Introduction
The enhancement of shrimp growth and immunity represents a crucial aspect of the prevention of economic losses. Consequently, bioproducts and microbial cells are regarded as superior alternatives for the management of animal health, facilitating accelerated growth and enhanced disease resistance. Saccharomyces cerevisiae (either in single-cell protein or whole cell form) has been reported to contain various immunostimulatory compounds such as β-glucans, nucleic acids, nucleotides, and chitin, as well as mannan oligosaccharides (MOS) (Navarrete and Tovar-Ramrez 2014. Nucleotides are low molecular weight intracellular and immunomodulatory compounds that serve as the basic building blocks of DNA and RNA. They play a crucial role in various physiological processes in living organisms, including cell division, modulation of cell growth, regulation of the immune system, and maintenance of gut health (Biswas et al. 2012). Therefore, this study evaluated different levels of nucleotide addition on growth performance, ammonia and nitrite stress, and return on investment (ROI) of Pacific white shrimp, Penaeus vannamei, raised in an intensive synbiotic system.
Materials and Methods
A control diet (C) containing commercial ingredients from the Brazilian feed industry was formulated to meet the recommended nutritional requirements for juvenile Penaeus vannamei. This diet was designed to be isoproteic, 360 g kg���1, and isoenergetic, 4400 kcal kg���1. Furthermore, three test diets were formulated with the inclusion of a commercial nucleotide source to replace wheat flour in the control diet at levels of 75, 150, and 300 mg nucleotides per kg feed���1, designated N75, N150, and N300, respectively. The commercial nucleotide source consists of 15% RNA (nucleotides) from the yeast Saccharomyces cerevisiae, 50% crude protein, 15% ash, and 8% water (Biotide Extra, Biorigin, S��o Paulo, Brazil). The study was conducted over 60 days at the Laboratorio de Carcinicultura (LACAR), of the Departamento de Pesca e Aquicultura (DEPAq) at the Universidade Federal Rural de Pernambuco (UFRPE), Brazil. The system was prepared in twelve fiberglass tanks filled with 600 L of seawater (32 gL���1) filtered to 250 μm and chlorinated with 30 ppm of active chlorine using sodium hypochlorite. During the feeding trial, the shrimps were fed three times daily (8.00 am, 01:00 pm, and 5:00 pm) with the experimental diets detailed in Table 1. Initially, the daily feeding rate was 8.0% of body weight, gradually reduced to 4.7% over 60 days. The feeding rate was adjusted daily based on estimated shrimp feed consumption and mortality (Van Wyk et al. 1999). Shrimp weight (n = 20) was monitored every 10 days to determine growth and to adjust the amount of feed amount. At the 30 days and at the end of the experimental period, all shrimp were counted and final weight, feed conversion ratio (FCR), survival and yield were determined. At the end of the growth trial, a total of 30 shrimps (n = 10 per replicate) were randomly collected from each experimental treatment and subjected to a stress test for ammonia nitrogen (NH3-N) levels in the rearing water (Oliveira et al. 2022; Pimentel et al. 2022). Subsequently, 24 shrimps (n = 8 per replicate) were used for the NO2-N stress test. In each test, experimental units containing 10 L of water (salinity 30 ± 0.6 g L-1, water temperature 28 ± 0.4 °C, and pH 7.5 ± 0.2) were used. The NH3-N concentration was achieved by applying a stock solution of 10 g L-1 of NH4Cl (PA) and the NO2-N concentration was achieved by applying a stock solution of 49.25 g L-1 NaNO2 (PA). Nucleotide costs and feed processing were considered alongside shrimp performance to determine the return on investment (ROI) according to Phillips and Phillips (2019). Feed production costs in this trial included feed at US$1.03 per kilogram, nucleotide at US$6.50 per kilogram, and shrimp priced at US$3.50 per kilogram, with an estimated production for 1 ha. ROI =(Net benefit with nucleotide addition ��� Net benefit in diet control)���difference in total expenses Nucleotide × control.
Results
Significant differences between treatments were found for final weight, growth (g week���1), yield kg m���3, with higher values observed in the nucleotide addition treatments. FCR was lower in all nucleotide treatments than in the control. Significant differences between treatments were also found for survival at day 60. With regard to ammonia nitrogen stress tests, the survival of P. vannamei with nucleotide addition was between 77.8 and 80.6% and the control 75%, although without significant diferences. Similarly, for nitrite nitrogen stress tests, survival was between 79.2 and 83.3% for nucleotide treatments and 70.8% for control, although without significant differences. For the dietary nucleotide treatments, the income from shrimp sales minus the cost of feed ranged from US$ 29,244 ha���1 to US$ 30,542 ha���1 for N75 and N300, respectively. Alterations such as an increase in villus height (significant findings obtained in this study and not discussed here) result in an expansion of the surface area available for nutrient absorption, which can consequently increase the efficiency of digestion and nutrient absorption in aquatic animals. In this study, it was observed that the height of villi, gut folds and enterocytes was significantly higher (data not discussed) in the nucleotide addition treatments than in the control treatment, which may indicate that the shrimp fed with nucleotide had a higher nutrient absorption capacity, reflecting in production performance and consequently ROI.
Conclusion
The addition of nucleotides (75, 150, and 300 mg kg feed���1) to the diet of P. vannamei reared in an intensive synbiotic system improved growth, survival, and yield compared to the control treatment. The addition also improved gut health (FH—fold height; VH—villus height; and EH—enterocyte height) and return on investment. The most recommended dose is 150 mg nucleotide feed kg���1, as it gave the highest ROI.
References
Biswas G, Korenaga H, Nagamine R, Kono T, Shimokawa H, Itami T, Sakai M (2012) Immune stimulant effects of a nucleotide-rich baker's yeast extract in the Kuruma shrimp, Marsupenaeus japonicus. Aquaculture 366–367. https:// doi. org/ 10. 1016/j. aquac ulture. 2012. 09. 001.
Navarrete P, Tovar-Ram��rez D (2014) Use of yeasts as probiotics in fish aquaculture. Sustain Aquacult Tech 5:145–172. https:// doi. org/ 10. 5772/ 57196
Phillips PP, Phillips JJ (2019) ROI Basics, 2nd edn. ATD Press, Alexandria, p 213
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