Changes in the digestive enzymes and hormones of gilthead seabream larvae (Sparus aurata, L. 1758) fed on Artemia nauplii enriched with free lysine

Variations in digestive enzymes and hormones during the larval development of gilthead seabream (Sparus aurata) fed on live prey (Artemia nauplii) enriched with free lysine were investigated for 16 days (from day 24 to day 40). Prior to initiation of the experiment, newly hatched larvae were transferred from incubators to fiber glass tanks (300 l) with black walls and fed the same diet until day 24. Each experiment was performed in triplicate. The amount of free lysine in the Artemia nauplii was increased by adding a 5.3 mM free amino acid solution to the culture water during a 16-h enrichment period. Larvae were sampled four times at four-day intervals, for 16 days. Larvae fed on the control group had lower growth than those fed on the lysine group at the end of the study but the difference was not significant (P > 0.05). The highest trypsin activity and lap/leu–ala peptidase ratios were observed in the control group (P < 0.05). Significant differences between bombesin activities of the treatment groups were not found in the fifth minute after the start of feeding (P > 0.05) but were significant in the fifteenth minute (P < 0.05). Significant differences between the CCK levels of the treatment groups were found (P < 0.05). In conclusion, three main ideas are presented in this study. First, Artemia nauplii can successfully be enriched with free lysine. Second, bombesin (GRP) and cholecystokinin (CCK) activity can be stimulated by free lysine. Third, the mechanisms controlling adaptation of the activity of the trypsin to the amount of dietary protein were not activated until day 40.