Dietary impact of algal and artificial diets, fed at different feeding rations, on the growth and fatty acid composition of Tapes philippinarum (L.) spat

The effect of lipid supplementation and algal ration on growth and fatty ac id composition of juvenile clams, Tapes philippinarum was investigated. A. diet of Tetraselmis suecica was supplemented with a lipid emulsion and fed at a daily ration of 0.5, 1.0 and 1.5% WW DW-1 day(-1). A mixed algal diet of Isochrysis galbana (clone T-Iso) and T. suecica (1:1 on DW basis) and st arvation functioned as positive and negative control treatments, respective ly. T. suecica, which contains eicosapentaenoic acid (20:5n - 3) but no doc osahexaenoic acid (22:6n - 3), was supplemented with 50% (% of algal dry we ight) of a 22:6n - 3-rich emulsion. Higher growth rates of T. philippinarum were associated with higher T. suecica feeding rations. Lipid supplementat ion resulted in significantly better growth rates of T. philippinarum at a T. suecica ration of 0.5, 1.0 and 1.5% WW DW-1 day(-1). The growth promotin g effect of lipid supplementation was significantly reduced during 4 consec utive weeks or by increasing the algal feeding ration. Increasing T. suecic a rations were associated with a decrease in the % composition of 22:6n - 3 while the 20:5n - 3% composition was hardly affected in the total lipids o f T. philippinarum. On the contrary, the absolute concentration of 22:6n - 3 (mu g per gram dry weight, mu g g DW-1) in the total Lipids of T. philipp inarum was similar at all T. suecica rations while the 20:5n - 3 concentrat ion significantly increased. The major effect of lipid supplementation on t he fatty acid composition of the clams, was a significant increase of the 2 2:6n - 3% composition in the total lipids, polar lipids and triglycerides. In contrast with 20:5n - 3, the effect of Lipid supplementation on the abso lute 22:6n - 3 content (mu g g DW-1) was more pronounced at a higher T. sue cica ration. After a starvation period of 4 weeks, the concentration (mu g g DW-1) of all fatty acids was significantly reduced, except the n-7 monoen oic fatty acids (MUFA) and the non-methylene interrupted dienoic fatty acid s (NMID). The concentration (mu g g DW-1) of the NMID in starved clams and clams fed different diets was similar to the concentration in the initial s ample. This indicated that growing spat was actively synthesising NMID and they were selectively retained in unfed animals. The results indicated that either dietary lipid or 22:6n - 3 was a growth-limiting factor when T. sue cica was fed at a ration of 0.5, 1 and 1.5% WW DW-1 day(-1). A selective re tention in starved clams and a preferential accumulation in fed clams was o bserved for 22:6n - 3 but not for 20:5n - 3 which indicated a greater impor tance of 22:6n - 3 compared to 20:5n - 3. (C) 1999 Elsevier Science B.V. Al l rights reserved.