Immunocompetence of Penaeus monodon under acute salinity stress and pathogenicity of Vibrio harveyi with respect to ambient salinity

Abstract

Salinity is an important environmental factor which usually goes unnoticed in shrimp growout systems. In the present study an attempt was done to analyse the physiological and immunological responses of Penaeus monodon to Vibrio harveyi infection under acute salinity stress. Shrimps were challenged with V. harveyi under 5‰ salinity stress, 35‰ salinity stress and optimal salinity (15‰) conditions by intramuscular injection. A control was maintained without bacterial challenge. Haemolymph was collected from shrimps soon after salinity change before V. harveyi challenge (post salinity change day (PSD) 0), on post challenge day (PCD) 2, 7 and 10. Immune variables viz., total haemocyte count (THC), phenol oxidase activity (PO), Nitroblue tetrazolium salt (NBT) reduction, alkaline phosphatase activity (ALP), acid phosphatase activity (ACP) and metabolic variables viz., total protein (TP), total carbohydrates (TC), total free amino acids (TFAA), total lipids (TL), glucose (Gl) and cholesterol (Ch) were then determined. One way ANOVA followed by Duncan's multiple comparison of the means revealed significant differences between shrimps subjected to different salinity conditions and then challenged with V. harveyi (P <0.05). Post challenge survival was significantly higher in shrimps maintained at 15‰ and acute salinity stress reduced the survival rate. However, pathogenicity of V. harveyi was slightly higher at 35‰ than at 5‰. At the onset of salinity stress, parameters viz., THC, NBT and ACP significantly reduced and TC increased at 5‰ and 35‰. Following V. harveyi challenge, an overall increase could be observed in metabolic variables on PCD2 and immune variables on PCD2 and 7 in P. monodon maintained at 15‰ compared to the control. Even though an increase in immune response and metabolic variables could be obtained on post challenge days in shrimps under salinity stress compared to the control, it was considerably low in comparison with shrimps maintained at 15‰. Regression analysis proved that NBT, ALP, TP and TL could be proposed as potential stress indicators to evaluate shrimp health status. In brief, it may be concluded from the study that acute salinity changes evoke physiological responses that affect the immunocompetence and metabolic performance of P. monodon against V. harveyi challenge, thereby increasing the susceptibility to infection. Moreover higher salinity enhanced the pathogenicity of V. harveyi.