Here, we investigated the reproductive biology (frequency and distribution patterns of ovigerous females, fecundity, reproductive output and egg size and volume) of Macrobrachium amazonicum in four populations living in Neotropical reservoirs, so as to verify whether this process was affected by the 2014–2016 ‘El Niño’ event. The following two phenotypes were identified: in two populations, small-size (SS) females (CL: 9.05 and 10.06mm) carried few eggs (X: 141 and 219) with a larger volume (X: 0.188 and 0.212mm3), whereas, in the other two populations, large-size (LS) females (CL: 15.89 and 14.51mm) carried many eggs (X: 814 and 846) with a smaller volume (X: 0.130 and 0.143mm3). All populations showed continuous reproduction and similar reproductive output. ‘El Niño’ did not directly affect both phenotypes, which is a reflection of the reproductive plasticity observed in this species. This phenomenon acted as a stressing factor, modifying the reproductive patterns of the species in a space–temporal scale. Ovigerous females showed specificity in habitat use, with SS females being associated with macrophytes in deeper environments (refuges), and LS females being associated with shallow sandy areas (shelter and nursery). The high variability of reproductive aspects is better explained by intrinsic factors and could be implying an incipient speciation.

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The bioecology of freshwater prawns can be understood by studying their reproductive biology. Thus, the aim of this paper was to determine and compare the reproductive potential of four freshwater caridean prawns collected in the Amazon region. For two years, we captured females of Macrobrachium brasiliense, Palaemon carteri, Pseudopalaemon chryseus and Euryrhynchus amazoniensis from inland streams in the municipality of Itacoatiara (AM). At the laboratory, we measured the biometric variables total length, carapace length, abdomen length and total weight, and recorded the number of eggs (NE) by direct counting. There was a significant difference for all variables between the species. The females of M. brasiliense were the most abundant and had the largest sizes and NE. However, the fecundity index shows that P. carteri has the highest reproductive potential of the four species. Despite the differences in size and weight, all the species showed low fecundity, which is a characteristic of continental palemonidean K strategist prawns.