Foraging ecology of the southern oyster drill Thais haemastoma (Gray): constraints on prey choice

The effect of shell size and density on the foraging ecology of the southern oyster drill Thais haemastoma (Gray) was studied because populations of these gastropods at two sites along the Louisiana coast differed in mean shell length, but always possessed aggregated dispersion patterns. In laboratory experiments, small snails (\u3c30 mm shell length) could feed on small mussels (Ischadium recurvum Rafinesque) with wet mass \u3c2 g or small oysters (Crassostrea virginica Gmelin) of \u3c 36 g found in clumps, but they had less success on large mussels, solitary oysters with thicker shells, or the clam Rangia cuneata (Gray). When drills fed in groups, total numbers of prey eaten increased for each prey category, although per capita consumption rates increased only for prey categories with longer handling times such as oysters. Larger oyster drills spent less time handling prey and consequently had greater consumption rates for most prey types. Based on a per capita index of prey profitability (dry tissue consumed per prey/handling time·drill density-1), small oyster drills were predicted to prefer mussels. Large oyster drills (\u3e 30 mm shell length) were predicted to prefer R. cuneata, but also to take a wider variety of prey species when feeding in groups. Prey profitability declined as the density of drills feeding increased, except for large, hard to handle prey like large mussels and oysters. Group feeding thus appears profitable only for large difficult-to-handle prey. Time-lapse videotaped feeding trials, with all three prey types available, were used to determine if oyster drills selected the predicted prey species. Small drills feeding singly preferred oysters, but did switch to the predicted prey, mussels, when feeding in groups. Large drills preferred R. cuneata, and had a somewhat broader diet when feeding in groups, as predicted. We suggest southern oyster drills select prey in an optimal fashion, but that predator size and density can set important constraints on prey choice. We also suggest that T. haemastoma may severely crop I. recurvum and R. cuneata at high salinities and limit these prey to low salinity areas of the Gulf coast. © 1987