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 acid 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 starvation functioned as positive and negative control treatments, respectively. T. suecica, which contains eicosapentaenoic acid (20:5n - 3) but no docosahexaenoic acid (22:6n - 3), was supplemented with 50% (% of algal dry weight) of a 22:6n - 3-rich emulsion. Higher growth rates of T. philippinarum were associated with higher T. suecica feeding rations. Lipid supplementation 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 promoting effect of lipid supplementation was significantly reduced during 4 consecutive weeks or by increasing the algal feeding ration. Increasing T. suecica 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 of 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. philippinarum was similar at all T. suecica rations while the 20:5n - 3 concentration significantly increased. The major effect of lipid supplementation on the fatty acid composition of the clams, was a significant increase of the 22:6n - 3% composition in the total lipids, polar lipids and triglycerides. In contrast with 20:5n - 3, the effect of Lipid supplementation on the absolute 22:6n - 3 content (mu g g DW-1) was more pronounced at a higher T. suecica 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 monoenoic fatty acids (MUFA) and the non-methylene interrupted dienoic fatty acids (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 sample. 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. suecica was fed at a ration of 0.5, 1 and 1.5% WW DW-1 day(-1). A selective retention in starved clams and a preferential accumulation in fed clams was observed for 22:6n - 3 but not for 20:5n - 3 which indicated a greater importance of 22:6n - 3 compared to 20:5n - 3. (C) 1999 Elsevier Science B.V. All rights reserved.