Abstract
Sessile bivalves belonging to the genus Isognomon (Lightfoot, 1786) are native to tropical and subtropical intertidal and infralittoral zones. Recently, they were introduced to the Mediterranean Sea as nonindigenous species. Here, we report the presence of two Isognomon taxa, the West Atlantic Isognomon bicolor (CB Adams, 1845) and the Indo-Pacific Isognomon legumen (Gmelin, 1791), in the neighbouring Islands of Chalki and Alimia in the South Aegean Sea. DNA barcoding and phylogenetic analyses were used for their identification, which proved essential for the genus Isognomon because due to its highly irregular and habitat-dependent shell shapes, their sole morphological identification was problematic. Additional field observations during two successive years of hard substrata structure supplied information on the habitat preferences of these taxa, providing useful ecological insights into their establishment in the area.