To have or not to have a penis? Phally polymorphism in minute, hermaphroditic gastropods from the Vertiginidae family

Abstract Phally polymorphism in snails offers an excellent opportunity to study the evolutionary basis of sexual polymorphism in animals. Phally polymorphism occurs when two or three sexual morphs co-occur in populations of hermaphroditic organisms. Euphallics have a fully developed and functional penis, hemiphallics have a reduced and non-functional penis, while aphallics have no penis. All morphs can self-fertilize, but only euphallics can donate sperm during copulation. In this study, we investigated the geographical pattern of phally polymorphism in minute Vertigo species by testing snails from Polish and Swedish populations. Considering the principle of resource allocation, we checked whether sexual morph reflects the size of the shell. Although the Swedish Vertigo snails tend to be smaller than the Polish ones, our results show no clear geographical pattern of sexual morph balance in the studied populations. Aphallics of V. angustior were bigger than the euphallics; conversely to V. substriata, where euphallics were bigger than aphallics. Such a result suggests a species-specific resource allocation towards the given sexual function. Since hemiphallics of V. antivertigo were smaller than euphallics, we discuss the possible trade-off when faced with resource depletion and the failure of attaining sexual maturity of euphallics.