Daylength, temperature and nutrient control of tetrasporogenesis in Asparagopsis armata (Rhodophyta)

The formation of tetrasporangia in Falkenbergia-phase tetrasporophytes of Asparagopsis armata Harvey (Rhodophyta : Bonnemaisoniales) isolated from introduced populations in Ireland and Italy, and from native populations in Victoria, Australia, was examined in temperature-controlled water-baths at 2°C intervals from 11–23°C and at 1- or 0.5-h daylength intervals from 8–12 h. At a daylength of 8 h, Irish plants formed tetrasporangia only at (15-)17(−21)°C, Italian plants at 17–21dgC and Australian plants at 13–17°C. However, after an incubation period of 5 wk, only the Italian consistently showed 100% reproduction under these conditions. At 17°C, the critical daylength for tetrasporogensis is 8–9 h for the Irish and Australian strains and 9–10 h for the Italian strain. These results indicate that all three isolates differ in their response to environmental variables. It is likely that the Mediterranean and Irish populations thus represent separate introductions into the Northern Hemisphere and that Australia was not the source of these plants. The abundance of gametophytes of this species at Messina, Sicily, in April and May is explained by a reproductive “window” of daylength and temperature for tetrasporogenesis from mid-November to late January. In Victoria, Australia, temperatures are inductive all year but daylength at the water surface is never