In the last two decades Pinna nobilis, the largest bivalve mollusk endemic to the Mediterranean Sea, has recolonized the tidal flats of some coastal lagoons along the Italian Adriatic coast. In this study, we investigate the influence of tides on the spatial distribution and density of a P. nobilis population developing on a tidal flat of the Venice lagoon (NE Italy) by exploiting remote sensing technologies. Our results show that there is a threshold topographic elevation (about 0.5 m below mean sea level for the studied tidal flat) above which the number and duration of emersions become limiting factors of P. nobilis abundance. Above this elevation, the population density decreases sharply. Densely populated areas tend to occur in tidal flat depressions, where the duration and frequency of emersions are low. We find, however, that the population density has large spatial variability in response to other factors, such as the seagrass percentage cover. The density of the shells increases with increasing seagrass percentage cover, and the dense P. nobilis population (0.8–2.68 N/m2) grows within the Cymodocea nodosa meadow that has a high seagrass percentage cover (>85%). However, within the seagrass meadow with the highest percentage cover, the bivalve preferentially colonizes the portion closest to the main channel, a possible source of nutrients. The shift to C. nodosa—dominated meadows in the Venice lagoon occurred during the last two decades has probably facilitated the observed colonization by P. nobilis. Our findings provide management information for protection and/or restoration of P. nobilis in coastal lagoons.
