Sexual dimorphism and geographic polymorphism in Raphitoma purpurea (Neogastropoda: Raphitomidae)

Raphitoma purpurea (Montagu, 1803) is a well-known marine gastropod species almost exclusively found in shallow water in the North-Eastern Atlantic, with rare records from the Alboran Sea and Western Mediter- ranean. Despite belonging to a very diversified and complex genus, this species is often considered unprob- lematic, with a distinct morphology and stable taxonomic history. After the field collection of eight mating male–female pairs and one female after egg deposition in Brittany, France, we performed a broad-scale characterization of the reproductive biology and ecology of R. purpurea using behavioural observations and shell and anatomical descriptions. We provide evidence of sexual dimorphism in the studied population of R. purpurea, with adult males being half the size of females. Furthermore, we gathered DNA-barcoding data of 252 raphitomid specimens in the Mediterranean and neighbouring Atlantic using a fragment of the cytochrome c oxidase subunit I, and present the most updated molecular assessment of raphitomids in the NE Atlantic and Mediterranean. Based on genetic data, we extend the distribution of R. purpurea in the Mediterranean: samples from Alboran Sea, France, Corsica, Italy and Croatia shared the genetic signa- ture of Atlantic samples. The Mediterranean specimens, morphologically clearly distinct from the others, correspond to the recently described R. andrehoaraui Pelorce & Horst, 2020. The specimens collected in the Alboran Sea shared morphological features with both the Atlantic and the Mediterranean morphotypes. Our results are compatible with a pattern of a single species, ranging from the northern Atlantic to the Mediterranean and consisting of two subspecies, R. purpurea purpurea (Montagu, 1803) in the Atlantic and R. purpurea andrehoaraui Pelorce & Horst, 2020 in the Mediterranean. The remarkable pattern discovered for this species suggests Raphitomidae as a model taxon for the study of sexual dimorphism and geographic polymorphism.