Size-specific aspects of the foraging ecology of the southern oyster drill Stramonita haemastoma (Kool 1987)

Abstract

Size-related aspects of the foraging ecology of the southern oyster drill, Stramonita haemastoma (Kool 1987), feeding on the oyster Crassostrea virginica (Gmelin) in laboratory experiments were studied. Three sizes of oysters were exposed to three sizes of oyster drills. Small oyster drills (less than 35-mm shell length) could successfully feed in the laboratory only on small oysters (less than 50-g wet mass). However, even intermediate sized and large snails could not consume the largest oysters (greater than 150-g wet mass), indicating a size refuge. Both the number and dry mass of oyster prey consumed increased with increasing predator size. Larger oysters also attracted greater numbers of snails. Oyster handling times decreased, and both dry mass consumed and absolute growth rate increased with increasing predator size. However, growth efficiency declined, indicating relative maintenance costs increased with predator size. A field experiment using caged oyster drills and oyster prey also suggested greater feeding rates in larger snails. Only seven percent of the oysters in an intertidal oyster reef were larger than 150 g wet mass. However, sizes of snails at exposed sites were smaller, with up to 39 percent in the smallest size category used in the laboratory experiments. At exposed sites, many oyster drills may not be able to consume oysters greater in size than the median size in the intertidal reef.