A team of researchers from several institutions in Mexico examined the use of supplemental microalgae Dunaliella sp in the feed of whiteleg shrimp (Litopenaeus vannamei) during a disease challenge from the bacteria Vibrio parahaemolyticus. The group published its results in the Journal of Invertebrate Pathology.
“To evaluate the effectiveness of Dunaliella sp. as an immune-stimulant for the improved survival of L. vannamei against V. parahaemolyticus infection, concentrations of 1.5, 2, 2.5 and 3% of Dunaliella sp. were included in the diet,” said the researchers in the study. “Shrimp in these groups were evaluated for levels of protein, glucose, lactate, triglycerides, and cholesterol in the shrimp hemolymph as indicators of physiological condition and for proPO and PO as indicators of the immune response.”
The group found that adding 3% of the microalgae dunaliella sp. to the feeds offered the best improvement in survival rate. However, several other parameters assessed for stress level determination did not provide clear results.
“Bioactive compounds present in Dunaliella sp. culture under stress conditions (low nitrates and phosphates) can enrich the survival rates and physiological and immune conditions of L. vannamei against diverse pathogens, such as V. parahaemolyticus,” the researchers concluded.
Why microalgae supplements?
Although demand for shrimp increases, world production remains static and aquaculture is being used to fill the gap, said the researchers. However, the shrimp aquaculture industry is facing several disease challenges, including from a bacterial pathogen that causes early mortality - Vibrio parahaemolyticus (Vibrio).
The bacterium has caused major losses for the industry, they said. Antibiotic therapy has been used to control the disease, but new products are needed to reduce the reliance on antibiotics.
Several strategies have been used to improve the innate immune system of shrimp, they said. These include offering vitamins, plant extracts, lipopolysaccharides (LPS) and β glucans.
Attention recently has focused on the use of microalgae as a source of metabolites that offer antimicrobial activities, they said. “Dunaliella sp. is known to produce large quantities of β-carotene; β-carotenes act as immune-stimulants capable of improving the immature immune system, preventing infection by opportunistic and secondary pathogens in shrimp,” they added.
Past work has examined different forms of microalgae for use in shrimp diets when facing a disease challenge from white spot syndrome virus (WSSV), said the researchers.
Shrimp also have been assessed for several parameters during stress challenges, they said. But it is hard to establish “normal values.”
Microalage used for diet supplementation was raised in nutrient stress conditions and beta-carotene levels were assessed, the researchers said.
In the feeding trial, shrimp were given one of four test diets, they said. The diets offered 36% crude protein and offered in 2mm pellets.
The trial feeds included 1,5%, 2%, 2.5% and 3% of the microalgae, they said. Two groups of control shrimp, one infected and one not, were given diets without the microalgae.
Shrimp were fed for 20 days and then challenged with the pathogenic bacteria, said the researchers.
Shrimp were checked for levels of protein, glucose, lactate, cholesterol and triglycerides at 0, 12, 24, 36 and 48 hours post-infection and mortality was noted, they said. Activity of prophenoloxidase (proPO) and phenoloxidase (PO) also was established.
Infected shrimp saw a rapid increase in mortality, said the researchers. After 48 hours post infection mortality rates were more than 80%.
However, shrimp getting the diet with 3% of the microalgae had the highest survival rates of any infected group at 30.9%, they said. The positive control group had the lowest survival rate at 10.2% and the negative control had no mortality.
All groups of shrimp showed a decrease in protein and glucose levels over time, they said. No significant difference among diets was seen for lactate concentration, and cholesterol levels were similar for all diets and at all times checked.
Differences were seen in triglyceride levels, the researchers said. The positive control group had the lowest levels, and shrimp getting the 1.5% diet the highest.
Although there was variation during infection, there were no significant differences for proPO activity, said the researchers. PO activity did vary based on time, increasing post infection and reaching the highest levels at 36-post infection for shrimp getting the diet with 2.5% microalgae.
Source: Journal of Invertebrate Pathology
Title: Survival of Litopenaeus vannamei shrimp fed on diets supplemented with Dunaliella sp. is improved after challenges by Vibrio parahaemolyticus
Authors: D. Félix, J. Elías, Á. Córdova, L. Córdova, A. González, E. Jacinto, N. Aldaz, F. Mendoza, M. Zazueta