Litopenaeus vannamei juveniles energetic balance and immunological response to dietary protein

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 body weight/day [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 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.