South American Basins Research Articles

Influence of upwelling mantle magmas on cratonic crust implied from Vp/Vs beneath South America platform

SUMMARY The crust of the South American platform recorded imprints of dynamic processes related with the opening of the Central and South Atlantic but has not been well measured. Crustal structure can be retrieved from teleseismic receiver functions using H–κ stacking, but nearly parallel stripes of high stacking values existing in stacking images for seismic stations in sedimentary area cause difficulties in identifying solutions. We show that some seemingly spurious stripes that do not point to any layer solution are helpful in the identification of the solution position. With the aid of the auxiliary stripes, we retrieved thicknesses and Vp/Vs of sedimentary and crystalline crust for 65 permanent stations of the Brazilian Seismographic Network and six new portable seismic stations in Brazil and Uruguay. The resulted sedimentary thickness and Vp/Vs exhibit a good correlation with the Phanerozoic sediments in the South American basins. The crust of Paraná–Etendeka Large Igneous Province (LIP) had been expected to be more mafic since it had ever been penetrated by mantle magma in the Cretaceous related to the south Atlantic opening. However, we found very low Vp/Vs (1.67) in the crystalline crust beneath the LIP, implying a more felsic crust and that no significant mafic intruding/underplating has occurred in the region. The more felsic crust may be formed in a special evolution early than the magmatic event, or during the magmatic event by releasing crustal volatiles. The resulted sedimentary thickness and Vp/Vs ratios exhibit a good correlation with the Phanerozoic sediments in the South American basins, which implies that Triassic–Jurassic and Cretaceous magmatism did not cause significant metamorphism in sediments formed before the magmatic events.

Tracking the Eurydesma Fauna transgression across southwestern Gondwana

The cold water Eurydesma Fauna, consisting of bivalves, brachiopods, and scarce gastropods, has been associated in several basins of Gondwana with the terminal phase of the Late Paleozoic Ice Age (LPIA). The Eurydesma Fauna-bearing transgression has been identified in basins of South America and southern Africa. In western Karoo Basin of South Africa four phases of ice growth and decay have been proposed for the Dwyka Group on the basis of interstratified glaciogenic diamictites overlain by non-glaciogenic, ice rafted debris free mudstones. Each of these phases is assigned to deglaciation sequences (sensu Visser, 1997) and has been proposed to be incomplete, third order sequences made up of a basal lowstand systems tract (LST) followed by a transgressive systems tract (TST) or exclusively by a thick TST. The correlation of these deglaciation sequences has been extended to the adjacent Aranos-Kalahari basin and to the Paraná Basin (Itararé Group) in South America. Recent high-precision UPb ages on single crystal chemical abrasion, isotope dilution-thermal ionization mass spectrometry (CA-IDTIMS) have helped to refine the diachronous demise of the LPIA and the age of the Eurydesma transgression. Tentative correlations with the glaciogenic section (Sauce Grande Formation) in the Ventana Fold belt (VFB) and Malvinas/Falkland Islands (Fitzroy Tillite Formation) are supported by radiometric ages, and similar facies associations respectively. This interbasinal correlation of deglaciation sequences and radiometric ages allowed to constrain the occurrence of this fauna in southwestern Gondwana to the Asselian.The postglacial nature of the Eurydesma Fauna transgression in South American basins, particularly in the VFB-Claromecó Basin of Argentina, is in contrast with its interglacial character in the Southern African basins, where the Eurydesma transgression caps the deglaciation sequence 3 and underlies the deglaciation sequence 4.

Stressing environmental conditions in the “petrified forest” from the Mata Sequence in the Triassic context of the Paraná Basin

