Temperature and daylength responses were determined in culture for isolates of the red alga Cystoclonium purpureum (Hudson) Batters from Nova Scotia (NS, Canada), Helgoland (HE, Germany), and Roscoff (RO, France). Most isolates survived temperatures of -1.5% -2° to 23° C, whereas 25° C was lethal. Only the RO-gametophytes died at 23°C. Optimal growth conditions were 10°-20°C in both long and short days for the NS isolates and 8°-15° C and 8°-18%degC at daylengths of > 12 h for the RO and HE isolates, respectively. Tetrasporophytes and gametophytes of the NS isolate reproduced at 10°-20°C in long and short days within 5 months. At lower temperatures reproduction was limited or slow. The European isolates formed tetrasporangia at 10°-20°C (HE) or 5°-18°C (RO), spermatangia at 5°-15%degC (HE) or 5°-20°C (RO), and carpospores at 5°-15°C (HE) or 10°-15°C (RO). Short days either blocked or delayed reproduction of the European isolates. The phenology of C. purpureum was studied at Helgoland and Roscoff, where similar seasonal patterns were observed. In early spring, growth was rapid and plants started to from reproductive structures. In summer, tetra- and carpospores were shed followed by degeneration of the upright axes while branched holdfasts persisted. New upright axes and juvenile plants were formed in autumn, but these remained small during the winter months. Published data indicate that the seasonal pattern at Nova Scotia is similar, although the onset of growth and reproduction is delayed until the end of spring. These observation correspond well with the results of the experiments. The life history of C. purpureum is regulated by temperature and daylength. In the eastern Atlantic, the limiting effects of short days confines growth and reproduction to spring and summer. In the western Atlantic, low winter temperatures alone bring about the same seasonal pattern. After plants have reproduced, uprights degenerate in spite of continuing favorable conditions. [ABSTRACT FROM AUTHOR]