Instituto Nacional de Ciência e Tecnologia de Adaptações da Biota Aquática da Amazônia

Anaesthetics are important in fish culture to reduce handling stress and mortality. Eugenol is a promising anaesthetic because of its low cost, efficacy, safety margin for fish and lack of toxicity to humans. The goal of this study was to establish a protocol using eugenol as a fish anaesthetic for tambaqui Colossoma macropomum (Cuvier), and provide information for regulating authorities on establishing safety dosage protocols for its use. Juvenile and sub-adult tambaqui were first individually exposed to doses of 35, 50, 65, 85, 100 or 135 mg L−1 eugenol for 10 min. A second experiment examined the effect of the duration of exposure to eugenol on the time required for recovery and survival of tambaqui. A eugenol dose of 65 mg L−1 was adequate to induce fish of both sizes into a surgical anaesthetic state, and recovery time was similar for dosages up to 100 mg L−1. Exposure to the ideal dose (65 mg L−1) for up to 30 min did not cause fish mortality. Fish blood glucose values were similar for all the tested eugenol doses as well as with the benzocaine control. The results show that eugenol is an efficient and safe anaesthetic for tambaqui.

Tambaqui Colossoma macropomum (Cuvier) is a fish of primary importance in Amazon aquaculture. It has been described as an acid-resistant species that moves seasonally between white (muddy) water and black water rivers and enters the extremely dilute acidic areas of flooded jungle to feed during the rainy season. To analyse the pH tolerance of this species, tambaqui were exposed to three water pH levels for 40 days (pH 4.0, 6.0 and 8.0). The water was acidified slowly over 3 h, allowing the fish to acclimate. A similar protocol was used to adjust water pH to 8.0. No mortality was observed during the exposure period. Several haematological parameters were significantly changed in alkaline-exposed animals, with significant decreases in haematocrit (20%), haemoglobin concentration (8%) and red blood cells (12%). Tambaqui showed severe blood variations when exposed to alkaline pH. Fish final weight, condition factor and specific growth rate (SGR) was inversely proportional to a pH increase, and SGR were higher for fish reared in acidic water. The relative insensitivity of tambaqui to low pH confirms its acid tolerance and is in accordance with its natural occurrence in black water habitats.

Abstract The Balbina hydropower dam in the Central Amazon basin, established in the Uatumã River in the 1980s, is emblematic for its socio‐environmental disaster. Its environmental impacts go far beyond the reservoir and dam, however, affecting the floodplain forests (igapó) in the downstream area (dam shadow), which have been assessed using a transdisciplinary research approach, synthesized in this review. Floodplain tree species are adapted to a regular and predictable flood pulse, with high‐ and low‐water periods occurring during the year. This was severely affected by the operation of the Balbina dam, which caused the suppression of both the aquatic phase at higher floodplain elevations and the terrestrial phase at lower floodplain elevations (termed the ‘sandwich effect’). During the period of construction and reservoir fill, large‐scale mortality already occurred in the floodplains of the dam shadow as a result of reduced stream flow, in synergy with severe drought conditions induced by El Niño events, causing hydraulic failure and making floodplains vulnerable to wildfires. During the operational period of the dam, permanent flooding conditions at low topographical elevations resulted in massive tree mortality. So far, 12% of the igapó forests have died along a downstream river stretch of more than 125 km. As a result of flood suppression at the highest elevations, an encroachment of secondary tree species from upland (terra firme) forests occurred. More than 35 years after the implementation of the Balbina dam, the downstream impacts caused massive losses of macrohabitats, ecosystem services, and diversity of flood‐adapted tree species, probably cascading down to the entire food web, which must be considered in conservation management. These findings are discussed critically, emphasizing the urgent need for the Brazilian environmental regulatory agencies to incorporate downstream impacts in the environmental assessments of several dam projects planned for the Amazon region.

