Comparison of rates of feeding and biodeposition of the American oyster, Grassostrea virginica Gmelin, fed different species of phytoplankton

Abstract

The feeding and biodeposition rates of small (2.4 cm) and large (5.2 cm) Crassostrea virginica were differently affected in experiments using flowing systems by moderate (292 μ C/1 sea water) and high (487 μ C/1 sea water) concentrations of the following phytoplankton: the diatoms, Thalassiosira pseudonana (Hasle & Heindal) ( Cyclotella nana), Skeletonema costatum (Greville) Cleve, Nitzschia closterium (Ehrenberg) Wm. Smith, and the green alga, Dunaliella tertiolecta Butcher. The average feeding rate of the oysters on the different phytoplankton was: Dunaliella < Nitzschia < Skeletonema < Thalassiosira. In contrast, oysters fed on Thalassiosira had the lowest rate of biodeposition (1.626 × 10 −3 g ash-free dry wt/g dry flesh wt per day) but oysters cultured on Nitzschia showed the highest rate of biodeposition (2.952 × 10 −3 units). The response of the feeding rate of the two sizes of oysters to food concentration differed according to the algal food source. Oysters fed on Thalassiosira and Skeletonema exhibited a higher feeding rate at a higher food level: in contrast, oysters fed on Nitzschia showed no significant differences and those cultured on Dunaliella a depressed feeding rate at the higher food concentration. We observed no differences in feeding rate between small and large oysters fed on Skeletonema and Dunaliella, but small oysters had a higher feeding rate than large oysters when cultured on Thalassiosira and a lower feeding rate when fed on Niitzschia. The smaller size of Thalassiosira and larger size of Nitzschia might account for these differences in the feeding efficiency of small oysters. The biodeposition rates of both sizes of oysters were also affected by food concentration of the different algae. At the high food concentration with Skeletonema, Nitzschia, and Dunaliella, all oysters showed a marked increase in the rate of biodeposition, but oysters fed on Thalassiosira exhibited no great change. On all species of phytoplankton, the larger oysters produced greater biodeposition than the smaller oysters. These differences in response of feeding and biodeposition rates are important to our knowledge of food chain dynamics and may be used to optimize aquaculture systems.