Nutrition of Aquatic Animal Larvae (I)

At present, the understanding of the nutritional requirements of fish and crustacean larvae is still in its early stages. Understanding the nutritional requirements of larvae and achieving larval culture is the basis for aquaculture. Generally speaking, newly hatched Artemia nauplii, various rotifers and copepods have been used as excellent feed sources for fish and crustacean larvae. Artemia nauplii can be obtained by hatching dormant eggs collected from the natural environment. Due to periodic or occasional shortages of natural resources, the supply of dormant Artemia eggs cannot meet the demand; the increase in the price of Artemia eggs will lead to an increase in the production cost of aquaculture. In addition, the nutritional quality of artificially hatched Artemia nauplii varies due to different Artemia egg harvesting seasons and regions, which is a long-standing problem. This problem can be solved to a certain extent by using different methods to eliminate the nutritional deficiencies of Artemia and rotifers.

In the past 25 years, although there have been significant achievements in the development of larval micro-granular feeds, biological feeds are still the first choice for the cultivation of aquatic animal larvae. Much knowledge about the use, cultivation and nutritional value of biological feeds is already known to the industry. However, the use of biological feeds significantly limits our understanding of larval nutritional requirements and the interactions between nutrients. In addition, understanding larval nutrition is a rather complex process. The anatomical structure and distribution of specific enzymes in the digestive tract of aquatic animal larvae usually change within a few weeks, and nutritional requirements also change with different stages of larval development.

Fortification of biological feeds has improved the industry’s understanding of essential nutrients. However, accurate quantification of larval nutritional requirements and in-depth understanding of the mechanisms of nutrient interactions must be based on complete formula feeds, and the growth rate, survival rate and physiological parameters of larvae must be similar to those of biological feeds. Determining nutritional requirements through formula feeds requires the use of chemically defined feed ingredients, which can strictly control the nutritional content of the feed. These data from experimental feeds must eventually be converted into feed formulas that can be used in actual commercial production. Research in the past decade has demonstrated that the huge difficulties faced in the production of formula feeds can be overcome, and that formula feeds can be used as the sole source of nutrition for some aquatic animals throughout the larval stage. However, many fish and crustacean larvae still rely more or less on certain phytoplankton and zooplankton-derived biological feeds. The understanding of larval nutritional physiology is closely related to the success of commercial formula feed development, and the larval nutritional requirements data established through formula feeds can serve the formulation and production of commercial feeds.

Digestive enzymes

Undoubtedly, the satisfaction of the nutritional needs of larvae is affected by the types and secretion of digestive enzymes at different stages of development. Studies have found that during the development of crustacean larvae, the types and activities of digestive enzymes in the body change. These changes correspond to changes in the type of food ingested (plant-based, animal-based or omnivorous). In the early stages of development of decapod larvae, trypsin activity is dominant, followed by amylase activity. In comparison, lower trypsin activity seems to be adapted to the longer intestinal retention time and effective absorption of food.

