the Paraguay River can be possible, as, in Aquidabán and Tebicuary rivers, the golden mussel has not achieved those basins yet.In our study, L. fortunei was found in higher pH values than in previous studies (Pessotto & Nogueira, 2018), but with lower values of conductivity.Despite being less flexible to environmental conditions than Corbicula fluminea; L. fortunei also presented a wide range of tolerance to different conditions.
The presence of invasive bivalves in protected areas is a silent threat in Paraguay.The occurrence of the two most invasive bivalves of South America (Corbicula fluminea and Limnoperna fortunei) in three protected areas of Paraguay is an indication that these areas are not fully efficient to protect the freshwater fauna.The invasive bivalves affect not only their native counterparts but may also affect the whole environment, affecting transparency and plankton dynamics (e.g., Boltovskoy et al., 2009;Darrigran & Damborenea, 2011;Sousa et al., 2014;Ferreira-Rodríguez et al., 2018).Protected areas are usually developed based on the needs of plants and terrestrial vertebrate animals (Fagundes et al., 2016;Frederico et al., 2018), and freshwater species usually have different requirements.Thus, the existing protected areas are not always suitable for freshwater animals and environments.Our surveys were not directed to protected areas, and probably the invasive bivalves occur in other areas of Paraguay.The management plans of those areas must be updated to consider the presence of invasive species and develop strategies to mitigate their impacts on native fauna.Moreover, the occurrence of invasive bivalves in sympatry (Table 1) can potentialize their negative effects.
Detailed distribution is important to establish proper strategies to deal with invasive species and for the conservation of native species.Several species of native mussels occur in Paraguay and Paraná basins (Cuezzo et al., 2020), although the information is still limited to old records (Schade, 1965;Quintana, 1982).Recent efforts done in the state of Mato Grosso, upstream of Paraguay, showed a rich and ecologically important fauna of native freshwater mussels (e.g., Colle & Callil, 2012;Olivera-Hyde et al., 2020;Santos et al., 2020Santos et al., , 2021)).Pereira et al. (2014) considered the Paraguay Basin at Mato Grosso as of high diversity, but the Paraguayan portion as of medium richness.However, it is probably an artifact of low sampling effort as were observed for invasive species, but further investigation is needed to reveal the real diversity of native freshwater mussels in Paraguay.

Figure 1 .
Figure 1 .Invasive bivalves in Paraguay.A -Clusters of Limnoperna fortunei in an exposed rock.B -Specimen of Corbicula fluminea collected in Paraguay.Scale bar = 1cm.

Figure 2 .
Figure 2 .Distribution of invasive bivalves in Paraguay.The circles are the records from sampled sites and the squares the records from the Museo Nacional de Historia Natural del Paraguay.A -Freshwater ecoregions, B -Terrestrial ecoregions, C -Protected areas.

Table 2 .
Environmental variables range in the site with invasive bivalves in Paraguay, except for C. largillierti which occurred in one area.

Table 1 .
Species of invasive bivalves found occurring in sympatry in Paraguay.