Genetic insights into the non-indigenous copepod Pseudodiaptomus marinus Sato, 1913 (Calanoida: Pseudodiaptomidae) in the Mediterranean Sea

The calanoid copepod Pseudodiaptomus marinus Sato, 1913, native to the Indo-Pacific region, has rapidly expanded its range in European waters since its first detection in the Adriatic Sea in 2007, raising questions about its introduction pathways and population dynamics. In this study, we investigated the genetic diversity and phylogeography of P. marinus in the Mediterranean Sea, focusing on four Italian coastal areas, using a dual-marker approach based on the mitochondrial cytochrome oxidase subunit I (COI mtDNA) and the internal transcribed spacer of the ribosomal cistron (ITS) genes. Molecular data allowed the reconstruction of the phylogenetic relationships within the Pseudodiaptomidae family and the characterization of the species P. marinus across geographically distant areas. COI analyses revealed three main Pseudodiaptomidae clades corresponding to freshwater, brackish, and marine species, while all P. marinus sequences clustered together, showing no evidence of cryptic species. Haplotype networks indicate repeated human-mediated introductions, with Italian populations connected among themselves and to other introduced areas, though higher haplotype diversity was observed in putative native regions. ITS analyses confirmed the absence of clear biogeographic structuring among Italian populations, suggesting that some ITS sequences from Korean populations, within or near the presumed native range of P. marinus , may require further verification, although the observed divergence could also reflect native-range genetic diversity or distinct lineages within a broader species complex. Overall, our findings provide an overview of the genetic diversity of P. marinus across different Mediterranean areas, offering new insights into its characterization and suggesting possible introduction pathways in European waters. These findings underscore the value of integrated molecular approaches in supporting and enhancing monitoring programs for non-indigenous species, while also improving our ability to anticipate their potential spread under future environmental changes scenarios.