In recent years, species richness and diversity in aquatic ecosystems has declined as environments are increasingly impacted by anthropic actions. Freshwater prawns are among the aquatic organisms most capable to survive in a heterogeneous environment. Due to factors such as the distance from an estuary or dam, Macrobrachium amazonicum populations in rivers do not show amphidromy. So, the present study investigated the influence of environmental conditions on life-history traits of the Amazon river prawn, M. amazonicum, in two distinct and unconnected aquatic systems, with different salinity, a dammed river and an estuary, in the eastern Brazilian Amazon. The biological characteristics of M. amazonicum populations in the two environments were compared and related to environmental parameters, which differed significant differences between the two environments and between seasons. Dissolved oxygen, precipitation, and temperature varied most significantly with the seasons in both the estuary and river. M. amazonicum prawns in the estuary were larger and heavier than those in the river during rainy periods. The mass-length ratios and condition factor varied significantly between the M. amazonicum populations in the estuary and river, with negative allometric growth (grows faster in length than in weight) predominating in both populations, and condition factor was better in the estuary for males and in the river for females. The relative frequencies of occurrence of the different female maturation stages and the male morphotypes were related to precipitation and turbidity in both environments, and also to salinity in the estuary. In these two distinct aquatic systems, the abiotic parameters determined by the seasonal precipitation cycle profoundly influenced the development of this crustacean, despite its ecological plasticity. Overall, the study showed that river damming triggered environmental changes in the freshwater river ecosystem and played a key role in determining the life-history characteristics of M. amazonicum in these contrasting aquatic systems.

Study area

The data were collected bimonthly from June 2017 to May 2018 in two different and unconnected aquatic systems in the eastern Amazon region of northern Brazil: estuarine stretch (01°04’17.3” S, 48°18’36.3” W) approximately 19 km long, with a mean depth of 6–7 m; river upstream of the Tucuruí Hydroelectric Power Plant (03°48’22.9” S, 49°44’01.3” W), with an area of approximately 2,917 km² and maximum depths between 58 m and 74 m. Both sites have a hot, humid tropical climate.

Data collection

At each collection, the abiotic characteristics (temperature, salinity, pH, turbidity, and dissolved-oxygen concentration) of each site were measured in situ using a Horiba U-50 multiparameter water-quality meter. Precipitation data were obtained from the database of the Brazilian National Meteorological Institute (INMET, 2017). The precipitation regime has four distinct seasonal periods: rainy-dry (June–August), dry (September–November), dry-rainy (December–February), and rainy (March–May).

Capture of specimens

Three sampling points were established at each site for the capture of M. amazonicum specimens. At each point, 10 wooden shrimp traps, known locally as “matapis”, baited with grated babaçu (Orbignya speciosa) fruit pulp were set out. The traps were set at a depth of 1–2 m for a standard period of 12 h at both sites. The collected prawns were transported to the laboratory, where they were identified based on an appropriate taxonomic reference and sexed. The total length (TL) of each specimen was measured (in centimeters), its total mass (TM) was determined (in grams), and the gonads were removed and fixed in Bouin’s solution for 24 h.

Light microscopy

The fixed gonads were processed histologically for embedding in paraffin to obtain a series of 5-μm sections, which were hematoxylin-eosin stained and then analyzed and photographed under an Eclipse Ci-S light microscope fitted with a Nikon S-Ri1 (Japan) digital camera. The ovarian stages of females were classified as immature, maturing, mature and reorganized according to the shape, color, and histology of the ovaries. Male morphotypes were classified at Translucent claw (TC), Cinnamon claw (CC), and Green claw (GC) according to their macroscopic characteristics, color, spination, and  germ cell organization in the testis.