Quantitative beach research. I. The “left-right-phenomenon”: sorting of Lamellibranch valves on sandy beaches

At several places on the sandy beaches of the Dutch coast unequal quantities of L- and R-valves of Lamellibranchs were found to be present. This sorting effect may be due to the general asymmetry of the two valves (Spisula subtruncata, S. solida, Donax vittatus, Angulus tenuis), or to a difference in fragility of the valves (Macoma balthica, Angulus fabula), or to the fact that one of the valves has a projection (Mya arenaria). It was found that the L-R-phenomenon resulting from the general asymmetry shows regular changes in intensity during the tidal phases. In the tidal area experiments were carried out with stained valves, and studies were made regarding the differences in “behaviour” of single left and right valves in the water near the beach, while on the beach the orientation of left and right valves in the points of the compass was determined. The results obtained lead to the conclusion that the sorting of the asymmetrical valves is effected by water-streams flowing obliquely up and down at the base of the slopes of the beach and the banks; these processes occur underwater by the interplay of water-currents (particularly the tidal streams) and the oscillatory wave-movements near the bottom.