The regulatory loop between gut cholecystokinin and tryptic enzyme activitcxuy in sea bass (Dicentrarchus labrax) larvae is influenced by different feeding regimes and trigger substances

• We investigated the influence of feeds on tryptic enzyme activity in larval sea bass.
• Larvae fed the microdiet showed a higher tryptic enzyme activity during ontogeny.
• We observed a daily endogenous rhythm of CCK in all fed and food-deprived larvae.
• Higher enzyme activity in larvae fed Artemia might indicate a better stimulation.
• Reduced tryptic enzyme activity indicates a limit in diurnal digestive capacity.

Improving the understanding of digestive physiology in first feeding larvae is a prerequisite for advancing diet formulations and feeding protocols. In marine fish larvae that lack a stomach at first-feeding trypsin represents the main proteolytic enzyme. CCK is one of the key regulators of digestive enzyme secretion in adult vertebrates and current knowledge suggests that it is also involved in early stages of teleosts, although this may vary between species. Here, we investigated the influence of Artemia and a commercial microdiet on the ontogenetic development of tryptic enzyme activity as an indicator for digestive capacity in first-feeding sea bass. In order to examine the regulation and feedback mechanisms in the digestive tract we followed the response of gut CCK and tryptic enzyme activity during a one-day observation depending on the feeding regime at 23 days post hatch. Larvae fed the microdiet showed a higher tryptic enzyme activity, probably as an adaptation to the higher content in complex protein in the diet. The plant protein phytohemagglutinin (PHA), added to the microdiet as a potential stimulator for the digestive system, did not induce elevated tryptic enzyme activity nor was it beneficial for growth. This was possibly due to adverse effects of too high doses. We observed an endogenous rhythm of CCK over the day, independent of the dietary treatment or short-term fasting. Higher tryptic enzyme activity in larvae fed Artemia during the day might indicate a better stimulation by live prey in the digestive tract or the superiority of a discontinuous feeding schedule in this group. We suggest that a reduction in tryptic enzyme activity after several feeding events indicates a limit in diurnal digestive capacity. Sea bass larvae are apparently able to adapt to the feeding schedule by synchronizing the tryptic enzyme activity like adult fish.