Growth and lipid composition of scallop juveniles, Placopecten magellanicus, fed the flagellate Isochrysis galbana with varying lipid composition andthe diatom Chaetoceros muelleri

The effect of feeding the flagellate Isochrysis galbana (Parke; clone T-Iso ) of modified lipid composition on the growth and lipid composition of juve nile scallops [Placopecten magellanicus (Gmelin)] was investigated in the s pring of 1993. I. galbana grown in 85-liter cage culture turbidostats under conditions of nitrogen limitation had a significantly higher total lipid c ontent than when grown under nutrient-replete conditions. This was due main ly to a doubling in the amount of less unsaturated triacylglycerol in the c ells. The concentrations of methyl and ethyl ketones were also greater in n itrogen-limited cells. Diets of nitrogen-limited I. galbana and nutrient-re plete I. galbana grown in continuous and semi-continuous cultures were comp ared. Scallop juveniles were batch fed daily, and measurements of ingestion were determined. Samples of juveniles were removed periodically for determ ination of organic weight. The juveniles did not grow when fed nitrogen-lim ited or nutrient-replete I. galbana alone; however, when each diet was supp lemented with 20% of the diatom Chaetoceros muelleri (Lemm.), there was a s ignificant increase in growth in the juveniles receiving the nitrogen-limit ed I. galbana compared with juveniles on other diets. In comparison with I. galbana, C. muelleri provided a rich source of carbohydrates and the essen tial fatty acid 20:4 omega 6. This study shows the importance of providing optimal dietary levels of omega 3 and omega 6 polyunsaturated fatty acids, as well as less unsaturated fatty acids and carbohydrates.