Effect of prey size on the energy budget of a predatory gastropod, Thais haemastoma canaliculata (Gray)

Abstract

Energy budgets of the southern oyster drill, Thais haemastoma canaliculata (Gray), fed diets restricted to either (1) hatchery-reared oyster spat 1–2 cm in length, or (2) wild-caught juvenile oysters 4–5 cm in length, were determined at constant temperature (21°C) and salinity (7.5%.). Mean scope for growth (total energy available for growth and reproduction) was positive in both diet groups; however, oyster drills fed small oysters had a significantly greater mean scope for growth than oyster drills fed large oysters. There was an interaction between predator and prey size. Small oyster drills (< 750 mg flesh dry wt) were unable to feed efficiently on the larger oysters, therefore mean weight-normalized scope for growth of the small oyster drills in the small oyster diet was approximately four times greater than small drills in the large oyster diet group. Weight-normalized scope for growth of large oyster drills (> 750 mg dry wt) was identical in both oyster diet groups. Metabolic energy losses were slightly greater for both large and small oyster drills fed small oysters than oyster drills fed large oysters, but this difference did not affect scope for growth differences between the two diet groups. Especially for small individuals of T. haemastoma, considerable energetic benefits exist for selecting optimum-sized bivalve prey, simply because prey size can limit ingestion rates, and hence scope for growth.