The species of Indo-West Pacific origin, Charybdis hellerii, was recently introduced to the Brazilian fauna. The size of individuals, sex-ratio, sexual maturity, molting and spawning period of this species was characterized in order to better understand their occupation, reproduction
and development in invasive areas in the western tropical south Atlantic. The sex ratio was 1:1.2, and estimated size at sexual maturity was 35.0 mm of carapace width. Individuals in the intermolt phase were predominant, and no crabs in the process of ecdysis were captured. Ovigerous females
were present throughout most of the year, with the peak of spawning activity during the winter. The studied population, characterized by moderate densities of adult, juvenile and ovigerous female specimens, and the temporal and spatial sampling along the western Atlantic, principally along
the Brazilian coast, confirm the hypothesis that C. hellerii is well established in this area, and has arrived in recent times. The rapid expansion along the southwestern Atlantic Ocean as well as the potential relationships with other brachyuran species should be monitored in order
to anticipate possible negative impacts.
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