Influence of hydrodynamic forces on population structure of Pinna nobilis L., 1758 (Mollusca: Bivalvia): The critical combination of drag force, water depth, shell size and orientation

The effects of hydrodynamics on size, shape and distribution of benthic organisms are still not completely understood. Benthic organisms that inhabit wave-swept environments usually have small sizes and ethological adaptations to reduce drag and increase resistance force. Water speeds produced by waves in intertidal habitats can be more than 10 times higher than those in subtidal environments. However, comparatively small water speeds can produce high drag forces (F_d) on large subtidal organisms.Pinna nobilis is a subtidal epibenthic large bivalve-mollusc endemic to the Mediterranean Sea, a common inhabitant of Posidonia oceanica meadows. It lives partially buried in the seabed and shows a characteristic population structure. Small individuals are usually located at shallow sites whereas large individuals are only observed in deeper levels or sheltered locations. Also, some populations show a common orientation of the shell. These features are widespread throughout the Mediterranean, but their causes are unknown.The present work is a study on the relationship between population structure of P. nobilis and the habitat hydrodynamics. The main factor considered was Drag force due to the water flow produced by waves. Drag forces (F_d) supported by two populations located at different depths in the same P. oceanica meadow were estimated according to both the size and orientation of shells. Also, F_d acting on the individuals during the greatest storm recorded in the zone in the previous 9 years, were calculated. Drag coefficients (C_d), necessaries to estimate F_d, were estimated in the towing tank of the “Ecuela Técnica Superior de Ingenieros Navales (ETSIN)” of the Polytechnic University of Madrid.The results show significant differences in F_d acting in both populations. Despite the important increment of water speed with wave shoaling, individuals of the shallow population (SP), located at 6 m depth, withstand less F_d than those of the deep population (DP), at 13 m depth. The main reasons of this F_d reduction in SP are both the small size of the individuals and their common orientation, having the dorso-ventral side of the shell towards the main water flow. This fact, together with previous data showing higher mortality, less density of individuals, and less maximum asymptotic length, evidence that selective pressures regulate these population parameters, producing a trade-off between hydrodynamics, shell size and orientation, for each shore type and water depth.Combining the data of F_d supported by each population for different wave types, approximate values of the optimal mean F_d and the maximum dislodgement force withstood by P. nobilis were estimated (< 9 Newton (N) and ≈ 45 N respectively).