Immune responses of Litopenaeus vannamei to thermal stress: a comparative study of shrimp in freshwater and seawater conditions

The effects of hypothermal (22-16 degrees C) and hyperthermal (22-28 degrees C) stress on the immune system responses of the shrimp Litopenaeus vannamei cultured in either freshwater or seawater were measured and compared. The following immune system indicators were measured for comparison: total hemocyte count (THC), activity of phenoloxidase (PO), nitric oxide synthase (NOS), superoxidase (SOD), and malondialdehyde (MDA) content. Thermal stress significantly decreased THC in both freshwater and seawater shrimp within 6-12 h (P < 0.05). After hypothermal stress, all shrimp had a significantly lower THC level than their prechallenge levels (P < 0.05). Under both types of thermal stress, the activity of PO, NOS, and SOD first increased and then decreased. After 48 h of thermal stress, shrimp PO and NOS activity decreased in both freshwater and seawater. After 48 h of thermal stress, the reduction in the SOD activity in the hemolymph of freshwater shrimp was greater than that in seawater shrimp. During exposure to stress, the MDA content in freshwater shrimp was significantly higher than in seawater shrimp, which demonstrated that lipids in freshwater shrimp were more susceptible to peroxidation than those in seawater shrimp, particularly at low temperatures. Large temperature fluctuations, particularly sudden cooling, should be avoided when rearing L. vannamei because of high rates of lipid peroxidation and decreased immunity. These effects are more marked in freshwater than in seawater.