On the sandy beaches of some Dutch North Sea islands an unequal distribution was observed of lamellibranch valves perforated by species of Natica and of unperforated valves. The percentage of perforated valves increased from lower to higher levels on the slopes of the sandbanks and the beach.Observations on the beach and experiments in the laboratory led to the hypothesis that this “hole effect” is caused in the following way:
When water runs over a perforated valve the presence of the hole allows water to stream under part of the edge. Consequently the ring of sand supporting the edge becomes less stable, and the valve can be easily transported.
It is concluded that this “hole effect” will be observable in the case of shells which, normally or accidentally, have holes or other openings when they lie on the sand (e.g. Fissurella, Haliotis, many Brachiopoda, Solenidae and Chlamydae, and valves with abrasion holes).
The consequences of such sorting phenomena to sedimentological and other types of marine research are briefly discussed.
