Species without dimorphic secondary sex characteristics easily visible to humans, such as diference in size or morphology, are expected to experience low levels of sex-specifi c selection. However, monomorphism in classic visible traits could be a result of genetic or physiological constraints that prevent the sexes from reaching divergent fi tness optima. Additionally, biochemical and molecular work has revealed a variety of less easily observed phenotypes that none-the-less exhibit profound dimorphism. Sex- specifi c selection could act on these more subtle, less visible, traits. We investigate sex-specifi c selection in the polygynandrous dusky pipe sh (Syngnathus oridae), which lacks size, color, and morphological dimorphism. Using experimental breeding populations, we revealed that although males and females have similar opportunities for sexual selection, only males experience signi ficant sexual selection pressures on body size. We also investigated patterns of sex-biased and sex-specifi c gene expression in gonads, livers, and gills, and tested whether genes with highly divergent expression patterns between the sexes are more likely to be tissue specifi c, and therefore relieved of genetic constraints. We. Sex-bias in gene expression was widespread, although the reproductive organs had the most sex-biased and sex-specifi c genes. Sex-specifi c selection on gene expression in gills was primarily related to immune response, whereas the liver and gonads had a wide variety of cellular processes, as well as reproductive proteins, showing sex-biased expression. These sex-biased genes are likely less constrained by pleiotropy, as they were more organ-speci fic in their expression patterns. Altogether, we nd evidence for ongoing and historical sex-specifi c selection in the dusky pipe sh.