Variations in dissolved oxygen levels are common in the Amazonian aquatic environments and the aquatic organisms that inhabit these environments developed a variety of adaptive responses to deal with such conditions. Some Amazonian fish species are tolerant to low oxygen levels and the cichlid Astronotus ocellatus is one of the most hypoxia-tolerant species. Herein, we aimed to unveil the biochemical and molecular responses that A. ocellatus presents when submitted to hypoxia. Hypoxia indicators were measured, such as plasma glucose, plasma lactate, hepatic glycogen and relative transcript levels of prolyl hydroxylase 2 (phd2) and hypoxia-inducible factor-1α (hif-1α) in juveniles of approximately 50 g exposed to 1, 3, and 5 hours of hypoxia (0.7 mg O2.L-1), followed by 3 hours of recovery in normoxia (6 mg O2.L-1). Fish exposed to hypoxia reduced liver glycogen levels within 3 hours of hypoxia, when comparing with 1 hour, and increased plasma glucose and lactate. Under the same condition, phd2 transcripts levels increased in gills, but decreased in liver. In contrast, hypoxia did not affect relative gene expression of hif-1α in both tissues. Based on the transcription pattern of phd2, these results showed that liver and gills of A. ocellatus have different molecular strategies to cope with environmental hypoxia.

ABSTRACT Ephemeral aquatic habitats, such as temporary pools and roadside ditches, are characterized by cyclical flooding and drying that create highly dynamic conditions for fish community assembly. These environments can exhibit high spatial and temporal variability in community structure driven by local topography, distance to source habitats and interannual variation in rainfall, which affects connectivity dynamics. As such, they provide unique opportunities to test hypotheses about the processes shaping metacommunities, particularly through the partitioning of beta diversity into turnover and nestedness components. Here, we investigate these processes in ephemeral aquatic habitats of the Atlantic Forest, where we sampled 36 temporary pools and 36 roadside ditches throughout 2024, recording 1,545 individuals from 20 species. Fish communities’ composition did not differ significantly between habitats or hydrological periods, but beta diversity was consistently high and predominantly influenced by turnover. This suggests spatial processes such as dispersal limitation and colonization history are key drivers of community structure in those temporary aquatic habitats. We tested whether beta diversity components were associated with local environmental (temperature, dissolved oxygen, pH, volume) and spatial (distance to the nearest stream) predictors. During the dry period, nestedness was positively correlated with differences in volume for temporary pools, indicating that greater habitat availability promotes species accumulation under low connectivity, where dispersal occurs possibly only by overland displacement. Additionally, turnover was associated with differences in distance to the nearest stream in roadside ditches during the dry period, likely reflecting differential colonization by stream-associated species. During the wet period, differences in pH were correlated with turnover in roadside ditches, consistent with Mass Effect dynamics under higher connectivity conditions. Meanwhile, in temporary pools during the wet period, turnover increased with differences in distance to the nearest stream, suggesting colonization by highly tolerant and capable of dispersal species in more isolated sites. These contrasting relationships may also reflect the spatial organization of habitats, with linear ditches facilitating directional dispersal along roads, whereas spatially dispersed pools depend on more stochastic colonization. Thus, pools act as seasonal refugia for tolerant or amphibious species during low connectivity, and for stream-dwelling species during high connectivity, whereas ditches function as more persistent nodes that facilitate dispersal during wetter periods. This seasonal complementarity helps explain the observed temporal and habitat-related variation in beta diversity, highlighting that effective conservation must target the entire floodable mosaic, rather than isolated sites, to maintain this dynamic metacommunity.

Introduction Temporary freshwater habitats are characterized by cyclical flooding and drying that create highly dynamic conditions for fish community assembly. These environments can exhibit high spatial and temporal variability in community structure driven by local topography, distance to source habitats and interannual variation in rainfall, which affects connectivity dynamics. As such, they provide unique opportunities to test hypotheses about the processes shaping metacommunities. Here, we investigate these processes in temporary freshwater habitats of the Atlantic Forest. Methods We sampled 36 temporary pools and 36 roadside ditches throughout 2024, recording 1,545 individuals from 20 species. We tested if community composition differs between habitat type and rainfall periods and whether beta diversity components were associated with local environmental and spatial predictors. Results Fish community composition did not differ significantly between habitats or hydrological periods, but temporal variation in spatial beta diversity was consistently high and influenced by turnover. During the dry period, nestedness was positively correlated with differences in volume for temporary pools and roadside ditches. Additionally, during the wet period, turnover in ditches was correlated with differences in pH. In temporary pools during the wet period, turnover increased with differences in distance to the nearest stream. Discussion High turnover for both habitats and periods suggest spatial processes such as dispersal limitation and colonization history are key drivers of community structure in those temporary aquatic habitats. Nestedness correlations with differences in volume in the dry period indicate that greater habitat availability promotes species accumulation under low connectivity, where dispersal occurs possibly only by overland displacement. Meanwhile, turnover correlation with differences in pH for ditches in the wet period is consistent with mass effect dynamics under higher connectivity conditions, with pH acting as a proxy for stream water influx. Turnover correlation with differences in distance to the nearest stream for pools during the wet period suggest colonization by highly tolerant and capable of dispersal species in more isolated sites. Overall, our findings emphasize that beta diversity in temporary aquatic habitats is primarily structured by habitat availability and spatial processes interacting with hydrological dynamics, underscoring that effective conservation must target the entire floodable mosaic rather than isolated sites to maintain this dynamic metacommunity.