This study deals with the Piscina Outcrop, a Rhaetic reddish conglomeratic sandstone containing silicified logs which corresponds to the Mata Sequence in the southernmost Brazilian Paraná Basin. We performed ground-penetrating radar (GPR) analysis and the results ratified previous sequence stratigraphy analysis indicating that these deposits correspond to architectural elements exclusively related with the filling processes of fluvial channels. The erosive unconformity identified at the boundary between the Mata Sequence and the underlying rock packages could represent a considerable depositional hiatus. The integration of the GPR results with sedimentological and paleovertebrate data from a nearby outcrop at slightly lower topographic level indicated that the layer with the fossil woods is superimposed through an erosive unconformity to the stratigraphic levels corresponding to the Hyperodapedon Assemblage Zone of Carnian age from the basal portion of the Candelária Sequence. The wood assemblage was composed exclusively of araucarian coniferous stems identified as Agathoxylon (cf. Agathoxylon). The wood growth patterns were mainly characterized by woods showing indistinct growth rings, with latewood mostly restricted to one cell in each ring, suggesting weak seasonality and short-term, periodic droughts. Rare specimens with absent growth rings were also present. Oscillations in growth ring width and the common occurrence of false rings were likely linked to plant exposure to climatic instability. Anatomical evidence pointed to active biochemical responses to intense and extensive biological attack of fungi and arthropods. The dominant presence of carbon in the elemental composition of a dark substance filling many tracheid lumens suggested a very likely affinity with polyphenol remnants in biochemical barriers. The set of wood signatures seems to be linked to regional stressing conditions rather than to the paleolatitudinal location when compared to other key South American basins during the Rhaetic. An allochthonous taphocoenosis is the most likely taphonomic interpretation for this wood assemblage. The logs were probably incorporated into the stream through the collapse of riparian areas and buried in the fluvial channel by high-energy flows after biotic and abiotic decay processes during lifetime and secondarily through exposition before entombment in a river channel.

New U–Pb geochronology for the Central Atlantic Magmatic Province, critical reevaluation of high-precision ages and their impact on the end-Triassic extinction event

The end-Triassic extinction (ETE) event represents one of the ‘big five’ episodes of mass extinction. The leading hypothesis for the cause of the ETE is the intrusion of voluminous magmas of the Central Atlantic Magmatic Province (CAMP) into carbon-rich sediments of two South American sedimentary basins, around 201.5 Ma. The timing of dikes and sills emplacement, however, must be considered in light of age models from CAMP rocks occurring in North America. In this work, we present new high-precision ages for critical samples in NE Brazil (201.579 ± 0.057 Ma) and Canada (201.464 ± 0.017 Ma), in order to evaluate how the South and North American magmatic events compare at the 100-ka level, and to the ETE timing. We also discuss inter-laboratory reproducibility of high-precision CAMP ages, including the 230Th disequilibrium corrections that are made to zircon U–Pb dates. Our findings in this newly discovered extension of the CAMP large igneous province in NE Brazil support the hypothesis that the CAMP may be responsible for the ETE through the triggering of greenhouse gas release from magma-evaporite interactions (contact metamorphism) in the South American basins.

Assessing climate change impact on flood discharge in South America and the influence of its main drivers

A warmer atmosphere is able to hold more water which consequently intensifies the hydrological cycle. The projected increase in extreme precipitation has been associated with greater floods; however, most recent studies have argued that the reduced soil moisture could be causing the opposite effect. We aim to understand how the hydrometeorological variables affect flood discharge and what the projections are for South America, a vulnerable continent that has been barely studied regarding flood trends. We used climate data from Eta simulations nested in 4 global climate models (BESM, CanESM2, HadGEM2-ES, MIROC5) as input for the MGB-SA hydrological model to yield flood discharge estimates. Then we were able to project the climate impacts on extreme precipitation, antecedent soil moisture, and flood discharge for large rivers (>1,000 km2) and understand how these variables are related. Our results showed a strong sign that antecedent soil moisture is expected to be reduced in most of the continent except in Southeastern South America (SESA). On the other hand, there are mixed signs for rarer precipitation and a clear spatial pattern for 2-year precipitations (RP2), which is expected to increase on the SESA and west Amazon and decrease on Central South America (CSA). For basins>100,000 km2, results indicate a negative change sign for 2-year precipitations, meaning that rainfall events that generate ordinary floods in large South American rivers are expected to decrease in the XXI century due to climate change. The change signs for flood discharge and extreme precipitation are spatially similar but more basins show a decrease for flooding than for rainfall. While only half of the South American basins are expected to present reduced 2-year precipitations, nearly 70 % of the rivers present a negative sign for 2-year floods, which can be attributed to the reduced antecedent soil moisture.

