THE MODE OF ACTION OF ARTEMIA IN ENHANCING UTILIZATION OF MICRODIET BY GILTHEAD SEABREAM SPARUS-AURATA LARVAE

In recent years, a great deal of interest has been generated in the de velopment of a microdiet (MD) as an economic live food alternative for the larvae of commercially important species of marine fish. Despite the poor performance of microdiets (MD) when used exclusively to rear marine fish larvae, results were markedly improved when inert MDs were co-fed with live Artemia nauplii. This paper reviews the influence of Artemia on the enhancement of the ingestion, digestion and assimilati on of MD during co-feeding with an emphasis on work carried out at the National Center for Mariculture (NCM) in Eilat, Israel. The accumulat ing information and findings at the NCM suggest two possible modes of influence by Artemia nauplii on the ingestion, digestion and assimilat ion of MD during co-feeding: (1) the remote influence on MD ingestion by visual and chemical stimuli and/or (2) the direct influence of naup lii biochemical composition on larval digestion and assimilation. The MD ingestion rates in seabream larvae when exposed to both Artemia vis ual and chemical stimuli, at various concentrations of Artemia nauplii , increased up to 120% as compared to ingestion rates in larvae that w ere offered MD alone. The free amino acids (FAA) alanine, glycine and arginine and the compound betaine were identified as the chemical stim uli for gilthead seabream larvae from the 14 metabolites found in the Artemia rearing medium. The effect of exogenous enzymes on digestion a nd assimilation in gilthead seabream larvae was demonstrated when panc reatin supplementation to a MD enhanced its assimilation by 30% and si gnificantly improved growth while seabass (Dicentrarchus labrax) larva e were unaffected by a similar pancreatin supplementation suggesting s pecies-specific proteolytic ability during larval development. The Art emia body composition may also contain substances exerting an influenc e on larval digestion and assimilation. MDs supplemented with various fractions extracted from Artemia nauplii, i.e. neutral and polar lipid classes or a non-lipid fraction, separately and in combination, signi ficantly increased MD assimilation by 10-20% in22-d old larvae while t he effect of the supplemented fractions diminished with age. In a seri es of follow-up studies the results indicated that Artemia phosphatidy lcholine (PC), lysophosphatidylcholine (LPC) and the free fatty acid ( FFA) fraction influenced significantly (P < 0.05) MD assimilation comp ared to the other lipid classes. The consumption of Artemia nauplii ma y also increase the production of bombesin, a hormone influencing dige stion. When Artemia nauplii were given as the sole food to the larvae, the presence of bombesin increased by 300% as compared to the levels that were found in larvae given only a MD. (C) 1997 Elsevier Science B .V.