The size at the onset of sexual maturity (SOM) was determined in four hololimnetic populations of Macrobrachium amazonicum using morphometric, physiological and functional criteria. Male prawns from two populations analyzed had hypertrophied chelipeds and large body proportions, showing the presence of four morphotypes, i.e. large-size phenotype (LS). However, the other two populations showed smaller males without morphotypes, i.e. small-size phenotype (SS). The development of sexual weapons in males modulated the mating system and SOM in this species. It was verified that there is a synchrony between physiological and functional maturities in SS males. On the other hand, functional maturity in LS males occurred after physiological and morphological maturities. In females, we verified synchronization between morphological and functional maturities. The results obtained indicated that, in both sexes, physiological maturity anticipates the others criteria. There were no differences between the sequential scheme of sexual maturity for female phenotypes, being: physiological → functional morphological. However, the evaluation of sexual maturity in males must take into account the phenotype of population, since that was modulated by functional maturity. The sequential scheme for LS males is: physiological → morphological → functional, while for SS males it is: physiological = functional → morphological.

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 10.06 mm) carried few eggs (X: 141 and 219) with a larger volume (X: 0.188 and 0.212 mm3), whereas, in the other two populations, large-size (LS) females (CL: 15.89 and 14.51 mm) carried many eggs (X: 814 and 846) with a smaller volume (X: 0.130 and 0.143 mm3). 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.