Screening And Identifying Pathogenic Microorganisms Of The Oyster Crassostrea Ariakensis

The oyster, Crassostrea ariakensis, is extensively cultivated in estuarine areas of southern China seas, especially in the Pearl River valley of Guangdong Province, and represents an important economic activity. The oyster aquaculture has grown rapidly, however, annual mass mortality (nearly 60～80%), usually from February to May and November to December of every year, has re-occurred in major reared regions since 1992. The causes of mortality outbreaks have not been understood, so it is important to reveal them for the sustainable development of this industry. Previous researches showed that microorganismal pathogens in the host were major causative agent of mass mortality in the Pacific oyster Crassostrea gigas and the eastern oyster Crassostrea virginica etc. The present study was carried out to screen and identify the pathogenic microorganisms associated with mass mortality of the oyster C. ariakensis. Challenge experiments were conducted by injection into healthy cultured oysters C. ariakensis with potential virus, RLO and 11 strains of bacteria, which were isolated from the moribund oysters during oyster mortality outbreaks occurring on rearing site. Our experimental results showed that the mortalities of the oysters caused by potential virus and RLO were not significantly different from those of the control, while the mortalities among groups injected by four strains of the bacteria were significantly higher than the control group, two strains of Vibrio alginolyticus, one strain of Vibrio proteolyticus and one strain of Vibrio carcariae, were identified by the Biolog bacteria identification system and 16S rDNA sequence comparison respectively. The mainly damaged organ of the oyster C. ariakensis was digestive gland.The absorptive cells in the wall of digestive gland tubules were broken and shed, furthermore, the tubule wall collapsed, and the digestive gland were extensively damaged. The drug sensitivity testing indicated the four strains of vibrio were resistant to penicillin G, ampicillin and Erythromycin, and sensitive towards chloramphenicol and norfloxacin.