Growth in bivalve molluscs: Nutritional effects of two or more species of algae in diets fed to the American oyster Crassostrea virginica (Gmelin) and the hard clam Mercenaria mercenaria (L.)

Each of 15 experimental groups of American oysters and hard clams was fed one of 15 diets composed of various mixtures of four species of algae (Carteria chui, Isochrysis galbana, Platymonas suecica, Thalassiosira pseudonana) for 6 weeks. Growth of hard and soft tissue was not correlated with gross chemical (protein, lipid, carbohydrate, ash) or amino acid composition of the diets but was instead related to the presence or absence of particular algal species in the diets. Growth was generally least in diets containing large proportions of C. chui and greatest in diets containing large amounts of I. galbana or T. pseudonana. Diets containing both I. galbana and T. pseudonana invariably supported greater growth than diets consisting solely of either species, indicating synergism in the relative food values of the species. P. suecica was a relatively poor food when used as the sole component of the diet, but its food value was markedly increased when fed in combination with either (or both) I. galbana or (and) T. pseudonana. Two contrasting hypotheses explaining this interaction are presented.