Abstract
SUMMARY The life history of the brown alga Chnoospora implexa J. Agardh (Chnoosporaceae, Scytosiphonales) from Japan was studied in laboratory cultures. This species showed a heteromorphic and diphasic life history, alternating between erect gametophytes and discoid sporophytes. The gametophytes were dioecious and produced isogametes. The zygotes developed into sporophytes at 20°C under long-day conditions, which formed plurilocular zoidangia. Zoids released from the plurilocular zoidangia developed again into sporophytes that always formed plurilocular zoidangia at 20°C and 25°C in long-day conditions, and mainly unilocular zoidangia at 25°C in short-day conditions. Zoids released from unilocular zoidangia developed into dioecious gametophytes. At 15°C zygotic erect thalli were formed and were revealed to be diploid by microspectrofluorometric measurements of nuclear DNA contents. The development and reproduction of unfused gametes were similar to those of zygotes. Some strains showed a direct-type life history; gametophytic thalli were produced, but not via a sporophytic phase.
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