Geometric morphometric analysis reveals that the shells of male and female siphon whelks Penion chathamensis are the same size and shape

Secondary sexual dimorphism can make the discrimination of intra and interspecific variation difficult, causing the identification of evolutionary lineages and classification of species to be challenging, particularly in palaeontology. Yet sexual dimorphism is an understudied research topic in dioecious marine snails. We use landmark-based geometric morphometric analysis to investigate whether there is sexual dimorphism in the shell morphology of the siphon whelk Penion chathamensis. In contrast to studies of other snails, results strongly indicate that there is no difference in the shape or size of shells between the sexes. A comparison of P. chathamensis and a related species demonstrates that this result is unlikely to reflect a limitation of the method. The possibility that sexual dimorphism is not exhibited by at least some species of Penion is advantageous from a palaeontological perspective as there is a rich fossil record for the genus across the Southern Hemisphere.