No whirlwind romance: typhoons, temperature and the failure of reproduction in Caulacanthus okamurae (Gigartinales, Rhodophyta)

The red alga Caulacanthus okamurae Yamada occurs commonly on the coasts of South Korea, but reproductive plants have rarely been found in the field. S easonality of field populations was examined and the temperature responses of growth and reproduction were determined in laboratory cultures. In the f ield, monthly average temperatures of surface seawater ranged from 12.7 deg reesC +/- 0.73 (SD) to 21 degreesC +/- 1.19. Monthly mean biomass in 25 x 2 5 cm quadrats (n = 3) ranged from 11.2 +/- 9.6 g m(-2) (SD) to 148.8 +/- 19 .2 g m(-2) in wet weight with minimal biomass in February 1995 and maximal values in August, In culture, the growth of vegetative plants and germinati on of tetraspores of C. okamurae occurred at temperatures from 13 to 27 deg reesC. Tetrasporangia, gametangia and cystocarps were formed at 19 degreesC and above, but not at 13 or 16 degreesC, indicating that reproduction is d irectly affected by seawater temperature. On the basis of these data, seawa ter temperature was above the critical temperature for reproduction from Au gust to October (20-21 degreesC) yet only three plants with tetrasporangial branches were found in the field. In September the biomass of C. okamurae declined sharply following severe storm damage. Thus, the absence of reprod uctive organs in field populations of C. okamurae may occur because they in habit a seasonally stressful habitat, in which typhoon damage removes branc hes before the water temperature becomes favourable for reproduction.