Hydrological data and modeling to combine and validate precipitation datasets relevant to hydrological applications

Study RegionForty-one river basins in Brazil and neighboring countries in South America. Study FocusIn large river basins, on countrywide or continental scales, it is often difficult to have consistent and accurate long time series of spatially distributed precipitation data available. However, these are needed to calibrate hydrological models and to run hydrological simulations continuously in real-time streamflow forecasting. In this study, we assess two real-time precipitation products based on rain gauges and satellite data (TRMM-MERGE and CPC-NOAA) for their use in streamflow forecasting in the hydropower sector in Brazil. To take advantage of each precipitation data source and derive a unique dataset, a methodology is proposed to combine, extend, and validate the datasets. We consider the discharges at the river basin outlets as an independent and robust reference for hydrological applications. Observed discharges are used to quantify precipitation uncertainties and to weight the blending, while discharges obtained from hydrological modeling are used to validate the final precipitation product. New Hydrological Insights for the RegionThe proposed blending method, which uses the uncertainty of the original datasets to define the weighting factors, was efficient in generating a precipitation product that performs better than each dataset separately when used to force a hydrological model. The use of the double-mass curve correlation to extend the time series of the datasets beyond their common period allowed us to produce long time series of precipitation for South American basins and hydrological applications. The study shows that it is possible to rely on river discharge data and hydrological modeling to select and combine different precipitation products in the region and presents a step-by-step methodology to do so.

Damming shapes genetic patterns and may affect the persistence of freshwater fish populations

Abstract River damming threatens freshwater biodiversity worldwide by isolating populations, reducing both gene flow and genetic variation, and affecting the ability of a given population to adapt to environmental changes. Herein, we employed a microsatellite DNA marker panel and a multitemporal approach to assess the effects of damming on the genetic diversity of a long‐lived species from both contemporary and historical perspectives. To accomplish this, we used the migratory catfish Pseudoplatystoma corruscans as a model and sampled it in two South American basins. We tested whether: (1) populations from dammed and undammed environments would display different levels of genetic diversity; (2) ecosystem size would influence the level of genetic diversity; and (3) projected whether fish populations would maintain their current genetic diversity in the long term. Our findings reveal that P. corruscans populations from dammed landscapes with smaller ecosystems showed smaller estimated effective population size, lower genetic diversity, and higher relatedness coefficient than in intermediate ecosystem size in both dammed and undammed environments. We also noticed changes in estimated gene flow and effective population size from both contemporary and historical perspectives, probably because of dams. Predictive genetic diversity analysis revealed a critical scenario for the long‐term persistence of P. corruscans in dammed environments with smaller ecosystem sizes. Our study suggests that anthropogenic barriers have substantially impacted genetic diversity and genetic structuring of the migratory fish P. corruscans. Thus, the formation of conservation policies is urgently needed to avoid local extinctions of this species in fragmented riverscapes. We highlighted the effectiveness of large ecosystems and connectivity between populations in conserving genetic diversity of migratory fishes.

Fossil diversity and taphonomy of glacial and post-glacial lower paleozoic strata, NE Paraná Basin, Brazil

Unlike other South American basins, the boundary between Ordovician and Silurian in the Paraná Basin, Brazil, has yet to be studied in detail. This article reports the occurrence and taphonomic analysis of a fauna composed of the organophosphatic brachiopods Kosoidea australis and ?Palaeoglossa sp., the mollusks ?Paleoneilo sp. and ?Cuneamya sp., besides rynchonelliform brachiopods and the ostracods Harpabollia sp. and Satiellina paranaensis. This assemblage indicates a Hirnantian age for the top of the Iapó and the base of the Vila Maria formations, northeastern border of the Paraná Basin. Taphonomic signatures and facies associations point to a pro-glacial environment, at the transitional shoreface, represented by the last sedimentary facies of the Iapó Formation. The overlying transgressive strata represent the beginning of the Vila Maria Formation, in which fossils and lithofacies indicate predominant post-glacial conditions in an offshore setting. Two taphofacies were delineated. One features size- and type-selected fossils, deposited in rain-out facies originated from the melting glacier. The second, with autochthonous to parautochthonous fossils, represents a post-glacial offshore setting. The low faunal diversity points to marine connections between North and South African basins, and with South American basins.

Population genetics of the endangered catfish Pseudoplatystoma magdaleniatum (Siluriformes: Pimelodidae) based on species-specific microsatellite loci

ABSTRACT The Neotropical catfish genus Pseudoplatystoma comprises eight species of large size, widely distributed in South American basins. The endangered species P. magdaleniatum is endemic to Magdalena basin (Colombia), experiences high fishing pressure and its population genetics is relatively unknown. To study the genetic status and structure of P. magdaleniatum, 25 species-specific polymorphic microsatellite loci were developed using next-generation sequencing and then tested in samples collected in the Magdalena-Cauca basin. Based on 15 of these loci, P. magdaleniatum showed a high number of alleles per locus (9-10), high values of observed (0.762-0.798) and expected (0.770-0.791) heterozygosities, recent reduction of population size and gene flow. These findings constitute a baseline to measure potential changes in genetic diversity and structure of this commercially important species in a basin undergoing high anthropogenic activities.

