Thermal conditions of overwintering for tropical green mussel Perna viridis in Uranouchi Inlet, Tosa Bay, Shikoku, Japan in 2010

Abstract

The green mussel Perna viridis, a sessile bivalve of tropical origin, was first introduced into Japanese waters in 1967. This mussel was present in Uranouchi Inlet on the south coast of Shikoku by the 2000s and was found to be widely distributed there in 2008. The survival rates of overwintering P. viridis and the corresponding water temperature regimes, were monitored at three stations in the inlet during the 2009–2010 winter season. At each station, 100 mussels of different size classes were initially deployed in suspended net boxes, and a water temperature data-logger was employed. The winter survival rates were 98% at St. 1 near the mouth of inlet and 95% at St. 2 inwards from the mouth, but only 1% at St. 3 at the head of the inlet. The lowest monthly mean water temperatures were 15.3°C at St. 1 (February), 14.1°C at St. 2 (January), and 10.2°C at St. 3 (January). Dividing each month into thirds, the lowest mean temperatures for these shorter intervals were 14.4°C at St. 1 (early February), 13.4°C at St. 2 (early February), and 9.8°C at St. 3 (middle January). At Sts. 1 and 2, water temperatures below 12°C occurred only once, ephemerally, during this study, while at St. 3, the water temperature dropped below this level frequently, for maximum continuous periods of 1017.5 hours below 12°C, 428.5 hours below 11°C, and 90.3 hours below 10°C. The overwintering capacity of P. viridis thus seems to be limited by the degree and duration of colder environmental temperatures.