Mortality and impact on life activity of Akoya pearl oysters caused by red tides of dinoflagellate Karenia mikimotoi

Recently, red tides caused by the dinoflagellate Karenia mikimotoi in pearl-culturing areas such as Ago Bay, Mie Prefecture have raised concerns about their effects on the Akoya pearl oyster Pinctada fucata. We assessed the mortality of juvenile oysters in red tide seawater and investigated the effect of the dinoflagellates on valve movement of mature oysters. Of juvenile oysters, 7.5% died after 36 h in seawater with a K. mikimotoi density of 1x10(4) cells/mL and all oysters died after 24 h with 6x10(4) cells/mL. Frequency of valve movement of mature oysters rose with increase of cell density, and became significantly higher than that of the control at 3x10(3) cells/mL.