Evaluation of a panel of microsatellite markers to study their applications in Serrapinnus notomelas and to reveal the genetic diversity in Hyphessobrycon eques

Jewel tetra (Hyphessobrycon eques) is a freshwater fish found in several rivers and basins in South America. The present study is the first study to create a panel of microsatellite markers for detecting genetic diversity in H. eques and evaluating the application of these markers in Serrapinnus notomelas. In total, 44 individuals were genotyped from the natural (WIL, n = 20) and stock in captivity (CAP, n = 24) population. Moreover, 19 microsatellite markers were obtained, of which only 8 loci presented a high degree polymorphism. In total, 45 alleles were detected, ranging from 126 bp (Hype2G2) to 420 bp (Hype2E2). The Hardy–Weinberg equilibrium (p < 0.05) revealed significant difference in one locus in WIL (Hype1G4) and three loci in CAP (Hype1F4, Hype2C3, and Hype2G2). Null alleles (p < 0.05) were present in only one locus (Hype1G4). The WIL and CAP populations revealed high genetic diversity during FST analysis. The cross-amplification test for S. notomelas revealed that only two loci (Hype2C3 and Hype2G2B) presented satisfactory transferability results. The developed microsatellite primers will be useful in studying the genetic diversity and population structure of H. eques in wild populations and fish farms in the Brazilian and other South American basins.

Otolith shape as a classification tool for Chinook salmon (Oncorhynchus tshawytscha) discrimination in native and introduced systems

Chinook salmon (Oncorhynchus tshawytscha) are widely distributed across the globe, with native stocks in the North Pacific Ocean and self-sustained populations in both the Northern and Southern hemispheres. In their native range, Chinook salmon face many conservation and management challenges, including depleted stocks, loss of genetic diversity, and hatchery influences, whereas naturalized range expansion poses a threat to novel ecosystems. Therefore, ways to improve stock discrimination would be a useful tool for fishery managers. Here, we evaluated otolith shape variation in Chinook salmon as a potential tool for stock discrimination using wavelet coefficients and Fourier harmonics in three case studies at multiple spatial scales. We adopted a simple Classification Tree model that used otolith shape variation to separate Chinook salmon groups. We found best performance of the model occurring between hemispheres, followed by Oregon basins, within-watershed Elk River, Oregon, and lastly among South American basins. Otolith shape analysis is a promising tool for stock discrimination if used in conjunction with other methods to better understand plasticity of anadromous species that use pan-environmental systems.

New palynostratigraphic data of the Irati (Assistência Member) and the Corumbataí formations, Paraná Basin, Brazil, and correlation with other south American basins

This research presents the palynostratigraphy of organic-rich shales from the Irati and the Corumbataí formations, Paraná Basin (PB), Southeastern Brazil, as part of an unconventional hydrocarbon source rock and CO2 reservoir assessment study. Thirty-four samples from the Corumbataí Formation and the Assistência Member of the Irati Formation were collected in the states of Goiás (northern border of the PB), São Paulo and Paraná (eastern and southern border of the PB, respectively). The acquired data allowed to establish a comprehensive palynostratigraphic study across the basin where a total of 18 pollen genera (34 pollen species), seven spore genera, four microplankton genera (1 species), and Chlorophyceae algae species where identified. The palynostratigraphic analysis also reveals a clear dominance of bisaccate pollen grains such as Corisaccites alutas, Lueckisporites virkkiae, and Weylandites lucifer. The Lueckisporites virkkiae zone was identified in the upper part of the Irati Formation (Assistência Member) and the lowermost part of the Corumbataí Formation, indicating a Kungurian to Roadian age for this part of the succession. Differences in the Guttulapollenites hannonicus and Tornopollenites toreutos biostratigraphic ranges, recovered in the Corumbataí Formation, suggest an earlier development of these species in the Paraná Basin during the middle Permian. Therefore, to evaluate the differences in the first occurrences of key species within the Paraná Basin, a close palynostratigraphic correlation between the main Guadalupian-Lopingian South American Gondwana basins is tentatively established.

