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Recent theoretical studies suggest that food webs are size-structured with top predators coupling different energy sources, but evidence for this hypothesis is still scarce, especially in highly diverse tropical rivers. Here, we explored the association between body size, trophic position, and the use of allochthonous and autochthonous basal production sources in the Volta Grande rapids of the Xingu River, a major clearwater tributary of the Amazon River, during the period prior to operation of the Belo Monte Hydropower Plant (BMHP). This section of the river contains a maze of channels with rocky shoals that support dozens of endemic fishes, mollusks and other aquatic taxa that may be impacted by flow regulation by the Pimental Dam located upstream. During the low-water season, we surveyed fish, crustaceans, mollusks, sponges, aquatic and terrestrial insects, zooplankton, and basal production sources to obtain tissue samples for analysis of stable isotope ratios of carbon (δ13C) and nitrogen (δ15N). Biomass of most aquatic organisms appears to be largely supported by riparian vegetation, highlighting the importance of the lateral connectivity between aquatic and terrestrial habitats. Unlike expected, we did not observe a gradual increase in coupling of energy pathways with increasing body size and trophic position. Findings provide a baseline for trophic ecology of this river under the natural flow regime for future impact assessments, and also indicated that more complex food web models, potentially including additional functional traits (e.g., gut length), are needed to describe resource and habitat use in highly diverse tropical ecosystems.

Niche partitioning occurs among coexisting populations to reduce the effects of competitive exclusion among species of similar niche. The aim of the present study is to verify the trophic niche partitioning and feeding behavior between two mustelids, the Giant otter and the Neotropical otter, through the dry and rainy season hydrologic of the Lower Xingu River. Our results suggest that the diets of both mustelids are composed primarily of fish of the family Anostomidae (Headstanders). Despite extensive niche overlap, our results indicate partitioning is facilitated by differences in niche breadth, with potential implications for conservation of both species in the case of declines in prey abundance and diversity. Both species inhabit an area recently impacted by completion of the Belo Monte Hydropower Plant, resulting in large changes to the hydrologic regime. Thus, our results provide important information for conservation efforts regarding the feeding behavior and co-occurrence of both species, as well as providing a baseline for monitoring future health of these mustelid populations. The present study is the first to test the hypothesis of niche partitioning between these two mustelids outside a protected area in the Amazon.

This study provides new length‒weight (LWR) and length‒length relationships (LLR) for 25 indigenous fish species from Xingu River basin. Overall specimens were collected in different aquatic habitats in the Xingu River basin in quarterly collections between 2012 and 2018. The collections were carried out using distinct fishing gears, such as gill nets (20 to 180 mm mesh), seines (3 mm mesh), sieves (1 mm mesh), and casting nets (20 to 60 mm mesh). The present study shows the LWRs for 25 species, and the LLRs for 23 species. In addition, presents the new maximum total lengths for 20 species.

A new species of Geophagus sensu stricto is described from the Tapajos River basin, Brazil, elevating the number of species of the genus to 21. The new species is of commercial importance and is known in the aquarist trade as Geophagus 'red head'. The new species is diagnosed using an integrative approach, based on mitochondrial DNA analysis along with morphological evidence. The new species is distinguished from all congeners by the absence of markings on the head, the bar pattern composed by nine vertical bars on the flanks and the presence of distinct longitudinal bands in the caudal fin. Additionally, it shows a genetic distance of at least 2.0% in cytochrome b gene sequences from its closest congeners. Molecular analysis including most genera of Cichlidae from South America corroborates that the new species belongs to the group of Geophagus sensu stricto.

Pollution in aquatic ecosystems is rapidly becoming one of the world's greatest ecological challenges. Given their intermediate position between terrestrial and marine environments, estuarine systems are especially vulnerable to human pollution. Amazonian estuaries have unique characteristics, such as heterogeneous landscape intercalating tracts of vegetation with sandbanks and beaches. In the present study, we provide the first qualitative and quantitative data on litter retention in an Amazonian estuary, comparing vegetated and bare substrate areas. Overall, 12,003 items were recovered, with a mean ± SD density and weight of 1.69 ± 2.16 items/m2 and 78.08 ± 93.11 g/m2, respectively. Plastic was the principal material (80.97%) found. The highest number of items was found in the vegetated habitats (73.11%), indicating these areas as the most affected by plastic pollution. Our findings provide important insights for future research planning and implementation of effective public policies for conservation and management of these important ecosystems.

