New Records of Isognomon Species from Crete, Greece; Evidence from Adult Specimens and Additional DNA Barcodes

During the last two decades, non-indigenous species of the genus Isognomon have been increasingly reported in the Mediterranean Sea, yet their taxonomy and distribution remain poorly resolved because of pronounced shell plasticity and limited molecular reference data. In the present study, Isognomon specimens collected along the coasts of Crete (Greece) were examined, using an integrative approach combining adult morphometric analyses with mitochondrial (COI and 16S rRNA) and nuclear (28S rRNA) genetic markers. The results confirm the establishment of Isognomon bicolor in the eastern Mediterranean and document the presence of specimens morphologically consistent with I. australicus, which clustered within the unresolved I. legumen–I. aff. legumen complex in both mitochondrial and nuclear phylogenetic analyses. Morphometric analyses revealed pronounced shell plasticity, with shell width, hinge length, and nacreous height providing the greatest discriminatory power, although considerable overlap between taxa limited reliable identification based solely on shell morphology. Molecular analyses confirmed the identity of I. bicolor and further supported its eastward expansion within the Mediterranean basin. In contrast, the concordance between mitochondrial and nuclear datasets indicates that the unresolved taxonomy of the I. australicus–I. legumen complex is unlikely to result solely from the use of mitochondrial markers. These findings expand the available morphometric and genetic data for Mediterranean Isognomon populations and highlight the need for integrative taxonomic approaches, incorporating additional nuclear and genomic data to resolve species boundaries.