The mode of action of Artemia in enhancing utilization of microdiet by gilthead seabream Sparus aurata larvae

Abstract

In recent years, a great deal of interest has been generated in the development 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 assimilation of MD during co-feeding with an emphasis on work carried out at the National Center for Mariculture (NCM) in Eilat, Israel. The accumulating information and findings at the NCM suggest two possible modes of influence by Artemia nauplii on the ingestion, digestion and assimilation of MD during co-feeding: (1) the remote influence on MD ingestion by visual and chemical stimuli and/or (2) the direct influence of nauplii biochemical composition on larval digestion and assimilation. The MD ingestion rates in seabream larvae when exposed to both Artemia visual and chemical stimuli, at various concentrations of Artemia nauplii, increased up to 120% as compared to ingestion rates in larvae that were offered MD alone. The free amino acids (FAA) alanine, glycine and arginine and the compound betaine were identified as the chemical stimuli for gilthead seabream larvae from the 14 metabolites found in the Artemia rearing medium. The effect of exogenous enzymes on digestion and assimilation in gilthead seabream larvae was demonstrated when pancreatin supplementation to a MD enhanced its assimilation by 30% and significantly improved growth while seabass ( Dicentrarchus labrax) larvae were unaffected by a similar pancreatin supplementation suggesting species-specific proteolytic ability during larval development. The Artemia body composition may also contain substances exerting an influence 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, significantly increased MD assimilation by 10–20% in 22-d old larvae while the effect of the supplemented fractions diminished with age. In a series of follow-up studies the results indicated that Artemia phosphatidylcholine (PC), lysophosphatidylcholine (LPC) and the free fatty acid (FFA) fraction influenced significantly ( P < 0.05) MD assimilation compared to the other lipid classes. The consumption of Artemia nauplii may also increase the production of bombesin, a hormone influencing digestion. 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.