The effect of lipid supplementation on growth and fatty acid composition of Tapes philippinarum spat.
The possible use of emulsions as an artificial lipid supplement to live algae for seed of the Manila clam, T. philippinarum, was investigated. The uptake and assimilation of an emulsion, rich in docosahexaenoic acid (DHA, 22:6n-3) and eicosapentaenoic acid (EPA, 20:5n-3) and fed at 20 or 40% of the algal dry weight, were verified analytically by fatty acid analysis of the clams and their diets. Dietary requirements for n-3 highly unsaturated fatty acids (HUFA) were examined by supplementing Dunaliella tertiolecta, which contains no polyunsaturated fatty acids longer than 18:3n-3 and Tetraselmis suecica, which contains EPA but only trace amounts of DHA. An algal mixture of Isochrysis galbana (clone T-Iso) and Chaetoceros neogracile (1:1, on a dry weight basis) was used as the control diet. After 4 weeks, lipid supplementation resulted in a significant increase in the DHA level in the seed compared with those fed on non-supplemented Dunaliella (from 9.5 to 19.8 and 22.0% at a supplementation of 20 and 40%, respectively) or Tetraselmis (from 3.4 to 24.8 and 26.9%, at a supplementation of 20 and 40%, respectively) diet. Feeding solely D. tertiolecta resulted in a significantly lower daily growth rate (DGR) compared with those fed on T. suecica or the mixed algal diet. Lipid supplementation improved the DGR of clams fed on D. tertiolecta while hardly any effect was detected in those fed on T. suecica. The poor nutritional value of D. tertiolecta was indicated by the continuous decrease of the DGR and resulted in a DGR that was no longer significantly different from the starved ones at the end of the experiment.