The effect of hydrodynamics on shell orientation and population density of Pinna nobilis in the Gulf of Oristano (Sardinia, Italy)

Pinna nobilis is the largest endemic bivalve of the Mediterranean Sea, declared protected since 1992. Although hydrodynamic stress induced by waves is known to influence density, size and orientation of P. nobilis, the effect of other hydrological features is unknown. This paper considers a P. nobilis population living within a Posidonia oceanica meadow in the Gulf of Oristano (Sardinia, Italy). We hypothesize that spatial differences in density and orientation of P. nobilis may be related to significant wave height (HS), wave direction (DW), bottom current direction (DBC) and bottom current speed (SBC). A population of P. nobilis was investigated at different sites and its distribution was correlated to hydrodynamics by means of a numerical modeling approach. The spatial distribution was patchy, with a density of 0.06–6.7 ind. 100 m− 2. A non-uniform distribution of shell orientations (OS) was demonstrated in 4 sites out of 6. DBC and SBC were the main factors affecting OS, while waves had little influence. A SBC of 0.07 m s− 1 appears to be the threshold for inducing specimen directionality with shells aligned to the current and the ventral side exposed to the flow. This suggests that feeding strategy is a key factor in determining OS, in addition to drag minimization. We also highlighted the role of adjacent lagoons in supporting high densities as a result of high food availability. These findings demonstrate the usefulness of modeling techniques in explaining the spatial distribution pattern of P. nobilis and in contributing to our knowledge of its ecological traits.

► Shell orientation and density of Pinna nobilis were correlated to hydrodynamics.
► Bottom current direction and speed affected shell orientation.
► At currents > 0.07 m s− 1, specimens exposed the ventral side to the flow.
► Feeding strategy determined shell orientation in addition to drag minimization.
► Proximity to lagoons and gyres seemed to influence density of individuals.