Effects of baitworm digging on the soft-shelled clam, Mya arenaria, in Maine: Shell damage and exposure on the sediment surface

Experiments conducted during the fall of 1997 on an intertidal flat in Main e determined the extent of shell damage and exposure of Mya arenaria on the sediment surface resulting from commercial bloodworm (Glycera dibranchiata ) digging. We conservatively estimate that worm diggers dig up and expose o n the sediment surface approximately 6% of the greater than 2 mm fraction o f the clam population each time they turn over the sediment. Twenty percent of the clams had at least one valve damaged. Fifteen percent of intact cla ms exposed were found with their siphon up (normal living position), 41% wi th their siphon down, and 44% were horizontal on the sediment surface. Larg e clams (5.7 cm average shell length) placed on the sediment surface in the siphon up position reburied faster and to greater depths than those in hor izontal or inverted positions. Small clams (2.7 cm shell length) buried fas ter than large clams, and those placed horizontally or with their siphons u p reburied faster than clams placed with their siphons down. We detected no difference in reburial patterns between large clams exposed on undug and r ecently dug sediment. Our recovery of large clams after 10 days, however, w as much greater (91.8%) from undug sediment than dug sediment (59.4%) and w e found twice as many clam shells exhibiting evidence of predation in the d ug than the undug area. Only about 50% of the small clams were recovered li ve. Shell damage of recovered dead clams indicated that predators consumed some missing clams. Our results suggest that baitworm digging negatively af fects the survival of Mya arenaria by directly damaging shells and by expos ing clams to increased risk of predation.