When morphological evidence became deceiving: Miocene diversification of eastern Atlantic porcelain crabs of the genus Pisidia (Decapoda: Porcellanidae)

Even though it is one of the most studied marine regions in the world, the biodiversity of the Mediterranean Sea is still incompletely characterised. In this study, we explore the taxonomy and evolutionary history of the porcelain crab genus Pisidia in the Atlanto-Mediterranean Province, where it has undergone a complex taxonomic history resulting in identification confusion and synonymisation. Through an integrated study using a multilocus phylogeny based on two mitochondrial genes and three nuclear genes, haplotype networks, alongside a morphological analysis and 3-D renderings of micro-CT X-ray images, we investigate the diversification patterns of species complexes in the region. As a result, we have identified five distinct lineages that correspond to presumed species of Pisidia in the eastern Atlantic and the Mediterranean, each well-differentiated by both molecular and morphological characteristics. Our time-divergence analyses suggest that interspecific diversification occurred during the Miocene, whereas intraspecific diversification took place during the Pleistocene. Paleoclimatic and paleogeographic events, such as the Messinian Salinity Crisis and the Pleistocene glacial cycles, played a significant role in the evolution of Pisidia in the Mediterranean and East Atlantic. Additionally, we found species sympatry in several locations in the western Mediterranean, which may explain the long-standing taxonomic debate arising from the coexistence of morphologically distinct species that were previously assumed to be conspecific.