Effects of temperature and salinity on growth, reproduction and survival in the introduced red alga Heterosiphonia japonica (Ceramiales, Rhodophyta)

We determined temperature and salinity responses in culture of a Norwegian isolate of the introduced red alga Heterosiphonia japonica. The species was eurythermal, surviving temperatures between 0degreesC and 30degreesC, with adult plants showing slightly higher temperature tolerance than carposporelings. Optimal growth conditions for carposporelings were 19-25degreesC and 30 psu. Growth rates at 20 psu and 30 psu did not differ significantly, but a marked reduction in the development of tetrasporangial stichidia was observed at 20 psu. At 15 psu, growth was poor and no stichidia were produced. Ten psu was lethal. Vegetative reproduction by fragmentation was common. Sporelings survived at least 40 days in darkness under various temperature conditions. The wide tolerance limits to temperature and salinity, high specific growth rates and effective propagation by fragmentation suggest that H. japonica will continue to disperse northwards and eastwards along the Norwegian coast, but that spread into the Kattegat and Baltic Sea will be controlled by low salinities and freezing winter temperatures. The life history of the Norwegian isolate was completed in culture, and a Polysiphonia type of life history was observed at 12degreesC and 17degreesC, whereas only tetrasporangial and vegetative specimens have been observed in field collections.