BURNETT, K.; ILIFF, S.; BURNETT, L.*: Chronic Sub-lethal Hypoxia Increases Susceptibility of Litopenaeus vannamei to Bacterial Challenge

Mortality rates of the aquacultured shrimp Litopenaeus vannamei injected with Vibrio parahaemolyticus are significantly increased when the bacterial challenge is accompanied by an immediate shift from well-aerated water to sub-lethal hypoxia (acute hypoxia). A more realistic scenario is that shrimp raised in intensive culture or in natural ecosystems would be exposed for longer periods of time to sub-lethal hypoxia (chronic hypoxia). The present study addressed the possibility that animals might adapt to chronic hypoxia, returning to a more disease- resistant status. Juvenile L. vannamei (1.0 – 1.5 g) were either maintained under normoxia or exposed to sub-lethal hypoxia (4% O₂, 2% CO₂ and pH of 6.8-7.0). After seven days animals from both exposure conditions were injected intramuscularly with an LD₅₀ dose (10⁵/shrimp) of V. parahaemolyticus. Both test groups were held under sub-lethal hypoxia for 48 hours and monitored for survival. Mortality rates were compared by a repeated measures two-way ANOVA. Chronic (7 day) hypoxia significantly increased mortality rates in challenged shrimp compared to acute hypoxia. Taken together with our previous studies (Mikulski et al., [2000] J. Shellfish Res. 19:301-311), these data show that both acute and chronic hypoxia increased susceptibility of L. vannamei to bacterial challenge. Furthermore, these shrimp did not successfully adapt to overcome the suppressive effects of hypoxia on disease resistance, rather the animals displayed higher mortality rates with increased time of exposure to hypoxia. (USDA CSREES 99-35204-8555)