Sex dimorphism and evidence of sexually selected traits: A case study on the killifish Aphaniops stoliczkanus (Day, 1872)

Sex dimorphism is found in many organisms and is an important source of intraspecific variation. Among freshwater fishes, all members of the Aphaniidae (killifishes, Cyprinodontiformes) are known for their pronounced sex dimorphism, and it has been proposed that sexual selection has played a role in their diversification. However, few studies have assessed their sex dimorphism in detail. Here, we use Aphaniops stoliczkanus (Day, 1872) to analyse morphological variability within and between sexes and within the total sample using statistical analyses of body morphometry, meristic traits and otolith data. We found that sex dimorphism (i) is significant in many body morphometric variables, but (almost) absent in meristic traits and otolith morphometry, (ii) is most marked in anal- and dorsal-fin lengths (larger in males), and in anal- and dorsal-fin positions (more anteriorly positioned in males) and (iii) does not alter the overall variance of the population unless the character concerned is highly divergent. Our data also suggest that anal- and dorsal-fin lengths in males of A. stoliczkanus are sexually selected traits. Together with literature data, this reinforces the idea that sexual selection plays a role in the diversification of aphaniid species. Our results are also relevant to work on fossils, in which morphological variability is often difficult to interpret.