Growth performance and biochemical composition of Ruditapes decussatus (L.) spat fed on microalgal and wheatgerm flour diets

Trials were carried out for different growth diets, consisting wholly or pa rtially of wheatgerm flour, for the spat culture of the little-neck clam, R uditapes decussatus (L.). The following diets were tested: diet A, consisti ng of 100% of the daily food ration of the microalga Isochrysis galbana, cl one T-ISO; diet B, consisting of 50% microalgae and 50% wheatgerm; diet C, consisting of 25% microalgae and 75% wheatgerm; and diet D, consisting of 1 00% wheatgerm. Diet A was taken as the reference diet and two control diets were added: suboptimal algal diet B, consisting of 50% of the daily ration of microalgae, and suboptimal algal diet C, consisting of 25% microalgae. Spat growth was determined by live weight, dry weight, organic content and length. Biochemical analyses of the diets and the spat were carried out at the beginning and the end of the 35-day experimental period. The growth ach ieved when 50% of the daily ration of microalgae was replaced by wheatgerm was similar to that obtained with control diet A, consisting of 100% microa lgae. As regards organic matter, the growth of the spat fed on this diet, d iet B, was double that shown by the spat fed on suboptimal algal diet B. Wh en 75% of the microalgae were replaced by wheatgerm the growth rates decrea sed, being only equivalent to 60% of those for reference diet A. Neverthele ss, the percentage of organic matter of this spat, indicative of the index of condition, was comparable to that of the spat fed on the reference diet. The total replacement of microalgae by wheatgerm produced a considerable d ecrease in spat growth, 20% of that obtained with reference diet A. The per centage of organic matter in the spat fed on the extreme diets, diet D and suboptimal diets B and C, was observed to be significantly less than that f ound in the spat fed on the other diets. Another characteristic of the diet s tested was a decrease in protein and lipid content, together with an incr ease in that of carbohydrates, which corresponded to the extent to which mi croalgae were replaced by wheatgerm. The biochemical composition of the spa t reflects a limitation in the lipid content of diets C and D, reflected in a lower lipid growth rate in the spat compared to that of other body matte r. (C) 1999 Elsevier Science B.V. All rights reserved.