Tandler, W.M. Koven, E. Hadas, S. Kolkovski
Microdiet (MD) research has yet to produce an artificial diet for larval marine fish, which will completely replace the requirements for live feed. This results from the fact that MD research has still to resolve the problems associated with changes in the feeding, digestive capacity and nutrient requirements in the developing larva.
Gilthead seabream (Sparus aurata) served as our model fish. We were always intrigued by the difference in the acceptance between live food and inert MD. By using radioactively labeled MD we were able to show that live feeds are ten fold as attractive as MD. In fact we demonstrated that MD ingestion in 21 d gilthead seabream larvae satisfied only the maintenance requirements. We then investigated the role of Artemia nauplii in co-feeding experiments and found that this procedure is successful as a result of the release of chemical stimuli and visual feeding cues by the Artemia nauplii. We showed that these cues account for more than double (120%) of the feeding rate as compared with larvae in the absence of these cues. Out of the compounds which are released by Artemia nauplii to the environment we tested 14, of which only the 3 free amino acids alanine, glycine and arginine and the compound betaine were found as potent attractants.
In earlier studies we showed that the absorption of MD in gilthead seabream could be improved by a dietary presence of digestive enzymes. We observed a 30% improvement in MD absorption in 20 d gilthead seabream larvae when given MD supplemented with 0.5% porcine pancreatic extract. In further experiments we showed that the supplementation of MD with porcine lipase fed to 45 day gilthead seabream larvae was associated with a 3-4 fold increase in labeled TAG absorption. However, the advantage of MD lipase supplementation of TAG absorption was limited to older larvae. We speculated that this results from a limited supply of lecithin to aid in fatty acid and diacyl glycerol transport via the enterocytes. In a recent study in which we supplemented MD with 2.5% phosphatidylcholine (PC), we clearly demonstrated a reverse age effect; with a
fold increase in the assimilation of 14C oleic acid into polar lipids (PL) in 20 d old guilthead seabream larvae as compared to insignificant effect of dietary PC in 35 day larvae. These observations clearly indicate that larval MD digestion and absorption are prime candidates for optimization via MD supplementation.
In later studies we showed that feeding stimulation was associated with an increase in the magnitude of the digestive endocrine response in terms of changes in bombesin synthesis. Moreover, we showed that this response was feed quality dependent with larvae fed live feed having thrice the level of body bombesin as compared with MD fed larvae. These results are indicative of the response of the whole digestive apparatus to dietary stimuli.
Finally, the incorporation of these findings in novel MD was associated with improved gilthead seabream larval growth. This is a demonstration that beyond nutritional requirements and micro particle design, MD supplementation with components which optimize the feeding and digestive physiological larval response will lead to MD which will completely replace live feed.
(National Center for Mariculture, PO Box 1212, Eilat 88112, Israel)