RESUMEN
Knowing the reproductive biology and population dynamics of invasive species are essential for environmental conservation and protection of native species. The success of these invasive species is directly linked to their reproductive strategy. Therefore, this study aimed to describe the reproductive cycles and evaluate population parameters of the invasive bivalves Mytilopsis sallei and Isognomon bicolor, and to estimate if those characteristics would favor their population growths in the northeast coast of Brazil. The bivalves were sampled monthly from June 2016 to May 2017, respectively from the Sanhauá River estuary and Jacarapé beach, State of Paraíba, Northeast Brazil. Through histological analyses, reproductive parameters were determined in order to identify sex, gonadal development, minimum size at maturity, and mean gonadal index. The asymptotic growth (L∞) and growth rate (K) parameters were estimated using the von Bertalanffy growth curve, and recruitment patterns and cohorts were projected based on shell length frequency distributions. Mytilopsis sallei presented more than 50% spawning individuals in most months, while animals showing gametogenic gonads were predominant during the season of greatest precipitation. Isognomon bicolor had ripe gonads (about 30%) and spawning individuals (more than 40%) in all months of the year, but unlike M. sallei, it had the highest concentration of ripe individuals in the months of greatest precipitation. Both species showed equal and high growth rates (K = 1.1 yr-¹) and analysis of the cohorts indicated that these populations are established and expanding. The results confirmed the great invasive potential of the two species in their local environments (estuary and marine) in Northeast Brazil and, therefore, their harmful potential for the conservation of native species and the environment in the invaded areas.