Trial Control of the Greater Clam Drill (Lunatia heros) by Manual Collection

Abstract

Soft-shelled clams (Mya arenaria) 38 mm. long were planted 172 per m.2 in three 10 m.2 plots in a sandy flat where none occurred naturally but where there were 6 drills per m.2 feeding on mussels (Mytilus edulis). After plantings, at least 300 drills entered each plot daily. After 12 days clam survival was only 19% in the plot where there was no protection; 31% where soil was raked before planting and exposed drills removed (84% efficient); 34% where this pre-planting clearance was combined with 8 post-planting, low-tide, manual collections of surfacing drills (3% to 5% efficient). These intense efforts had little effect and would be unjustified on public beds even if much more effective. Drills kill more than half their prey without boring their shells. A higher proportion of the drill population comes to the surface by night (6%) than by day (3%) and small drills are unexplainably scarce.