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Context: Macrobrachium amazonicum is a freshwater prawn of socioeconomic importance in the Amazon region, whose growth, reproduction and fecundity vary according to the environment. Aims: To determine which abiotic factors influence the size and fecundity of populations of M. amazonicum in estuary and river environments. Methods: To do so, we captured a total of 255 ovigerous females, 181 from the estuary and 74 from the river, and measured temperature, total dissolved solids, turbidity, salinity, pH, dissolved oxygen and precipitation. Key results: We found that turbidity, precipitation and dissolved oxygen explained the variation in M. amazonicum size, and that the estuarine females were larger and had more and smaller eggs than riverine females. Conclusions: Precipitation, turbidity and dissolved oxygen were found to bedetermining factors for the ecological plasticity of M. amazonicum females in different environments. Implications: Freshwater prawn species display different reproductive strategies for distinct environments, and understanding their fecundity is important to establish efficient management policies. In addition, this information should be useful to improve the cost–benefit ratio of freshwater prawn farming in the Amazon region. In this study, we sought to determine which abiotic factors influence the fecundity and size of populations of M. amazonicum in estuary and river environments. Precipitation, turbidity and dissolved oxygen are determining factors for the differences found in the fecundity and size of M. amazonicum females in different environments. This information should be useful for improving the cost-efficiency of prawn culture. (Image credit: Breno Silva and Juan Caldas.) [ABSTRACT FROM AUTHOR]

Oogenesis in fish is a process that involves cell proliferation, growth and maturation. In rivers blocked by hydroelectric dams, these factors can be altered and affect the reproduction mechanism of the species. Regarding this scenario, the present study aimed to describe oogenesis in two cichlids native to the Amazon basin, Geophagus argyrostictus and G. altifrons, during pre- and post-dam periods of the Xingu River. Females of both Geophagus species were captured and biometric measurements were taken. Afterwards, the fish were euthanized, and the gonads were removed and subjected to histological processing for light microscopy, scanning electron microscopy and immunoperoxidase staining. Oocyte diameter and zona radiata thickness were also measured. The ovaries appeared in pairs, where according to their shape, size, vascularization and cell organization, five stages were described for both species. In the post-dam period, there was a reduction in the mean diameter of the oocytes, especially type IV, and a decrease in the thickness of the zona radiata, mainly, in G. argyrostictus. In both species, there was a greater presence of oocyte atresia in the post-dam period. PCNA immunoreactivity was more intense in type I and II oocytes, while vitellogenin immunoreactivity occurred in the cytoplasm and follicular cells of oocytes III and IV. These data suggest that because of changes in the level/flow of the river, the gonads of these two species adjust to the new environment, with a decrease in mean diameter and zona radiata thickness of the oocytes, which can interfere with the reproduction of the animals., Competing Interests: Declaration of Competing Interest The authors declare that there are no conflicts of interest for the publication of this study., (Copyright © 2022 Elsevier B.V. All rights reserved.)