Sudden Unusual Mortality Syndrome (SUMS) in the eastern oyster Crassostrea virginica is characterized by rapid mortalities in near market size individuals and typically occurs in spring through fall in the northern Gulf of America. Due to the drastic economic consequences, where off-bottom oyster aquaculture operations may experience 70% or greater mortality of oysters nearly ready for the market, identification of the cause of SUMS is of utmost importance for the industry.
Experiments were performed to investigate potential microbial roles in SUMS. Oyster tissues were collected from fresh dead eastern oyster individuals during SUMS events. Tissues were homogenized and filtered to obtain three size fractions: D-EBV composed of eukaryotes, bacteria, and viruses (> 45 µm), D-BV composed of bacteria and viruses (< 0.45 µm), and D-V composed of viruses (< 0.2 µm). Apparently healthy tissues from oysters not experiencing mortality were used to generate a H-EBV fraction. Adult oysters from the same healthy population were anesthetized and injected with homogenate fractions or an artificial seawater (ASW) control and monitored in a flow-through system for mortalities.
Rapid mortality occurred in the first couple of days in D-EBV and D-BV treatments. These mortalities were prevented through inclusion of antibiotics in the injectate, suggesting a bacterial cause. After 6 weeks in the system D-V survival was high (83%) but dropped to 0% within 7 days (Fig. 1). These mortalities coincided with high water temperatures and a stretch of low salinity, with similar conditions occurring one to two weeks prior to mortalities. Survival in the ASW and H-EBV controls remained high (75%), indicating that water quality alone was insufficient to explain this rapid die-off. These results support the hypothesis of a multi-stressor syndrome and do not exclude the possibility of viral involvement in SUMS events.
In addition to the study outlined here, we will present data from follow-up studies and microbiome profiles from oysters experiencing SUMS mortalities.