Novel Microsatellite Markers for the Invasive Golden Mussel Limnoperna fortunei

The golden mussel (Limnoperna fortunei), a mollusc originating from Asia, has caused considerable environmental and economic damage in various Brazilian river basins and reservoirs. Genetic studies that focus on characterizing its invasive potential and the feasibility of control measures are required. The present study identified and characterized 13 new microsatellite markers (of which eight were polymorphic) in L. fortunei using enriched genetic libraries. The analysis performed on 48 individuals from three populations identified 32 alleles, ranging from one to seven alleles per locus. The size of the fragments ranged from 172 to 381 bp. The polymorphic information content values ranged from 0.114 to 0.768, with observed and expected heterozygosity values ranging from 0.043–0.938 to 0.123–0.805, respectively. Six loci showed significant deviation from the Hardy–Weinberg equilibrium, possibly influenced by the heterozygous deficit. The microsatellite primers developed will be useful in studies of the genetic diversity and population structure of L. fortunei and will also contribute to the adoption of control measures for this invasive species in Brazilian and other South American basins.

The invasive potential of tilapias (Osteichthyes, Cichlidae) in the Americas

The invasion of tilapia can result in substantial impacts on native communities. Thus, understanding the spatial dynamics of invasions may help prevent future introductions and mitigate impacts. This study estimated the environmentally suitable areas for occurrence of eight tilapia species (genera Coptodon, Oreochromis, Pelmatolapia, and Sarotherodon) in the Americas and their invasive potential using Ecological Niche Models (ENMs). The United States is the most invaded country, receiving all tilapia species. In South America, the southeast and south regions of Brazil are highlighted as the areas where two species are concentrated. The ENMs predicted that all tilapia species have high invasive potential in the Americas, and despite having more tilapias in North America, South and Central Americas are more susceptible to tilapia invasion. All South American basins were predicted to harbor tilapia species that have not yet arrived on the subcontinent. Our study evidences the need to implement management measures and governmental policies in the Americas to deal with problems caused by tilapia introductions. In North America, the focus is on the control of tilapia populations and in Central and South America priority should be given to contention of introduction processes.

Two sides of a coin: Effects of climate change on the native and non-native distribution of Colossoma macropomum in South America.

Climate change and species invasions interact in nature, disrupting biological communities. Based on this knowledge, we simultaneously assessed the effects of climate change on the native distribution of the Amazonian fish Colossoma macropomum as well as on its invasiveness across river basins of South America, using ecological niche modeling. We used six niche models within the ensemble forecast context to predict the geographical distribution of C. macropomum for the present time, 2050 and 2080. Given that this species has been continuously introduced into non-native South American basins by fish farming activities, we added the locations of C. macropomum farms into the modeling process to obtain a more realistic scenario of its invasive potential. Based on modelling outputs we mapped climate refuge areas at different times. Our results showed that a plenty of climatically suitable areas for the occurrence of C. macropomum occurrence are located outside the original basins at the present time and that its invasive potential is greatly amplified by fish farms. Simulations of future geographic ranges revealed drastic range contraction in the native region, implying concerns not only with respect to the species conservation but also from a socio-economic perspective since the species is a cornerstone of artisanal and commercial fisheries in the Amazon. Although the invasive potential is projected to decrease in the face of climate change, climate refugia will concentrate in Paraná River, Southeast Atlantic and East Atlantic basins, putting intense, negative pressures on the native fish fauna these regions. Our findings show that short and long-term management actions are required for: i) the conservation of natural stocks of C. macropomum in the Amazon, and ii) protecting native fish fauna in the climate refuges of the invaded regions.

Carboniferous postglacial faunas in the late Serpukhovian–Bashkirian interval of central-western Argentina

