Description: Food uptake follows rules defined by feeding behaviour that determines the kind and quantity of food ingested by fish larvae as well as how live prey and food particles are detected, captured and ingested. Feeding success depends on the progressive development of anatomical characteristics and physiological functions and on the availability of suitable food items throughout larval development. The fish larval stages present eco-morpho-physiological features very different from adults and differ from one species to another. The organoleptic properties, dimensions, detectability, movements characteristics and buoyancy of food items are all crucial features that should be considered, but is often ignored, in feeding regimes. Ontogenetic changes in digestive function lead to limitations in the ability to process certain feedstuffs. There is still a lack of knowledge about the digestion and absorption of various nutrients and about the ontogeny of basic physiological mechanisms in fish larvae, including how they are affected by genetic, dietary and environmental factors. The neural and hormonal regulation of the digestive process and of appetite is critical for optimizing digestion. These processes are still poorly described in fish larvae and attempts to develop optimal feeding regimes are often still on a ‘trial and error’ basis. A holistic understanding of feeding ecology and digestive functions is important for designing diets for fish larvae and the adaptation of rearing conditions to meet requirements for the best presentation of prey and microdiets, and their optimal ingestion, digestion and absorption. More research that targets gaps in our knowledge should advance larval rearing.

Description: 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.

Description: A protocol for combined analysis of cholecystokinin (CCK) levels based on radioimmunoassay (RIA) and fluorescence tryptic enzyme activity (FTA) was developed in order to accomplish a sensitive analysis of individual bodies and gut segments of fish larvae. Methanol was used for CCK extraction. The gut of herring larvae contained 8.9±1.2 fmol CCK/mg dry weight and in the post-larval Atlantic halibut the CCK levels varied significantly (p〈0.05) from 20.9±15.6 to 101.8±56.7 fmol/mg dry weight for separated intestinal and pyloric segments, respectively. Acid solution, 0.02 mol/l HCl–CaCl2 (pH 1.8), and alkaline solution, 0.1 mol/l Tris–0.02 mol/l CaCl2 (pH 8.0) were tested to prepare crude trypsin extracts from Coregonids and Atlantic halibut larvae. The tryptic activity of crude extracts prepared with acid solution was enhanced by a factor of 3.19±0.52 compared to the tryptic activity of crude extracts prepared with alkaline solution. The larval trypsins (from yolk sac larvae) were stable in methanol, preserving 88% of its starting activity after 6 days of storage. Based on the results, the method of extraction using methanol and acid solution (pH 1.8) was suitable for the combined analysis for CCK levels and FTA in gut segments or single larvae. The potential application of these analytical tools may allow a better understanding of the individual variability of gut functionality, nutritional condition and the feeding activity of developing fish based on their content of CCK and tryptic activity.