28 JUNE 2016 • WORLD AQUACULTURE • WWW.WAS.ORG FIGURE 1. Diagram showing the different elements involved in food digestion in fish. Digestion of food takes place during the time elapsed from ingestion to fecal elimination. The efficiency with which ingested nutrients are hydrolyzed and absorbed within the digestive tract is highly dependent on factors that determine gut transit time, such as food availability and feeding time. Circumstances are different during larval life than in the juvenile period. Fish larvae eat a large amount of food relative to body weight, usually more than half of its body weight per day, to support a very high growth rate as well as the anatomical and physiological transformations that occur during early development. Fish larvae apparently are able to eat continuously, the residence time of food within the gut is very short, the stomach is not yet formed and its function as a temporary holding area is not present. In contrast, juveniles apparently eat at discrete time intervals and eat less food – only a small proportion of their weight/day. Ingested food may spend some time in the stomach before being passed to the intestine and the intestine is long enough to control transit throughout the different segments. In larvae and juveniles, feeding behavior and prey availability in wild fish, or the feeding protocol in the case of cultured fish, may affect the efficiency of digestion and nutrient utilization. For this reason we need good knowledge of the complete feeding process during the whole daily cycle to optimize digestion efficiency, improve growth and increase fitness of the fish, while minimizing waste released to the water and production costs; that is, to have a more efficient and sustainable production process. Despite the previous comments, there are very few studies on the daily pattern of feeding in fish larvae. Studies on the wild larvae When is the best time to feed fish? This is an interesting question that does not have an easy answer. Nevertheless, feeding protocol is a relevant issue to consider in rearing fish larvae and the subsequent growout of juveniles because it may affect fish growth, survival and robustness, and production costs. There are many different possibilities to feed fish stocked in a production unit, i.e. one or several meals a day, in the morning, in the afternoon, during the day, during the night, continuously, self-feeding, and more. The answer to the question about the best time to feed fish may differ among species and between larvae and juvenile stages. Each species has a defined feeding behavior as a consequence of evolution and adaptation to a particular environment. Animals and plants have developed biological rhythmicity as an adaptation to the predictable changing environmental conditions of daily, lunar and yearly cycles. The basis of endogenous regulation mechanisms of this physiological adaptive response to daily changes is a group of genes called clock-genes, the expression of which oscillates according to a circadian rhythm by means of an autonomous feedback loop of positive and negative transcriptionaltranslational factors. This selfsustaining molecular clock plays a determining role in synchronizing many physiological processes, particularly feeding (Fig. 1). In aquatic animals, these genes and therefore the circadian rhythm of biological processes are mainly synchronized by daily alternation of light and dark periods, but also by food availability and accessibility that may also have daily and seasonal periodicities. For those reasons, feeding is a biological activity highly dependent on natural cycles. Intake and Digestion of Food by Fish Larvae and Juveniles: A Strategy Planned Over Twenty-four Hours Manuel Yúfera FIGURE 2. Daily patterns of clock gene expression as a function of gut content (grey line) in gilthead seabream larvae. Gut contents oscillate in phase with the positive elements of the molecular clock (clock [brown line] and bmal [orange line]) and opposite to negative elements (cry [dark yellow line] and per [green line]). Dph: days after hatching.
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