Molecular and morphological data provide evidence for only one alien species of pearl oyster in the Mediterranean Sea

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Taxonomic identification of alien bivalve species in the Mediterranean Sea can be challenging because of high morphological variability and the occurrence of cryptic species complexes, as in the case of pearl oysters. While the presence of the Lessepsian species Pinctada radiata is well-established in the eastern Mediterranean Sea, the presence of P. fucata was recently suggested based on morphological data. In this study we performed an integrative assessment based on morphological and genetic data on pearl oysters collected across the Mediterranean Basin. Molecular species delimitation and phylogenetic analyses allowed a straightforward taxonomic assignment of all collected specimens to P. radiata. These specimens show the entire suite of morphological character states considered diagnostic of either P. fucata or P. radiata by previous studies. This finding clearly demonstrates that these morphological characters have no taxonomic value, and their variability observed in specimens from the Mediterranean Sea represents (part of) the intraspecific variability of P. radiata. While no evidence has been found for the presence of P. fucata, the earlier occurrences of P. radiata from the eastern and southern Mediterranean Sea are complemented with verified occurrence in the western and northern regions, demonstrating a further spreading of this non-native species throughout the Mediterranean Sea. This study clarifies the taxonomic identification and geographical distribution of pearl oysters in the Mediterranean Sea and substantiates the importance of molecular identification of alien bivalves characterized by extensive variation in shell characters.