The rostrum shows a large variation across caridean shrimps; however, our knowledge about the biological significance of this morphological structure is very limited, and information on its genetic control is completely absent. The present study concentrates on an unusual rostral dimorphism in a population of small Mediterranean caridean shrimp and combines laboratory and field observations. Analysis of lab-reared offspring supports the hypothesis that the post-larval elongation of the rostrum is controlled by a single genetic locus, with the long dentate rostrum representing the recessive state and the short larval-like rostrum representing the completely dominant state. The short rostrum is a sparsely distributed character; our results obtained from field studies suggest that it reduces the viability and probability of egg-bearing among large females but, unexpectedly, the specimens with a short rostrum show consistently more rapid sex differentiation and a significantly higher propensity to become males. Therefore, it has to be concluded that, under certain conditions, a single emergent character could influence the species evolution in a rather complex manner, thus affecting the life history, population structure and dynamics and mortality in certain subgroups. In turn, the genetic factors responsible for the different phenotypes would tend to be segregated through different subpopulations and size classes, thus partially escaping the negative selective pressure.