Cisterna, G.A., Sterren, A.F., López Gamundí, O. & Vergel, M.M., March 2017. Carboniferous postglacial faunas in the late Serpukhovian–Bashkirian interval of central-western Argentina. Alcheringa, ISSN 0311-5518.Typical glacial–postglacial sequences associated with the Late Palaeozoic Ice Age (LPIA) are recognized in the Calingasta-Uspallata Basin, central-western Argentina, particularly in the Hoyada Verde and El Paso formations (late Serpukhovian–Bashkirian) at Barreal Hill (San Juan province). Brachiopods and bivalves accompanied by gastropods, conulariids, nautiloids, corals and ostracods constitute the marine assemblages of the El Paso Formation. They are assigned to the Aseptella–Tuberculatella/Rhipidomella–Micraphelia (AT/RM) fauna, characterized by two fossil assemblages: Aseptella–Tuberculatella, identified in the lower fossiliferous interval, and Rhipidomella–Micraphelia in the upper. The development of the different invertebrate assemblages within the El Paso Formation, and their relationship with coeval suite in the Hoyada Verde Formation, can be explained by a complex array of abiotic factors (substrate stability, turbidity, nutrient availability, variation in oxygen levels, poor circulation and salinity variations in the water column) that were directly related to glacial retreat dynamics and coastal configuration. A restricted palaeofjord setting is proposed for the depositional environment of the El Paso Formation in contrast to an exposed open marine coast with a gently sloping shelf for the Hoyada Verde Formation. The study of the postglacial fauna of the El Paso Formation and its relationship with the Levipustula fauna in the Calingasta-Uspallata Basin, help determine the main controls on the distribution of the postglacial faunas in other late Palaeozoic South American basins, such as the Tepuel Genoa Basin in Patagonia and the Tarija Basin in Bolivia.Gabriela A. Cisterna [gabrielacisterna@conicet.gov.ar], CONICET-UNLAR, Av. Dr. Luis M. de la Fuente s/n, La Rioja, 5300, Argentina; Andrea F. Sterren [asterren@unc.edu.ar], CICTERRA (CONICET – Universidad Nacional de Córdoba), Av. Vélez Sarsfield 1611, X5016GCA, Córdoba, Argentina; Oscar López Gamundí [orlg2003@yahoo.com], P1C Consultants 1121 Banks Street, Houston, TX 77006, USA; María del Milagro Vergel [vergelmar@tucbbs.com.ar], CONICET – INSUGEO – Facultad de Ciencias Naturales e Instituo Miguel Lillo (UNT), Miguel Lillo 205, 4000, San Miguel de Tucumán, Argentina.

Paleoictiofauna da Formação Pimenteira (Devoniano), Bacia do Parnaíba, Pi, Brasil

The Pimenteira Formation extends through the east border of the Parnaiba Basin and corresponds to the Middle and Upper Devonian (Eifelian–Frasnian). This formation possesses as characteristic fauna an invertebrate association composed mostly of gastropods, brachiopods and trilobites. However, there are also rare vertebrate remains, normally isolated and poorly preserved. The vertebrate fauna have been studied since the middle of the twentieth century and until then, had not shown a considerable taxonomic variety, only comprising Ctenacanthus and Machaeracanthus spines. In this study, besides discussing the fossil fish material already known for this formation, we describe new fossil remains, including the description of: a new Machaeracanthus spine; the first spines of climatiiform acanthodians, one related to Ptomacanthus and the other to Climatius enodicosta; an Antarctilamna-like spine; and fragments of Ctenacanthus kegeli. Lastly, we present the possible first record of Placodermi for the Pimenteira Formation and the Parnaiba Basin. These occurrences expand considerably the vertebrate faunal composition of the Pimenteira Formation and helps to understand the connections among South American basins during the Devonian, due to the similarity between the Pimenteira Formation and other South American fossiliferous formations, especially those of Bolivia.

First occurrence of Anodontites tenebricosus (Bivalvia: Mycetopodidae) in the Tocantins-Araguaia basin, Brazil.

