Sexual Dimorphism in Fin Size and Shape in Bluefin Killifish

Sexual dimorphism provides insight into how trait optima differ between males and females, despite their shared genome. Measuring sexual dimorphism can help identify which traits have been shaped by sexual selection. While fish morphology has been widely described, fewer studies have quantified sexual dimorphism across all fin types-pectoral, pelvic, dorsal, anal, and caudal. Fins are often overlooked due to their small size, tendency to fold against the body, and poor preservation post-collection. In this study, we quantified sexual dimorphism in fin size and shape across all fin types in the bluefin killifish, <i>Lucania goodei</i>. We found striking sexual dimorphism in the dorsal and anal fins, particularly in area, ray length, and base length. In contrast, the pelvic, pectoral, and caudal fins showed moderate but detectable levels of dimorphism. In both dorsal and anal fins, males exhibited elongation of the posterior region relative to females. Dorsal and anal fin traits (i.e., area, ray length, base length) were also strongly correlated within both sexes, such that individuals with larger than average dorsal fins also have larger than average anal fins. This correlation is present for both males and females, suggesting shared developmental pathways, pleiotropy, or correlated selection between dorsal and anal fins. Overall, our results indicate that the dorsal and anal fins are key targets of sexual selection in males, likely reflecting their roles in courtship, competition, and/or external fertilization.