Species of the family Pinnotheridae constitute an ideal group for morphometric studies due to their complex morphological adaptations. These adaptations respond to the selective pressure of a symbiotic life style. This study describes the relative growth and morphometric features of the symbiotic pea crab Austinixa aidae (associated with the ghost shrimp Callichirus major), from the sandy beaches in the southwest Atlantic, Brazil. Significant differences were detected in the biometric proportions, particularly the chelar propodus length and carapace width, of each sex. These dimensions were also related to the size at which the individuals reached morphological sexual maturity (5.1 mm of carapace width for both sexes). Males and females were 2.4 times wider than long, which corresponds to the principal adaptation developed by Austinixa species to live in cryptic environments. Moreover, juveniles were proportionally more rounded. The changes in the biometric proportions of carapace length and width of A. aidae were more pronounced in males and females, adaptations that facilitate roaming within the galleries of their hosts.