(With 1 figure)Anodontites tenebricosus (Lea, 1834) typically inhabits lotic environments, partially buried in sandy or gravel substrate in South America waters ( Bonetto, 1967 ; Castellanos and Landoni, 1990). The species occurs in four Neotropical sub-regions, and, in Brazil, it has been mostly found in the Parana basin (Mansur and Pereira, 2006; Graf and Cummings, 2007 ; Bogan and Cummings, 2011; Troncon and Avelar, 2011).The Tocantins-Araguaia basin occupies 10.8% of Brazil, and reaches five Brazilian states and Distrito Federal. The Tocantins River rises in Goias state, Central Brazil, and its superior course is formed by two main tributaries, the rivers Maranhao and Parana. Nowadays, the water pumping for agriculture and industry uses are the mainly applications of the basin, turning critical the balance between water availability and its demand ( ANA, 2013 ).The field work was carried out at the Parana River (14° 46’ 17.89” S - 47° 20’ 18.68” W), on the surroundings of Alto Paraiso de Goias, Goias state (see Figure 1). The bivalves were collected by fishing net in September 2008, by the end of the dry season. A complete individual and three valves were identified and deposited in the Mollusk Collection of University of Brasilia (lot numbers CMUnB 417 and CMUnB 429).According to Pereira et al. (2013), A. tenebricosus occurs principally at the lower countries of South America, being recorded in the Uruguay, Plata and Parana basins, with its frequency decreasing northwards (Pereiraet al., 2013). Simone (2006) estimates its distribution along all South-American basins westerly to Andes, however, there was no record of the species in the Tocantins-Araguaia basin until now, and the Venezuelan register for the Orinoco river delta shows no connection to this hydrographic region.The Brazilian inland aquatic ecosystems are in real danger. For the range of Tocantins-Araguaia basin, the main threats to the freshwater biodiversity are poor water treatment, deforestation, construction of dams and canals, intensive cattle ranching and predatory fisheries (Tundisi, 2003). Most of these are pointed as the causes of local extirpation and extinction of many endemic species of freshwater mussels ( Bogan, 1993 ). The Brazilian Ministry of Environment (MMA) classified A. tenebricosus as “Vulnerable” (Amaral et al., 2008), but the IUCN ranks it as “Least Concern” (Bogan and Cummings, 2011). Therefore, this divergent situation urgently requires studies on freshwater mussels in the country.The new record of A. tenebricosus in the headwaters of the Tocantins River may be a sign that it can be found along the Tocantins-Araguaia basin, and this new-found data is an incentive for further fieldwork at the region. The biological and ecological features of the animal, as well as its relation with fishermen and other communities, must be investigated.

Potential distribution of the invasive freshwater dinoflagellate Ceratium furcoides (Levander) Langhans (Dinophyta) in South America.

Dinoflagellates of the genus Ceratium are predominantly found in marine environments, with a few species in inland waters. Over the last decades, the freshwater species Ceratium hirundinella and Ceratium furcoides have colonized and invaded several South American basins. The purpose of this study was to create a distribution model for the invasive dinoflagellate C.furcoides in South America in order to further investigate the basins at potential risk, as well as the environmental conditions that influence its expansion. This species is known to develop blooms due to its mobility, resistance to sedimentation, and optimized use of resources. Although nontoxic, blooms of the species cause many problems to both the natural ecosystems and water users. Potential distribution was predicted by using a maximum entropy algorithm (MaxEnt). Model was run with 101 occurrences obtained from the scientific literature, and climatic, hydrological and topographic variables. The developed model had a very good performance for the study area. The most susceptible areas identified were mainly concentrated in the basins between southeastern Brazil and northeastern Argentina. Besides already affected regions, new potentially suitable areas were identified in temperate regions of South America. The information generated here will be useful for authorities responsible for water and watershed management to monitor the spread of this species and address problems related to its establishment in new environments.

Systematics and morphology of Potamotrygon orbignyi (Castelnau, 1855) and allied forms (Chondrichthyes: Myliobatiformes: Potamotrygonidae).

The Neotropical freshwater stingray Potamotrygom orbignyi (Castelnau, 1855), and other similar "reticulated" species occurring in northern South American basins, were submitted to a thorough taxonomic analysis based on an extensive external and internal morphological study. The identity of P. orbignyi and the taxonomic status of the related nominal species Potamotrygon dumerilii (Castelnau, 1855), Potamotrygon reticulata (Günther, 1880), and Potamotrygon humerosa Garman, 1913, are defined. Taxonomic and morphological analyses revealed that P. reticulata and P. dumerilii fall within the range of variation found in P. orbignyi and were consequently treated as junior synonyms, corroborating previous works. The extensive variation in coloration observed in P. orbignyi could not be divided into consistent morphotypes; P. orbignyi is therefore a widespread species in the upper, mid and lower Amazonas basin, the Orinoco drainage, and in rivers of Suriname and the Guianas. Additionally, P. humerosa and Potamotrygon marinae Deynat, 2007 were found to present characters that support their validity, and are redescribed based on newly collected material. Potamotrygon humerosa occurs predominantly in the mid and lower Amazonas River and in lower reaches of many of its affluents, whereas P. marinae is known only from French Guiana and Suriname. Characters that proved valuable as diagnostic indicators, either in combination or as derived features, are primarily from coloration, dermal denticles and spines (morphology, development and distribution), meristic features (e.g. numbers of tooth rows, vertebrae and mesopterygial radials), morphometric proportions (e.g. snout length, tail width at base and length), and size at sexual maturity.