In the Amazon Basin, several species of herbivorous serrasalmid fishes inhabit rapids, but it is unknown if they partition food resources during the annual low-water period when fish densities are high within greatly reduced volume of aquatic habitat. We investigated the trophic ecology of juveniles and adults of three species, Myleus setiger, Ossubtus xinguense, and Tometes kranponhah, common in rapids of the Xingu River during the low-water period. Diets, stable isotope ratios of muscle tissue, and functional traits were analyzed for 59 specimens of M. setiger, 175 of O. xinguense and 215 of T. kranponhah. The three species overlapped in dietary and isotopic space, with adult O. xinguense being most divergent. Juvenile and adult T. kranponhah and juvenile O. xinguense, two groups with broad diets, had lowest trophic positions estimated from isotopic data. Adult O. xinguense had the highest trophic position despite having large amounts of Podostemaceae in the diet. High trophic overlap during the low-water period suggests that either food resources are not limiting, or niches are partitioned by other means. Differences in functional traits of the three serrasalmids could be associated with differential efficiencies of swimming and feeding within microhabitats that vary according to water velocity and/or structural complexity.

Myleus pachyodus, a new serrasalmid species, is described from the Rio Teles Pires and Rio Jamanxim, both tributaries of the Rio Tapajos in Mato Grosso and Para states, Brazil. The new species differs from all congeners by having a remarkable ontogenetic thickening of the teeth shifting from slender incisiform in juveniles to thick bulky teeth at standard length ≥ 100 mm, whereas congeners present slender, antero-posteriorly flat teeth throughout all life stages. The new species further differs from congeners by having 18-20 branched dorsal-fin rays, by the lack of a diastema between contralateral outer series of premaxillary teeth and by the presence of 16-25 prepelvic spines, with the anteriormost spine never reaching the vertical through the pectoral-fin origin.

Pacus of the genus Myloplus represent a formidable taxonomic challenge, and particularly so for the case of M. asterias and M. rubripinnis, two widespread and common species that harbor considerable morphological diversity. Here we apply DNA barcoding and multiple species discovery methods to find candidate species in this complex group. We report on one well-supported lineage that is also morphologically and ecologically distinct. This lineage represents a new species that can be distinguished from congeners by the presence of dark chromatophores on lateral-line scales, which gives the appearance of a black lateral line. It can be further diagnosed by having 25-29 branched dorsal-fin rays (vs. 18-24), 89-114 perforated scales from the supracleithrum to the end of hypural plate (vs. 56-89), and 98-120 total lateral line scales (vs. 59-97). The new species is widely distributed in the Amazon basin, but seems to have a preference for black- and clearwater habitats. This ecological preference and black lateral line color pattern bears a striking similarity to the recently described silver dollar Metynnis melanogrammus. RESUMO Pacus do gênero Myloplus representam um desafio taxonômico formidável, e particularmente o caso de M. asterias e M. rubripinnis, duas espécies amplamente distribuídas e comuns que abrigam uma considerável diversidade morfológica. Aplicamos aqui a tecnologia do DNA barcoding e múltiplos métodos de descoberta de espécies para encontrar possíveis espécies novas nesse grupo complexo. Registramos uma linhagem bem suportada que também é distinta morfológica e ecologicamente. Essa linhagem representa uma nova espécie que pode ser distinguida das demais congêneres por apresentar cromatóforos escuros nas escamas da linha lateral que conferem uma aparência de linha lateral preta. Ela pode ser adicionalmente diagnosticada por ter 25-29 raios ramificados na nadadeira dorsal (vs. 18-24), 89-114 escamas perfuradas do supracleitro até o final da placa hipural (vs. 56-89) e 98-120 escamas totais na linha lateral (vs. 59-97). A nova espécie é amplamente distribuída na bacia Amazônica, mas aparentemente possui preferência por habitats de água preta e clara. A preferência ecológica e o padrão de colorido escuro da linha lateral consistem em semelhanças impressionantes com o silver dólar recém descrito Metynnis melanogrammus.

