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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 freezedrying, altering the digestibility of the microalgae.

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The nutritional quality of several microalgal diets used in the seed culture of the littleneck clam, Ruditapes decussatus, was evaluated in the present study. The live diets tested were Isochrysis galbana, clone T-ISO, Tetraselmis suecica and Phaeodactylum tricornutum. Criteria used in the evaluation of the diets were acceptability, digestibility, growth and biochemical composition. The highest growth rate was registered in the seed fed the T. suecica diet, followed by the seed fed I. galbana, while the lowest one was obtained in the seed fed P. tricornutum. Food ingestion rates were higher in the seed fed Tetraselmis. This fact would partly explain the higher growth observed with this diet. The limited nutritional value of Phaeodactylum could be related to the low digestibility of its cell wall, the degree of acceptability of this diet being similar to or even higher than that of the other two. The low protein content of Tetraselmis seems to indicate that R. decussatus shows a low requirement for proteins, which can be spared by carbohydrate, an abundant component in the cells of Tetraselmis. Lipid needs of this bivalve also seem to be rather low.

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The fatty acid composition of the Ruditapes decussatus spat fed on three different microalgal diets during 4 weeks was determined. The fatty acid pattern of each diet was also analysed. The diets used were Isochrysis galbana, clone T-ISO, Tetraselmis suecica, and Phaeodactylum tricornutum. The fatty acid composition of the spat was usually well correlated with that of the diet supplied. Major differences among spat cultures were found in 14:0, 16:0, 16:1n-9, 16:1n-7, 18:1n-9, 18:2n-6, 18:3n-3, 18:4n-3, 20:5n-3, 22:5n-6 and 22:6n-3 fatty acids. These differences were correlated with the particular fatty acid content of each diet supplied. It has been shown that R. decussatus spat have a very low capacity to elongate and desaturate linolenic acid to n-3 PUFA, so when 20:5n-3 or 22:6n-3 were not present in the diet, they were also absent, at least in measurable amounts, in the clams. The absence of any of the "essential" fatty acids, 20:5n-3 in T-IS0 or 22:6n-3 in Tetraselmis, did not limit spat growth, so their role as "essential" fatty acids might be a matter for discussion. Finally, the nutritive value of each diet was discussed in terms of its fatty acid composition.