Chemical compounds key to energy production, metabolism and signaling in cells
Immunity – the capacity of organisms to resist infections – is an important health management consideration for the commercial aquaculture industry because of the variety of bacterial and viral diseases that affect cultured species. Immunostimulation of farmed aquatic species is often done using various commercially available compounds, including glucans and nucleotides, which are incorporated as additives into formulated feeds.
Nucleotides are chemical compounds that play important roles in energy production, metabolism, and signaling in cells. They are essential to support all body functions and provide the natural building blocks for cellular growth and multiplication, and the biosynthesis of proteins and enzymes.
Nucleotide requirements often cannot be supplied fully from normal digestion of food, so the addition of free nucleotides in formulated aqua-feeds can be beneficial, especially during periods of stress to culture animals.
White shrimp trials
Several trials conducted in Ecuador by the authors with support from Lallemand, Inc. of Canada evaluated the effects of a diet supplemented with a specially formulated combination of nucleotides and nucleotide precursors extracted from the yeast Saccharomyces cerevisiae on growth, survival, and feed conversion in Pacific white shrimp.
In an eight-week trial, juvenile Pacific white shrimp (Litopenaeus vannamei) of 0.2-gram initial average body weight were held under stressful, low-salinity (3 ppt) conditions in three control and three experimental 60-liter tanks. All animals in both groups received a commercial feed with 45 percent crude protein, but the experimental group received feed supplemented with 2 kg nucleotide product per metric tons (MT) feed. Test results showed similar mortality levels in both groups, but animals that received the supplemented diet had significantly higher body weights (3.09 versus 1.41 g, P < 0.05, Fig. 1) at the end of the trial.
A 31-day trial with 5.5-gram shrimp stocked at 115 animals per liter in 60-liter tanks was carried out under a protocol similar to the one above. Animals fed the supplemented diet had 13.6 percent better growth and 4.0 percent higher survival rates than those in the control group.
In another experiment carried out in six control and five experimental earthen ponds, shrimp were stocked at 17 animals per square meter. After 31 days, no effect on growth was observed, but at 2.48, the animals that received nucleotide-supplemented feed had about 11 percent better feed-conversion that the control shrimp. The supplemented treatment also saw 31 percent survival – about 4 percent higher than the control.
Two bioassays carried out in Thailand with white shrimp fed a nucleotide-supplemented diet or a local commercial feed yielded similar results (Table 1). In a 110-day trial, animals receiving the supplemented feed had 14.3 percent higher survival, 27.3 percent better feed conversion, and 25.0 percent higher final weight than the control animals. In an 87-day test, shrimp fed the test diet had 38.5 percent higher survival and 23.2 percent higher final weight than the control group.
Lucien-Brun, Results of two studies with white shrimp, Table 1
|Parameter||Control Group (Commercial Feed)||Experimental Group (NSD)||Difference (%)|
|Trial 1 – 110 days|
|Final weight (g)||20.0||25.0||25.0|
|Trial 2 – 87 days|
|Final weight (g)||12.5||15.4||23.2|
In another experiment conducted in 60-liter tanks in Thailand, juvenile L. vannamei with 0.97-gram initial weight were maintained for 24 days at 31-ppt salinity. Control animals were fed three times per day with a local commercial feed, while test animals received a diet supplemented with 2 kg nucleotide product per metric ton (MT) of feed.
After five days, all animals were infected with White Spot Syndrome Virus. Results showed that the experimental animals had 90 percent survival compared to 27 percent survival in the control group. The test shrimp also had 57.7 percent higher weight gain.
Hybrid tilapia trials
Several additional trials were carried out in Thailand to evaluate the effect of nucleotide-supplemented diets on the growth, survival, and feed con-version of Oreochromis species hybrid tilapia. One 16-week trial used two batches of 50 juvenile tilapia. One batch served as a control, and the other received a diet supplemented with 2 kg nucleotide product per MT of feed. In weekly measurements of survival and growth parameters, results showed the animals fed supplemented feed were 6.6 percent longer, had 33.8 percent higher average body weight, and had 35.8 percent higher weight gain than the control fish. They also had 2.4 percent lower feed conversion and 5.6 percent higher survival.
In an 84-day trial, two batches of 50 juvenile hybrid tilapia each were used to assess the effects of nucleotide suplementation on immunoreaction. One batch served as a control, and the other received feed supplemented with nucloetides. All fish were vaccinated by immersion or intramuscular injection, then infected with virulent Aeromonas hydrophila, a heterotrophic gram-negative bacterium mainly found in very warm climates.
When infected with this bacteria, fish develop ulcers, tail and fin rot, and hemorrhagic septicemia, which can result in heavy mortalities. Results (Fig. 2) suggested that the nucleotides improved the immune response of tilapia after vaccination and significantly reduced mortality from the bacterial challenge.
(Editor’s Note: This article was originally published in the November/December 2007 print edition of the Global Aquaculture Advocate.)
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