Impacts Of Short-Term Exposure To Hypercapnic Hypoxia On The Susceptibility Of The Shrimp, Litopenaeus vannamei, To Vibrio parahaemolyticus     Karen G. Burnett, Stanley W. Iliff and Louis E. Burnett   Grice Marine Laboratory, University of Charleston, South Carolina     Under conditions of commercial production, aquatic organisms may be exposed to diurnal and seasonal changes in dissolved oxygen, pH and temperature.  In previous studies, our laboratory demonstrated that mortality rates of aquacultured shrimp Litopenaeus vannamei (1.0 – 1.5 g) injected with an LD50 dose of Vibrio parahaemolyticus (105/shrimp) significantly increased when the bacterial challenge was accompanied by an immediate shift from well-aerated water to sub-lethal hypercapnic hypoxia (4% O2, 2% CO2 and pH of 6.8-7.0).  A more realistic scenario is that shrimp raised in intensive culture might be exposed to sub-lethal hypoxia for several hours each day during peak production periods.  The present study addressed the possibility that animals might adapt to chronic hypoxia, returning to a more disease-resistant state.  Using the same infection model employed in prior studies, L. vannamei were exposed to hypercapnic hypoxia for four hours prior to challenge with an LD50 dose of V. parahaemolyticus.  Mortality rates in animals allowed to adapt to sublethal hypercapnic hypoxia for four hours prior to challenge were not significantly different from mortality rates in control animals that were maintained in normoxia, challenged with the LD50 dose of bacteria and returned to normoxic conditions.  These results are consistent with the possibility that shrimp may adapt to hypercapnic hypoxia within several hours of exposure, restoring normal levels of disease resistance.  The impacts of diurnal cycling in O2, CO2 and pH on disease resistance are being evaluated in the same infection challenge model.  (USDA CSREES 99-35204-8555).