Pinna nobilis, commonly known as the fan mussel or pen shell, is an endemic species in the Mediterranean Sea. The fan mussel population has recently been significantly endangered along the Mediterranean coast, mainly due to diseases. However, it has a critical role in the ecological system of the throughout the coasts of the Mediterranean. Therefore, it is important to investigate the interactions between P. nobilis and their enemies, parasites, symbiont in the ecological environment. Pea crabs are small crustacean and symbionts in a variety of invertebrates. They inhabit the mantle cavities of bivalve. The association between the pea crab, Nepinnotheres pinnotheres ( Decapoda), and P. nobilis (Bivalvia) from the Aegean Sea, Turkey was examined in this study. The crab samples were collected off Urla Karantina Island, Izmir Bay, Aegean Sea. The biometric characteristics of bivalve and crab in this coexistence were analyzed. 80% of P. nobilis was occupied by N. pinnotheres. The weight of pea crabs was recorded between 0.01 g and 3.87 g.