From Hulls to Caves: Insights into the Introduction and Expansion of Non-Indigenous Marine Bivalves of the Genera Isognomon and Malleus in the Eastern Mediterranean Sea

Although the Eastern Mediterranean Sea is a hotspot for marine bioinvasions, the accurate identification and monitoring of non-indigenous species (NIS) remain impeded by the ambiguous morphologies of species and limited regional genetic data. This study applied an integrative approach, combining morphological identification with DNA barcoding, to assess the taxonomy and expansion of bivalves from the genera Isognomon and Malleus in the Eastern Mediterranean Sea. Specimens were collected from a broad range of habitats, including marinas, ship hulls, reefs, and marine caves. Phylogenetic analyses revealed two distinct Isognomon species in the region: I. bicolor, frequently associated with artificial substrates and showing evidence of multiple introductions, and I. aff. legumen, restricted to cryptic natural habitats. A single species of Malleus cf. regula was also detected, clustering with sequences from neighboring Mediterranean regions. The study highlights the limitations of morphology-based taxonomy and the urgent need to enhance genetic reference databases, particularly with sequences from areas of nativity. As NIS increasingly expand from anthropogenic habitats into natural ecosystems, validated data are essential for risk assessment and conservation management.