White spot syndrome virus quantification in blue crab Portunus trituberculatus hatchery-produced larvae and wild populations by TaqMan real-time PCR, with an emphasis on the relationship between viral infection and crab health

The blue crab, Portunus trituberculatus, is one of the most important fishery resources in the Yellow Sea of Korea, but the wild stock of this species has been reduced due to over-fishing and the destruction of the natural habitat in this area. Hatchery-produced seeds of blue crab have been released into the sea to enhance stock, but information on viral infection in the larvae as well as wild crabs of this species is very limited. In the present study, TaqMan real-time PCR was applied to quantify white spot syndrome virus (WSSV) in hatchery-produced larvae and wild populations of P. trituberculatus in South Korea. Out of 140 P. trituberculatus zoea from seven commercial hatcheries, 96.4% were WSSV-positive. The mean WSSV copies were 6.0 per ng DNA, or 3216.0 per larva. In 222 adult crabs from four wild populations captured in different seasons, the WSSV-positive rate was 79.3%, and the WSSV load was 5.2 copies per ng DNA, or 2116.5 copies per mg tissue. Both the WSSV-positive rate and the load of the winter population were significantly lower than those of the other three populations. Statistical analysis showed no significant correlations between WSSV infection loads and growth (CL, CW, and BW). The results suggest that low viral load of WSSV may not affect the growth of P. trituberculatus in a wild environment.