High genetic diversity and lack of structure underly the invasion history of the non-indigenous oyster Dendostrea aff. crenulifera (Mollusca, Ostreidae) spreading in the Eastern Mediterranean Sea

Since the opening of the Suez Canal in 1869, hundreds of Indo-Pacific species have rapidly colonized the Mediterranean. Understanding the spatial and temporal patterns of this invasion is crucial for assessing its ecological impact. A notable example is the Lessepsian oyster Dendostrea sp., first discovered in Turkey in 1998 and later found throughout the easternmost Mediterranean, though its identity remained uncertain. This study clarifies the taxonomic identity and the introduction pathways of Dendostrea sp. using molecular analyses. Over 100 specimens from 25 sites in the Eastern Mediterranean, as well as Mauritius and Rodrigues (the native range), were sequenced for mitochondrial DNA (COI) and compared to 422 sequences from GenBank. Phylogenetic and species delimitation analyses identified the Mediterranean oysters as D. aff. crenulifera , conspecific with oysters from Rodrigues. The Mediterranean populations exhibited high genetic diversity, lack of phylogeographic structure, and showed no evidence of a founder effect. These findings suggest that D. aff. crenulifera entered the Mediterranean over two decades ago through multiple shipping-mediated introductions from its native range and successfully established, likely aided by the decline of native biodiversity. The observed genetic diversity pattern across the Mediterranean indicates high propagule pressure driving the species' invasion history, which likely underpins its establishment success by reducing the deleterious consequences of population bottlenecks and overcoming the so-called genetic paradox. This study underscores the value of molecular surveys in identifying taxonomically challenging non-indigenous species and uncovering their invasion histories.