Effects of a diet of a nitrogen-limited alga ( Tetraselmis suecica) on growth, survival and biochemical composition of tiger prawn ( Penaeus semisulcatus) larvae

The prasinophyte Tetraselmis suecica was grown with high (control) and low nitrogen (1760 and 176 μM nitrate) in continuous culture and fed to protozoea 1 Penaeus semisulcatus larvae, the first feeding stage of this prawn. This alga grown in the higher nitrogen medium was the better diet for the larvae: they developed faster and were heavier when fed the cells from this medium. In contrast to development, larval survival was not affected by the algal diet. The biochemical composition (protein, carbohydrate, lipid and individual fatty acids) of the algae and the larvae fed those algae were also measured. Carbohydrate increased three-fold in the lower nitrogen algae, while protein and lipid were reduced slightly compared to the control. There was a lower protein:energy ratio (0.1 to 0.2) in the lower nitrogen diets than in the control diets (ratio 0.3 to 0.4). Similarly, the n−3: n−6 ratio of fatty acids in the lower nitrogen diets (1.5) was half of that in the control diets (3.0). Although no differences were detected in the proportions of the gross biochemical components of the larvae fed the two diets, there were differences in the proportions of fatty acids. The essential fatty acid 18:3 n−3 was 1.6 times higher in the larvae fed the control diet, and this coincided with faster development to protozoea 2. The differences in the rate of larval development may be due to the higher protein:energy ratio of the control diets as well as to the different fatty acids and their ratios.