Abstract
Green mussel Perna viridis Linnaeus, 1758 is a tropical originated sessile bivalve. It had been introduced to Japanese waters in no later than 1967. The first occurrence of this mussel in Tokyo Bay was reported in 1985. It had spread quickly since then and is distributed widely now. The thermal condition for the overwintering of this mussel was investigated at the artificial seashore facilities in Yokohama Port of Tokyo Bay from 2010 to 2011. The survival rates of the mussel during the winter were 0.116 in 2010 and 0.387 in 2011 respectively. The lowest monthly mean water temperature of each year were 12.1°C (February, 2010) and 11.7°C (February, 2011), and the lowest mean temperature of the trisected period of the month were calculated as 11.4°C (Middle January, 2010) and as 11.3°C (Middle January, 2011). The durations of the lower temperature were estimated from recording times of each lower temperature level. It was suggested that the amount of durations of under 12℃ , 11℃ and 10℃ temperature levels were shorter than those of the extinction accrued year. The warm waters which are originated from Kuroshio current or freshwaters are considered to have influences for the overwintering of this mussel.
