On a remarkable sexual dimorphic trait on scales and fins of the old world Cyprinodontiformes (Actinopterygii: Aphaniidae)

Secondary sexual dimorphism is an interesting and stupendous subject of research study due to the fact that it is a result of sexual selection, mostly attributed to either female mate choice or male–male competition. This study aims to investigate the microscopic characterization of scales and fin rays in several aphaniids (Cyprinodontiformes: Aphaniidae) to find out the possibility of using these structures as secondary sexual dimorphism traits and discuss their taxonomic and evolutionary significance. Herein, the results reveal that (i) male individuals of aphaniid species of the genera Aphanius, Aphaniops and Paraphanius present contact organs in the forms of ctenus-like structure (in posterior margin of the scales) and spicule-like structure (in anal fin rays) that show sexual dimorphism of scales and fins; (ii) number, position, size and shape of contact organs provide taxonomic signal to distinguish members of the genus Aphaniops from two other genera; (iii) females of Aphaniops ginaonis also represent contact organs in scales but fewer than males and (iv) as the sexual dimorphism results from different sexual selection pressures acting on males and females, it provides evolutionary signals.