When reproductive and morphometric traits are compared with literature data from allopatric estuarine and inland populations living in the Amazon and Orinoco plains, shrimps from the Pantanal show conspicuous peculiarities: (1) maximum body size reaches only about one third to one half (males and females, respectively) of the maxima observed in other populations, suggesting shorter longevity; (2) females grow consistently to larger size than males, while the opposite pattern is typical of other populations; (3) different male morphotypes, associated with mate-guarding behaviour, are common in other populations, but absent in Pantanal shrimps; all males belong to the “translucent claw” type, showing pure-search mating behaviour; (4) minimum sexable size (appearance of smallest males with appendix masculina) and (5) minimum size of ovigerous females are smaller than in other populations; this indicates a heterochronic shift (predisplacement) of sexual maturation, which may compensate for a shorter life span, suggesting r-selection. In summary, our data show biologically relevant differences in life-history traits of shrimps from the Pantanal compared to M. amazonicum in other regions. All these differential traits persist also in long-term cultures maintained under identical constant conditions (common-garden experiments).