Onset of morphological sexual maturity, sexual dimorphism and relative growth of the freshwater crab Fredius ibiapaba (Decapoda: Pseudothelphusidae) from a humid enclave area of Northeastern Brazil

We investigate the onset of morphological sexual maturity, sexual dimorphism and relative growth of the freshwater crab Fredius ibiapaba. We collected the specimens bimonthly between August 2017 and June 2018. We measured the crabs to obtain the following morphometric variables: carapace width (CW); pleon width (PLW); propodus length (PL); propodus height (PH) and propodus width (PW). A total of 175 specimens of F. ibiapaba were analysed. The onset of morphological sexual maturity was estimated at 31.7 mm for males, and at 35.2 mm for females. We observed sexual size dimorphism in CL and PLW, with females larger than males. Positive allometry of cheliped variables was observed in juvenile and adult males, while females showed positive allometry of PLW in juvenile and adult. Finally, the growth of the variables was different between adults and juveniles in both sexes. The ecological information obtained in our study is unprecedented for the genus Fredius. The sexual size dimorphism with females larger than males, probably resulting from their reproductive biology. Additionally, the energy expenditure in cheliped growth in males, may be associated with success during agonistic disputes and the use of these structures to position females during copulation, since they are larger in body size.