Evaluation of freeze-dried microalgal diets for the seed culture of Ruditapes decussatus using physiological and biochemical parameters

This study evaluates the nutritional value of several species of freeze-dried microalgae used to feed the seed of Ruditapes decussatus. These microalgae were: Isochrysis galbana, clone T-ISO; Tetraselmis suecica; and Phaeodactylum tricornutum. The addition of antibiotics, kaolin and fresh microalgae to the freeze-dried diet was also evaluated, and a study was made of the nutritional value of one species of microalga in each of the four stages of preparation of the freeze-dried diet: culture; concentration; freezing and freeze-drying. Both physiological and biochemical criteria were used to evaluate the different diets: availability; acceptability; digestibility; and biochemical composition. The highest seed growth rate were achieved with a diet of Isochrysis, followed by Tetraselmis, whilst the growth rates achieved with a dried diet of Phaeodactylum were much lower. The differences observed between diets of Isochrysis and Tetraselmis were due to a greater ingestion of the cells of Isochrysis. The minimal nutritional value of Phaeodactylum appears to be related to the low digestibility of its cell wall. The growth rates of seed fed on a diet of dried microalgae were significantly lower than those achieved with fresh diets (Albentosa et al., 1996b) for each of the species of microalgae that were tested, and this appears to be related to the lower digestibility of the microalgae after they have been freeze-dried, The addition of antibiotic or kaolin to the freeze-dried diet did not result in a major increase of seed growth rates. The substitution of a small proportion of freeze-dried microalgae with fresh microalgae produced a significant increase in seed growth rates, although these were still lower than those obtained with a completely fresh diet. The loss of nutritional value of microalgae when they are freeze-dried may be due to some kind of process which occurs during freeze-drying, altering the digestibility of the microalgae. (C) 1997 Elsevier Science B.V.