) is a widespread Amazonian predator that harbors multiple genetic groups found in sympatry across different water types: black (acidic, tannin-rich, nutrient-poor), clear (neutral, nutrient-poor), and white (neutral, nutrient-rich, turbid). This makes it an excellent model to explore these interactions. Therefore, to assess the relative contributions of host genotype and environmental factors to gill microbiota composition, we analyzed 252 individuals sampled from 14 sites using Genotyping-by-sequencing to determine host genetic structure, and 16S rRNA gene metabarcoding to characterize the active bacterial communities in gill mucus and surrounding bacterioplankton. Our findings show that host genotype significantly affects gill microbiota composition, with distinct beta diversity and exclusive microbial taxa associated with different genetic groups, even when exposed to the same water type. Nonetheless, variation in microbiota composition was more strongly associated with site-specific environmental factors, particularly bacterioplankton communities. These results reveal a complex interplay between genetic and environmental drivers of fish-associated microbiota under natural conditions. They suggest a potentially important ecological relationship between bacterioplankton and host-associated microbiota and suggest that gill microbial communities may serve as sensitive biomarkers for detecting environmental disturbances in tropical freshwater ecosystems. IMPORTANCE: Fish gills are protected by a layer of mucus that hosts a community of beneficial bacteria essential for many vital functions. Understanding the factors that shape this gill microbiota is crucial not only for fish health but also for developing reliable microbial biomarkers to support habitat monitoring and conservation. In this study, we examined black piranhas from different genetic groups sampled across sites with contrasting water physicochemical parameters. This model provided a unique opportunity to disentangle the effects of host genetics and environmental conditions on gill microbiota composition under field conditions. Our findings show that although individuals from different genetic groups hosted distinct microbial communities, local environmental factors, especially the bacteria in the surrounding water, had a stronger influence. These results highlight the complex interplay between host and environment in shaping microbial communities and offer new insights into how fish and their microbiota respond to ecological variability in tropical freshwater ecosystems.

Abstract The aquatic biota of the Amazon is subject to considerable daily and seasonal fluctuations in its environment. The ability to adapt to changes in the environment, such as increased water temperatures, is essential for the survival and physiological performance of these organisms. The anticipated rise in temperature and CO 2 levels, projected to reach 6 °C and 50% above current levels by the end of the century, respectively, represents a significant and imminent threat to aquatic biodiversity. The objective of this study was to assess the effects of acute short-term exposure to elevated temperature on the brain mitochondrial respiration of two Amazonian fish species with contrasting trophic roles: Schizodon fasciatus , which represents a prey species at a lower trophic level, and Pygocentrus nattereri , a predatory species. Specimens of Schizodon fasciatus and Pygocentrus nattereri were collected and subjected to temperatures of 28 °C and 37 °C for a period of four hours. Brain mitochondrial respiration was quantified to analyze complexes I, I + II, III, IV, proton leak (H + Leak), RCR, and ETS. Additionally, the production of reactive oxygen species (ROS) was evaluated. The results revealed that exposure to higher temperatures caused elevated complex I activity in both species, with a more pronounced increase observed in S. fasciatus . Furthermore, significant increases in complex I + II and complex III were observed in S. fasciatus , while complex IV showed a decrease in both species. H + Leak exhibited a decrease in P. nattereri , whereas RCR demonstrated an increase in both species. ROS production displayed a decrease in P. nattereri but remained stable in S. fasciatus . The findings suggest that both fish species possess the capacity to adapt their brain mitochondrial function to higher temperatures, albeit with species-specific responses. These adaptations assist in maintaining brain energy production and minimizing oxidative stress, and emphasize potential imbalances in predator–prey relationships under changing environmental conditions.