Seasonal Macrofaunal Diversity in the Shells of Dead Pinna nobilis Linnaeus, 1758 in Southern Istria

The shells of dead Pinna nobilis individuals are important habitats in sedimentary coastal ecosystems, yet their ecological role is poorly understood. This study investigated macrofaunal communities associated with 80 P. nobilis shells from Soline Bay and Valovine Bay, northern Adriatic, analyzing variations in species abundance and biodiversity between shell orientations (vertical and horizontal) and across seasons. Shell dimensions were recorded, with larger shells and higher faunal abundance observed in Soline Bay compared to Valovine Bay. A total of 2225 individuals representing 183 species across 19 taxonomic groups were identified, with Malacostraca, Bivalvia, and Polychaeta being the most abundant. Vertically positioned shells hosted significantly more organisms than horizontally positioned ones, likely due to greater available surface area for settlement. Seasonal changes influenced organism abundance, with peaks in winter for Valovine Bay and spring for Soline Bay, correlating with environmental factors such as eutrophication. The most frequent species associated with the shells of dead individuals were the polychaete Sabella spallanzanii and the bivalve Rocellaria dubia, which can impact shell degradation. Despite their temporary nature, the shells of dead P. nobilis provide vital refuge and enhance biodiversity. The findings underscore the ecological importance of P. nobilis shells as biodiversity hotspots and highlight the need for their conservation and further study.