ENVIRONMENTAL-REGULATION OF DEVELOPMENT, LIFE-HISTORY AND BIOGEOGRAPHY OF HELMINTHORA-STACKHOUSEI (RHODOPHYTA) BY DAYLENGTH AND TEMPERATURE

The marine red alga Helminthora stackhousei (Clemente) Cremades et Per ez-Cirera [ = H. divaricata (C. Agardh) J. Agardh] from the west coast of Ireland has a heteromorphic life history in culture. Tetrasporangi a are formed on uniseriate, filamentous tetrasporophytes, mainly under short-day conditions, at (7-)9-17-degrees-C. Although photoperiodic i nduction occurs, night-breaks of 1 h in the middle of a 16-h night are ineffective in suppressing this response. Tetraspores form further un iseriate filaments which produce multiaxial gametophytic thalli, mainl y under long-day conditions at (7-)9-17-degrees-C. The critical daylen gth is not constant over this temperature range. Night-breaks promote the initiation of multiaxial thalli, and nine photoinductive long-day cycles induce a 50% response. At 16-degrees-C, equivalent photon expos ures at two irradiance levels promote development of multiaxial thalli in long-day regimes but continue to inhibit induction in short days. Tetrasporophytes and uniseriate gametophytes reproduce asexually by fr agmentation and monospores under all conditions. Multiaxial gametophyt ic thalli form monosporangia, gametangia, and develop carposporophytes regardless of photoperiod. Photoperiod and temperature as factors con trolling the developmental life history sequence are used to formulate models to explain the phenology of H. stackhousei. These factors are also considered in relation to the biogeography of the species in the eastern North Atlantic and Mediterranean.