Macrobrachium amazonicum (Heller, 1862) is the most important species for aquaculture native to South America. This study evaluates the phenotypic plasticity of females of Macrobrachium amazonicum with respect to the gonadal development, and determines the spawning type. Our study consisted of macro and microscopic analysis of the reproductive tract. Ovaries exhibited the following reproductive cells in developmental sequence: oogonia (OO) - mean diameter and standard deviation of 25.4 ± 6.5 μm; previtellogenic oocyte (PVO) - 61.7 ± 10.7 μm, vitellogenic oocyte (VO) - 113.9 ± 24.5 μm; and mature oocyte (MO) - 308.7 ± 56.3 μm. Ovaries increased in volume due to vitellogenesis and changing of basophilic to acidophilic composition. Follicular cells, atresic oocytes and postovulatory follicles were also analyzed. By combining macro and microscopic analysis, the ovaries of M. amazonicum were classified into six stages: Immature, In maturation, Mature, Spawned in maturation, Spawned and Resting. The ovarian development of M. amazonicum followed a standard pattern even among different populations. Considering our results and the evidence in literature, we conclude that spawning for this species is parceled or multiple, that is, synchronous in more than two groups (clutches of oocytes). Such observations provide basis for further studies addressing sustainable management strategies for species conservation and contribute to elucidate the biology of their specimens.

We analyzed aspects related to the relative growth and reproduction of females of Macrobrachium amazonicum (Heller, 1862) in a population of the semiarid Northeast Region of Brazil. Carapace (CL) and total length (TL) ranged 5.57–17.61 mm (mean 10.96 ± 2.07 mm) and 28.96–80.63 mm (mean 52.14 ± 9.13 mm), respectively. Relative growth analysis showed that total length increased in a higher proportion than carapace region (positive allometry). This region also increased in a higher proportion regarding weight (negative allometry) due to energy cost in reproduction. Of the three criteria used to determine size at the onset of maturity (SOM), the smallest size observed was obtained by physiological SOM I (based on advanced vitellogenesis): 8.22 mm CL, followed by functional SOM II (8.83 mm CL, based on ovigerous females) and physiological SOM III (9.48 mm, based on CL50). SOM was influenced by the population’s phenotype, and SOM I and SOM II showed synchronicity. The estimated maximum fecundity was 2,805 eggs per female, with a mean value of 690 eggs. There was continuous reproduction and the dry season favored the percentage occurrence of ovigerous females (September-December 2016 and July-August 2017). The smaller lengths of the individuals, compared to other female populations, were due to the characteristics of the environment and the intrinsic traits of the studied population. The negative allometry for the length-weight relation and the continuous reproductive period with seasonal variation was similar as that recorded for other Brazilian localities. This suggests that these last characteristics are a pattern for this species, which shows a latitudinal variation only in the sizes reached. The fast maturation and continuous reproduction of M. amazonicum females make them a potential species for commercial exploitation.