Light and scanning electron imaging confirm sexual dimorphism in scales and anal-fin rays of the genera Anatolichthys and Paraphanius (Teleostei: Cyprinodontiformes: Aphaniidae)

The investigation of secondary sexual dimorphism is a significant and exciting research topic since it has to do with sexual selection, which is connected to selecting a mate and male competitiveness. The purpose of this study was to investigate the microscopic characteristics of scales in several body parts and anal-fin rays of nine species of the old world cyprinodontiform fishes belonging to the genera Anatolichthys and Paraphanius to find out possible secondary sexual dimorphic traits and their importance in aphaniid taxonomy. Both light and scanning microscopy revealed that scales show general characteristics of aphaniid species and present large cycloid scales of several subtype forms including cycloid-true circular, cycloid-chordate, oval-reversed ovoid, polygonal-pentagonal, quadrilateral-square, oval-ovoid and oval-oblong in different body parts of fishes. Both genera had sexual dimorphism: (i) male individuals of all nine investigated species presented contact organs in the forms of ctenus-like structure in the posterior margin of the scales and spicule-like structure in the anal-fin rays; (ii) caudal-fin rays of both sexes lacked contact organs; (iii) the number, position, size and form of contact organs provided a taxonomic signal to identify the two genera; (iv) contact organs can be thought of as functional structures that keep male and female individuals in physical contact during courtship and display behaviour throughout the active phase of reproduction; (v) it can provide evolutionary signals because sexual dimorphism would come from various sexual selection pressures acting on both sexes.