A new species in the serrasalmid genus Myloplus is described from the middle Mazaruni River, Essequibo River basin, Guyana. The new species is similar to Myleus pacu and Myloplus planquettei in its silver to purplish black coloration and its overall morphology, and has a putative syntopic distribution with those species. The new species is, however, readily distinguishable from the other two by meristic counts of the unpaired fins and by differences in color pattern, primarily in males. Additionally, we provide comments on Myleus pacu and other species that have been confounded under that name. RESUMO Uma nova espécie de serrasalmídeo do gênero Myloplus é descrita do médio rio Mazaruni, bacia do rio Essequibo, Guyana. A nova espécie é similar a Myleus pacu e Myloplus planquettei pela sua coloração prateada a preto arroxeado e pela morfologia geral do corpo, e por ter suposta distribuição sintópica com essas espécies. No entanto, a nova espécie é prontamente distinguida dessas duas últimas espécies por contagens das nadadeiras não pares, e por diferenças no padrão de coloração, principalmente nos machos. Adicionalmente, fornecemos comentários sobre Myleus pacu e outras espécies que têm sido confundidas sob esse nome.

Summary This study provides length–weight relationships (LWRs) for 33 fish species sampled with several types of fishing gear in six expeditions between 2010 and 2011, during the dry and wet seasons in the middle and lower Cauca River Basin, Northern Colombia. The report represents the first reference on LWRs for 28 species and also provides the new maximum size for 15 species.

Two new species of Tometes from the Brazilian Shield rapids are described. Tometes kranponhah is endemic to the Xingu River basin, whereas Tometes ancylorhynchus occurs both in the Xingu and the Tocantins-Araguaia River basins. The two species are sympatric in the Xingu drainages and have many similarities in morphology and colouration. Major diagnostic differences are the dark pigmentation on the opercle of T. kranponhah and its distinct snout shape and arrangement of premaxillary teeth. In addition, T. kranponhah is a large fish that is abundant in the Xingu River, whereas T. ancylorhynchus is a medium-sized fish for which there are few records.

Summary Length–weight relationships (LWR) are presented for 27 freshwater fish species representing 11 families captured in the Trombetas River, a clearwater left margin tributary of the Amazon River. The allometry coefficient (b) of the LWR (W = aTLb) ranged from 2.51 to 3.49, with a median value of 3.00. This study represents the first reference on LWR for 23 species and new records of maximum lengths for 15 species.

A new species of Serrasalmidae, Tometes camunani, is described from the upper drainages of the rio Trombetas basin, Pará State, Brazil. The new species is distinguished from its congeners by having neurocranium with a slight concavity at the level of the frontal bone (vs. concavity absent, dorsal profile of neurocranium straight). It can be further distinguished from its congeners by having teeth with central cusp taller and acute (vs. central cusp shorter and with rounded edge in T. trilobatus), a terminal mouth (vs. upturned mouth in T. lebaili), and 12-26 prepelvic spines (vs. 0-9 in T. makue). The new species is strictly rheophilic like other species of Tometes, and occurs exclusively in the rapids of shield rivers, complex and fragile biotopes that are threatened by anthropogenic activities. An identification key to the species of the Myleus group is provided Uma espécie nova de Serrasalmidae, Tometes camunani, é descrita para as drenagens superiores da bacia do rio Trombetas, estado do Pará, Brasil. A espécie nova distingue-se dos congêneres pela presença de uma ligeira concavidade no neurocrânio na altura do frontal (vs. concavidade ausente, perfil dorsal do neurocrânio reto). Também pode ser adicionalmente distinguido dos seus congêneres por possuir dentes com a cúspide central mais alta e cume agudo (vs. cúspide central mais baixa e com cume arredondado em T. trilobatus), a boca terminal (vs. boca orientada para cima em T. lebaili), e 12-26 espinhos pré-pélvicos (vs. 0-9 em T. makue). A espécie nova é estritamente reofílica, como as demais espécies de Tometes, e ocorre exclusivamente nas zonas encachoeiradas dos rios de escudo, biótopos complexos, frágeis e ameaçados por ações antropogênicas. Uma chave de identificação para as espécies do grupo Myleus é apresentada