Soft shell blue crab (Callinectes sapidus) aquaculture is one of the oldest domestic aquaculture industries along the East and Gulf Coasts of the United States. Soft shell blue crabs are produced by culturing pre-molt (peeler) crabs in shallow dockside or land-based shedding systems until they shed their hard shell in order to grow. Soft shell production is most successful with peeler crabs harvested one to three days before molting. However, crabs are often held for 14 to 21 days. High mortality in shedding systems has always been a problem for the industry, and diseases, such as Callinectes sapidus reovirus 1 (CsRV1), can play a significant role. The prevalence of CsRV1 in dead peeler crabs ranges from 22 – 75% in blue crab shedding facilities in the Chesapeake Bay region.
To understand more about CsRV1, we conducted experiments examining the fate of naturally infected wild-caught crabs, the mortality rate in virus-injected juvenile (Florida) and adult (Louisiana) crabs, the potential for water-borne virus transmission, and the impact of salinity (1, 5, 15 ppt) on CsRV1 transmission among crabs in shedding systems. Crabs used in threshold experiments were kept in individual tanks until they died. Crabs used in transmission experiments were kept individually in cages in scaled-down recirculating systems. The cages prevented physical contact among crabs. Real-time qPCR was used to detect CsRV1 in crabs prior to the start of all experiments (Time 0) and at death or end of the experiment (Time Final).
Wild caught crabs with natural infections of CsRV1 died on average 13.8 days with viral loading greater than 500,000 copies of CsRV1 per mg of tissue. Injected, juvenile crabs showed significant mortality (50%-66%) when injected with virus loads between 106-107 copies of CsRV1 per mg of tissue after 18 days. Injected, adult crabs showed significant mortality (40%-100%) when injected with virus loads between 105-107 copies of CsRV1 per mg of tissue after 25 days. The absence of physical contact among crabs in the transmission study indicated that the transmission of CsRV1 among crabs is waterborne. Virus transmission and mortality was reduced in lower salinities (1 and 5 ppt) compared to higher salinity (15 ppt).
Information gathered from these experiments is being used in extension work with soft shell crab producers in an effort to increase production efficiency. We encourage producers to reduce the amount of time they hold crabs in their systems prior to molting, staying as close as possible to one to three days. We encourage the use of lower salinity water in recirculating systems and the discard of dead crabs into land based facilities to reduce the spread of the virus in their systems.