Originally published in International Aquafeed November - December 2014
Aquaculture production has greatly increased
over the last 20 years. In intensive production methods, decrease of water
quality, increase of stress, decrease of food quality, and increased bacterial,
viral or parasite infections can suppress the shrimp growth (Yousefian and
Amiri, 2009). The high susceptibility to stress and the rapid spread of
diseases in water have forced fish farmers to concentrate on maintaining their
fish in good health in order to achieve economic performance (Hoffmann, 2008).
For
a long time, the most common method for dealing with the occurrence of
bacterial infections in aquaculture was the administration of antibiotics.
However, aquaculture faces serious problems due to various adverse effects of
these drugs such as accumulation in the tissue, environmental microbial flora.
On
the other hand, to use antibiotic or vaccines for fish is expensive and in many
farms unavailable (Yousefian and Amiri, 2009). However, the use of substances
or nutrients incorporated into the feed to improve the survival rate, disease
resistance and growth of shrimp has been used more and more and successfully.
The Saccharomyces cerevisiae yeast used in
the fermentation of sugarcane to obtain ethanol could be an alternative. After
fermentation the yeast may undergo a process of separation and washing, and
then is stimulated to make an autolysis of its cell membrane, pouring their
intracellular contents into the medium. Also its RNA can be "broken"
into smaller fractions, for some specific enzymes, resulting in free
nucleotides and nucleosides. This product is highly digestible and with free
amino acids in its composition.
The
cell wall of yeast has a high β-glucans amount, which is an immuno-stimulant to
activate the T cells present in the intestine, triggering an activation of the
innate immune system. Shrimp are apparently entirely dependent on a
non-specific immune mechanism to resist infections (Hertrampf and Mishra,
2006). This stimulation of the immune system can leave it better prepared to
face possible infections by pathogens. The cell wall also contains
mannanoligosacharides (MOS) that agglutinate pathogenic bacteria.
Another
benefit of this hydrolyzed yeast is that the intracellular content is fully
available, ie, it offers high amounts of small chain polypeptides and free
amino acids together fully functional yeast cell walls. These nutrients are
readily available for absorption on the gut and metabolism utilisation. Among
these amino acids, there are high levels of glutamic acid (glutamine and
glutamate), which gives a large support to gut (as amino acid and energy
source), and also has excellent palatability, leading to an increase in feed
intake. The nucleosides guanosine monophosphate (GMP) and inosine monophosphate
(IMP) also contribute in improving the palatability. Stimulating the feed
intake, there is consequently a better resistance to the challenges and also a
higher growth rate.
The
free nucleotides from yeast can be used by the salvage pathway in cells (by
this metabolic pathway the body can synthesise nucleotides with less energy
cost as a result of the recycling of free bases and nucleotides from metabolic
degradation of nucleic acid from dead cells and / or from the diet), especially
in tissues with high cell turnover and limited capacity for synthesis of purine
and pyrimidine by via de novo (such
as intestinal epithelial cells, hepatopancreas cells, hemolymph cells and
immune system), where the requirement of these bases is high.
When
endogenous supply is insufficient for normal function, nucleotides become
semi-essential nutrients or “conditionally essential” (Carver and Walker,
1995). This is especially the case in certain disease states, periods of
limited nutrient intake or rapid growth (juvenile stages). Also dietary
nucleotides appear to be important to support optimal growth and metabolically
functions, such as lymphocytes and macrophages.
Some
studies have been made in recent years to study the effect of the addition of
nucleotides in the diet of shrimp, among these, Hertrampf and Mishra (2006) studied
the addition of 0.2 percent nucleotides in the diets of Penaeus monodon, which resulted in a significant
improvement in feed conversion ratio and decrease in mortality rate (38 percent).
The same authors studied shrimp larvae directly fed with nucleotides and then
compared with the feeding value of Artemia nauplii. In a
two-rearing-cycle experiment in Penaeus monodon larvae, Artemia completely
replaced nucleotides. In comparison to the Artemia group, the nucleotide group
has an improved survival rate of 7.4 percent in the first cycle and 18.4 percent
in the second cycle, respectively. These results show the importance of
nucleotide supplementation in shrimp diets, especially in the larval and
juvenile stages.
Nucleotides can combine nutritional as well as
sanitary benefits when added to feeds. The quality of the product, the proper
mixture and administration will determinate the results.
The use of hydrolyzed yeast as a source of free
nucleotides and nucleosides besides providing a considerable concentration of
these also possess other digestible nutrients to be readily used by the
metabolism, and the presence of cell wall with their structural indigestible
carbohydrates such as beta-glucans (immunostimulation) and MOS (agglutination
of pathogens). This set of functional nutrients is a powerful ally in improving
productivity in shrimp farming.
The Aquaculturists
This blog is maintained by The Aquaculturists staff and is supported by the
magazine International Aquafeed which is published by Perendale Publishers Ltd
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