Insufficient digestive enzyme activity was once considered the main reason for the difficulty in successful larval culture. Some researchers speculate that the digestive enzymes secreted by the larval digestive tract are far from meeting the needs, and effective food digestion mainly depends on exogenous enzymes provided by biological bait. However, studies have found that the enzyme activity obtained by eating Artemia is very low compared with the digestive enzyme activity measured in Atlantic white shrimp larvae. The growth of European sea bass fry aged 15-40 days was poor when fed with formulated feed, but it was not caused by insufficient endogenous enzyme activity. Garcia-Ortega et al. (1998) also believed that the biological feed consumed contributed little to the intestinal enzymes of the larvae. Lazo et al. (2000a) found that the digestive enzyme activity of American redfish fry was not related to the presence or absence of food, biological feed, and exogenous enzymes. The activity of trypsin from biological feed increased with the growth of the larvae, but it was only 17% of the total trypsin activity. Lazo et al. (2007) also found that the secretion of alkaline protease in American redfish fry was early, while the secretion of acidic protease was relatively late, and did not appear until the stomach was formed; the activity of trypsin, lipase and amylase did not appear until the fry opened their mouths to eat. The researchers speculated that the changes in the activity of some enzymes during the development of fry were determined by genetic mechanisms. However, there are many challenges in studying the digestive enzymes produced during the metamorphosis of the digestive tract. At present, there is still a lack of sufficient understanding of the types and secretion amounts of digestive enzymes that appear at different developmental stages of larvae, which has led to slow and inefficient research progress. The digestive ability of larvae to different ingredients in compound feed is likely to vary between species and at different developmental stages, which may be consistent with the developmental maturity of different parts of the intestine. Studies have found that trypsin activity varies between species and at different developmental stages, and it is believed that changes in trypsin activity are related to the beginning of the formation of a functional stomach. Even if the digestive system secretes sufficient amounts of digestive enzymes, the specificity of the enzymes may limit the digestion and utilization of specific feed ingredients. However, there are differences in the series of digestive enzymes secreted by fish and crustaceans in the early developmental stages, and the secretion levels and efficacy are limited; but after different stages of metamorphosis, the types of endogenous enzymes are continuously enriched and the secretion levels are continuously improved, so as to meet the needs of animal growth and survival.

The inability of larvae to survive and grow after ingesting formulated feeds may be due to the inability of the digestive enzymes they secrete to adapt to the physical or chemical properties of formulated feeds. For example, the cultivation of Macrobrachium rosenbergii larvae on micro-adhesive feeds produced by various processes has all ended in failure. These micro-granular feeds are easily ingested by larvae, leaving their intestines in a state of fullness. However, their growth rate and survival rate are significantly lower than those of the control group that ingested biological baits. These micro-adhesive feeds for Macrobrachium rosenbergii are improved from micro-granular feeds based on ovalbumin, which can be used to continuously cultivate multiple generations of small crustaceans, Moina polyspinosa, under sterile conditions. The use of yolk protein instead of ovalbumin as the main protein source to prepare the micro-adhesive feed for Macrobrachium rosenbergii can completely replace Artemia nauplii from stage 5 larvae through metamorphosis to the larval stage, and the growth rate and survival rate of Macrobrachium rosenbergii are not significantly different from those of the control group. Although the basic nutritional composition of the feed does not change much from ovalbumin to yolk protein, it may be because ovalbumin has been chemically modified, resulting in it being unable to be effectively digested and utilized by Macrobrachium rosenbergii larvae. It is not clear whether other crustaceans have this unique response.

In the early developmental stages of marine fish, very high peptidase activity can be detected in the body. This suggests that providing hydrolyzed protein may be an effective means of meeting the amino acid needs of marine fish larvae and juveniles in the early stages. As the intestinal tract of larvae and juveniles continues to develop and improve, the intestinal epithelial cells of the entire digestive tract are able to secrete a variety of digestive enzymes, which can be supplemented by other digestive enzymes to supplement the overall digestive function.

Changes in the types and activities of digestive enzymes during the early stages of Macrobrachium rosenbergii larvae are consistent with their strictly carnivorous characteristics. Changes in the types and activities of digestive enzymes have also been reported during the development of Atlantic white shrimp larvae. Digestive capacity is adapted to the development of digestive system structure and is also affected by changes in habitat and feeding habits during metamorphosis. In addition, digestive capacity may also be based on the quality of the food fed or ingested. Larvae and juveniles appear to metabolize phospholipids in the diet more efficiently than glycerides. Differences in the efficiency of lipid utilization by larvae and juveniles are determined by measuring lipid transport efficiency.

The study of the regulatory effects of diet on digestive enzymes has become a complementary means to understand the status of digestive enzyme activity during larval development. Certain nutrients can affect the expression of enzyme activity. Several nutrients have also been found to be the main factors regulating the expression of genes related to larval development. For example, it has been found that vitamin A in the diet is involved in the maturation of the intestine and pancreas of European sea bass larvae and juveniles, and the content and quality of lipids in the diet can affect the absorption pattern of lipids by the intestinal mucosa of European sea bass larvae and juveniles.