Effects of Anoxia on Histology, Bacteriology, Condition Index, Glycogen Levels, and Fecundity in the Eastern Oyster, Crassostrea virginica

Abstract

The eastern oyster, Crassostrea virginica, is well known for its commercial and ecological roles in Mobile Bay, AL. During summer months oysters may be exposed to hypoxic and occasionally anoxic conditions which in turn can cause mortality. This study analyzed the effects of anoxia on oyster condition under summer temperatures. Five hundred hatchery reared oysters ( = 48.05 ± 5.56 mm), were examined for the effects of anoxia on oyster condition. Oysters were exposed in the laboratory to anoxic (< 0.10 mg/L O2) conditions for intervals of 24 h, 48 h and 60 h at 25ppt salinity and 28ºC ± 1ºC. Condition index was evaluated prior to experimentation to establish a baseline for comparison to control and test oysters which were sampled at the 60 h interval and 4 weeks post experiment. Histology, glycogen levels, and bacteriology levels were evaluated for test oysters prior to the experiment to establish a baseline for comparison to samples taken during the experiment, at the 24 h, 48 h, 60 h intervals, and 4 weeks post experiment. Bacterial enumeration was done under aerobic and anaerobic conditions. Fecundity estimates were performed 18 weeks post-experiment on control and experimental oysters. Histological examination indicated that anoxia causes stress in oyster tissue. Digestive tubule lumen ratios increased significantly from the baseline to the 60 h interval. Condition indices were not significantly different (P<0.05) between baseline oysters ( =59.0±2.6) and test oysters at the 60 h interval ( =57.5±1.4). Similarly, there was no difference between the baseline oysters and the control oysters at the 60 h interval ( =55.9±2.16). Glycogen levels for 10 test oysters from all intervals did not differ from the baseline ( =72.5± 18.2) µmoles glucosyl units/g dry weight. The bacterial count for test oysters ( =1.11 x 106 CFU/ml) from the 60h interval was significantly higher than counts for the baseline oysters and the 24 h and 48 h intervals. The most common bacteria isolated were Clostridium sp which confirms previous studies on the natural anaerobic flora of oysters. No difference in fecundity could be established due to the lack of mature females in both the control and the experimental groups. These findings support previous reports that mortality in oysters during anoxic events is due to bacterial infection and not lack of endogenous fuels.