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 an d cornstarch diets on growth and biochemical composition of the seed o f the little-neck clam, Ruditapes decussatus. The seed of R. decussatu s, 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 ra te when cornstarch, which is 99% carbohydrate, was added to these diet s. Thus in the case of a daily ration of 0.5%, daily growth rates incr eased 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 wa s added. In the case of a ration of 1% I. galbana, the addition of ano ther 1% cornstarch lead to an improvement in daily growth rates, depen ding on the different weight class in question, of between 14.1 and 15 .5%. When compared to a daily ration consisting of 2% phytoplankton, w hich 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 term s 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%, respectivel y. These results improved noticeably when cornmeal, consisting of 10% protein and 90% carbohydrate, was used instead of cornstarch. In the c ase of a 2% phytoplankton diet, if we substituted an equivalent quanti ty of cornmeal for 50% of the phytoplankton. the growth rate in organi c matter was the same (99.0%) as those for the diet consisting of phyt oplankton alone. while growth rates in dry weight and live weight were 6.2 and 5.9% higher. respectively, than those of the phytoplankton di et. It would therefore appear that cornmeal (and to a lesser extent co rnstarch) can be successfully used :Is a partial substitute for phytop lankton in diets for the seed of R., decussatus and its use hatcheries and nurseries devoted to the culture of this species would lend to a considerable reduction of production costs. (C) 1998 Elsevier Science B.V.