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 Perez-Cirera [ = H. divaricata (C. Agardh) J. Agardh] from the west coast of Ireland has a heteromorphic life history in culture. Tetrasporangia are formed on uniseriate, filamentous tetrasporophytes, mainly under short-day conditions, at (7-)9-17-degrees-C. Although photoperiodic induction occurs, night-breaks of 1 h in the middle of a 16-h night are ineffective in suppressing this response. Tetraspores form further uniseriate filaments which produce multiaxial gametophytic thalli, mainly under long-day conditions at (7-)9-17-degrees-C. The critical daylength 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 exposures 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 fragmentation and monospores under all conditions. Multiaxial gametophytic thalli form monosporangia, gametangia, and develop carposporophytes regardless of photoperiod. Photoperiod and temperature as factors controlling 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.