Abstract
The present study was designed to evaluate the effect of dietary protein level on survival, assimilation efficiency and immunological condition of Litopenaeus vannamei juveniles using two protein levels in a range of optimal reported levels of 15% and 40% (equivalent to 15 and 40 g DP/kg BWd) and one extremely low (5% equivalent to 5 g DP/kg BWd). In order to reach this goal, effects of dietary protein level on energetic balance were recorded by ingestion rate ( I), respiratory rate ( R) and biomass production of L. vannamei during its growing process ( P). Energy lost from feces ( H) and urine products ( U) was calculated as ( H+ U)= I− R+ P and assimilated energy (As) as R+ P. At the end of the growth experiment, shrimp immune response, oxyhemocyanin (OxyHc), osmotic pressure and digestive gland glycogen were measured. Dietary protein level enhanced ingestion rate in shrimp fed 5 g DP/kg BWd compared to shrimp fed 40 g DP/kg BWd. However, daily growth coefficient (DGC, %)of L. vannamei juveniles was high in shrimp fed 40 g DP/kg BWd in comparison to shrimp fed 5 g DP/kg BWd. An inverse relation between wastes ( H+ U) and dietary protein level was observed, indicating that shrimp loose 81% of ingested energy when fed 5 g DP/kg BWd and only 5.6% when fed 40 g DP/kg BWd. A higher assimilation and production efficiency ( P/As) was obtained when shrimp were fed 40 g DP/kg BWd than that obtained in shrimp fed 15 or 5 g DP/kg BWd and an inverse relation between R and respiratory efficiency ( R/ AS) in relation to dietary protein levels was also observed. An increase in OxyHc was observed with increasing dietary protein levels indicating that shrimp accumulated protein as hemocyanin. A reduction of hemocytes occurred when shrimp were fed sub-optimal dietary protein levels indicating that zymogens contained in hemocytes, i.e., prophenoloxidase (ProPO) system, peneidins and their activities (phagocytosis, coagulation), were also reduced. A reduction on respiratory burst was observed indicating that sub-optimal dietary protein level affected the number of cells and the phagocytosis capacity of cells. Notwithstanding, the compensatory mechanism used by L. vannamei to respond nutritional stress, sub-optimal dietary protein level (5 and 15 g DP/kg BWd) induced not only a severe reduction in growth rate and assimilation efficiency but also in immune capacities.
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