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.

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Intraspecific morphological variation of Undaria pinnatifida sporophytes was investigated using two local strains from Taro and Okirai in the central and southern coastal areas, respectively, of Iwate Prefecture, northeastern Japan. Sporophytes of the two test strains were cultured at a commercial aquaculture site in Okirai Bay from December, 2006, to April, 2007. Thallus size as expressed by six parameters (blade length, stipe length, blade wet weight, stipe wet weight, blade width, and undivided blade width) differed between the strains under the same cultivation conditions and thallus density. Blade thickness and three morphological correlations (those between blade length and thallus length, blade wet weight and thallus wet weight, and undivided blade width and blade width) also differed between the strains. Cultured sporophytes of the Taro strain produced a longer and thicker blade than those of the Okirai strain whereas the latter produced a wider and more deeply incised blade, and a longer and heavier stipe than the former. These results show that variation in the morphology of local U. pinnatifida strains from Iwate Prefecture has a genetic basis.

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