Predatory behaviour of the naticid Euspira fortunei: why does it drill the left shell valve of Ruditapes philippinarum?

Field observations showed that at two sites in northern Japan, the invasive naticid gastropod Euspira fortunei preferentially drills the left shell valve of the bivalve Ruditapes philippinarum. Laboratory experiments revealed that when R. philippinarum was allowed to adopt its normal life position, there was a statistically significant preference for E. fortunei to drill the left rather than right shell valve. Further observations of the predatory behaviour of E. fortunei showed that: (1) because this species lifted the left side of its foot while catching R. philippinarum, the direction of the apex of this snail's shell usually coincided with that of the clam's anterior margin; (2) the snail turned its prey around the axis of the anterior and posterior margin; (3) and that if the snail drilled the right shell valve of R. philippinarum, the foot and siphons of the clam interfered with its predatory behaviour.