Sexual dimorphism in the cephalothorax of freshwater crabs of genus Aegla Leach from Chile (Decapoda, Anomura, Aeglidae): an interspecific approach based on distance variables

Some morphological studies based on the comparisons between two or three species of Aegla indicate that, as an expression of sexual dimorphism, the cephalothorax is longer in males and wider at the posterior end in females. To assess whether this is a general trend, we performed a series of covariational analyses relating the length and posterior width of cephalothorax in 1,242 specimens of both sexes from 17 of the 19 species of Aegla reported from Chile. By using a covariable homologous for all individuals, it was shown that the bias toward females in the posterior cephalothoracic width is greater than the bias toward males in the cephalothoracic length. Additionally, both variables show positive phylogenetic signal, both in each trait individually and their male-female relationship. For the entire Chilean clade of Aegla, both variables showed a narrow covariation pattern influenced by the sex of the individuals. Furthermore, the phylogenetic signal both in each variable individually and the female-male relationships derived from directional phylogenetic comparative analyses suggests that this relationship is inherited by shared common ancestry, restricting their adaptive potential to most probably directions of change. However, within certain limits, this variability has important reproductive and behavioral implications that are directly associated with the fitness of the taxon.