Carmen Navarro-Guillén

Effect of varying dietary levels of LC-PUFA and vegetable oil sources on performance and fatty acids of Senegalese sole post larvae: Puzzling results suggest complete biosynthesis pathway from C18 PUFA to DHA

Lipid nutrition of marine fish larvae has focused on supplying essential fatty acids (EFA) at high levels to meet requirements for survival, growth and development. However, some deleterious effects have been reported suggesting that excessive supply of EFA might result in insufficient supply of energy substrates, particularly in species with lower EFA requirements such as Senegalese sole (Solea senegalensis). This study addressed how the balance between EFA and non-EFA (better energy sources) affects larval performance, body composition and metabolism and retention of DHA, by formulating enrichment emulsions containing two different vegetable oil sources (olive oil or soybean oil) and three DHA levels. DHA positively affected growth and survival, independent of oil source, confirming that for sole post-larvae it is advantageous to base enrichments on vegetable oils supplying higher levels of energy, and supplement these with a DHA-rich oil. In addition, body DHA levels were generally comparable considering the large differences in their dietary supply, suggesting that the previously reported ∆4 fatty acyl desaturase (fad) operates in vivo and that DHA was synthesized at physiologically significant rates through a mechanism involving transcriptional up-regulation of ∆4fad, which was significantly up-regulated in the low DHA treatments. Furthermore, data suggested that DHA biosynthesis may be regulated by an interaction between dietary n-3 and n-6 PUFA, as well as by levels of LC-PUFA, and this may, under certain nutritional conditions, lead to DHA production from C18 precursors. The molecular basis of putative fatty acyl ∆5 and ∆6 desaturation activities remains to be fully determined as thorough searches have found only a single (∆4) Fads2-type transcript. Therefore, further studies are required but this might represent a unique activity described within vertebrate fads.

Involvement of cholecystokinin (CCK) in the daily pattern of gastrointestinal regulation of Senegalese sole (Solea senegalensis) larvae reared under different feeding regimes

Cholecystokinin (CCK) is an important regulator of pancreatic enzyme secretion in adult mammals and teleosteans. Although some studies have focused on the interaction between CCK and trypsin in marine fish larvae, little is known about the circadian patterns of the regulatory mechanism involving these two digestive components. In this study, we took advantage of the characteristic change from a diurnal to a nocturnal feeding habit that occurs in Senegalese sole (Solea senegalensis) post-larvae, to conduct an experiment where larvae and postlarvae were submitted to three different feeding regimes from mouth opening: continuous feeding, diurnal feeding and nocturnal feeding. The aim was to establish different daily feeding scenarios to uncover the operating mechanisms of CCK and tryptic enzyme activity over the 24-hourcycle to better understand the regulation of digestion in developing fish larvae. Results show a prevalence of simultaneous and opposing trends of CCK level and tryptic activity as a function of the postprandial time. This finding supports the existence of a regulatory loop between these two digestive components in pre- and post-metamorphic Senegal sole larvae. In addition, CCK level was also modulated by the gut content, tending to be lower when the gut is full and higher when is being emptied. Furthermore, larvae were able to synchronize digestive functions to very different feeding regimes, although it seems to be important having a diurnal feeding phase during pre-metamorphic stages for a proper development.

Ghrelin in Senegalese sole (Solea senegalensis) post-larvae: Paracrine effects on food intake.

Successful food consumption and digestion depend on specifics anatomical and behavioral characteristics and corresponding physiological functions that should be ready to work at the appropriate time. The physiological regulation of appetite and ingestion involves a complex integration of peripheral and central signals by the brain. Ghrelin is a peptide hormone involved in the control of energy homeostasis and increases food intake in mammals, however ghrelin has species-specific actions on food intake in fish. The aim of this study was to investigate whether this peptide has an orexigenic or anorexigenic role in Senegalese sole (Solea senegalensis) in order to improve the knowledge of the physiological basis underlying feeding activity. Feed intake was measured at several sampling points to determine the overall action time of the peptide and its effect in Senegalese sole food intake. Artemia protein digestibility and retention were determined in order to analyze the ghrelin effect in fed and fasted Senegalese sole post-larvae. Results suggested that ghrelin acts as orexigenic hormone in Senegalese sole, with a response time around 25min. Results indicated that Senegalese sole post-larvae are able to maintain absorption and retention capacities independently of feeding rate and nutritional status. Furthermore, the present study gives insight for the first time of the fate of the retained amino acids, being mainly used for protein accretion (86.79% of retained amino acids recovered in protein and FAA fractions).

Daily feeding and protein metabolism rhythms in Senegalese sole post-larvae

ABSTRACT Fish hatcheries must adapt larval feeding protocols to feeding behavior and metabolism patterns to obtain more efficient feed utilization. Fish larvae exhibit daily ingesting rhythms rather than ingesting food continuously throughout the day. The aim of this study was to determine the daily patterns of feed intake, protein digestibility, protein retention and catabolism in Senegalese sole post-larvae (Solea senegalensis; 33 days post-hatching) using 14C-labeled Artemia protein and incubation in metabolic chambers. Sole post-larvae were fed at 09:00, 15:00, 21:00, 03:00 and 09:00+1 day; and those fed at 09:00, 21:00, 03:00 and 09:00+1 day showed significantly higher feed intake than post-larvae fed at 15:00 h (P=0.000). Digestibility and evacuation rate of ingested protein did not change during the whole cycle (P=0.114); however, post-larvae fed at 21:00 and 03:00 h showed the significantly highest protein retention efficiency and lowest catabolism (P=0.002). Therefore, results confirm the existence of daily rhythmicity in feeding activity and in the utilization of the ingested nutrients in Senegalese sole post-larvae. Summary: This manuscript provides new knowledge regarding Senegalese sole post-larvae daily feeding behavior and protein metabolism, which are essential for the adequate development of feeding strategies for this species.

Variability in Digestive Enzyme Capacity in Early Stages of Marine Fish Larvae: Ontogenetic Variations, Biorhythms, Hormonal Control and Nutrient Sensing Mechanisms

In recent years, no substantial progress has been achieved to significantly improve survival rates of cultured marine fish larvae and make the viability in offspring production more predictable. Improving survival and quality of marine larvae in aquaculture industry beyond the present status requires consideration of nutritional quality of the feed but also an in-depth knowledge of larval physiology to account for biorhythms in digestion capacity, appetite and regulation of food ingestion and endocrine control of digestion processes associated to nutrient sensing and feeding activity. Since gut hormones are most likely stimulated and released in response to specific nutrients it is crucial to study the nutrient sensing mechanisms also in the gastrointestinal tract of fish larvae and the importance of nutrients as they are not only substrates, but active regulators of physiological processes. This chapter focuses on the importance of the daily and circadian rhythms in the digestion capacity in larval stages of fish, emphasising the potential variability of enzyme secretion during the day, addresses the specific role of the gut hormones cholecystokinin and ghrelin on appetite and digestive functions in fish larvae and provides a comprehensive consideration of the food sensing mechanisms in the gastrointestinal tract of fish larvae associated to specific nutrients. Knowledge in these research areas, through larval ontogeny, will be specifically beneficial for the development of stage-specific micro diets and replacement of live feed with formulated feed. Recommendations on future research aspects and approaches within these themes are discussed and suggested in the specific sections.