Effect of microalgal and inert (cornmeal and cornstarch) diets on growth performance and biochemical composition of Ruditapes decussatus seed

Research was carried out into the effect of phytoplankton, cornmeal and cornstarch diets on growth and biochemical composition of the seed of the little-neck clam, Ruditapes decussatus. The seed of R. decussatus, fed on daily rations of Isochrysis galbana (organic weight) of 0.5 and 1% of live weight of the seed, showed an improvement in growth rate when cornstarch, which is 99% carbohydrate, was added to these diets. Thus in the case of a daily ration of 0.5%, daily growth rates increased by between 33.5 and 32.3%, depending on whether we are referring to organic weight, dry weight or live weight, when 1.5% cornstarch was added. In the case of a ration of 1% I. galbana, the addition of another 1% cornstarch lead to an improvement in daily growth rates, depending on the different weight class in question, of between 14.1 and 15.5%. When compared to a daily ration consisting of 2% phytoplankton, which was considered to be the optimal ration for growth in the seed of these clams, the replacement of half the quantity of I. galbana by a quantity of cornstarch of equivalent weight gave a growth rate in terms of organic weight of 87.9% that of the phytoplankton diet, while the rates for dry weight and live weight were 89.6 and 87.9%, respectively. These results improved noticeably when cornmeal, consisting of 10% protein and 90% carbohydrate, was used instead of cornstarch. In the case of a 2% phytoplankton diet, if we substituted an equivalent quantity of cornmeal for 50% of the phytoplankton, the growth rate in organic matter was the same (99.0%) as those for the diet consisting of phytoplankton alone, while growth rates in dry weight and live weight were 6.2 and 5.9% higher, respectively, than those of the phytoplankton diet. It would therefore appear that cornmeal (and to a lesser extent cornstarch) can be successfully used as a partial substitute for phytoplankton in diets for the seed of R. decussatus and its use in hatcheries and nurseries devoted to the culture of this species would lead to a considerable reduction of production costs.