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The causal factors for observed differences in tetrasporangial periodicity in high- and midlittoral plants in an estuarine population ofRhodochorton purpureum (Lightf.) Rosenv. were analysed. In high littoral plants tetrasporangia were absent from early March to early November when tetrasporogenesis was blocked by long days (>c. 12 h) and, from mid-May on also by high temperatures (>c. 16°C). Midlittoral plants, in particular those growing at shaded sites, continued to form tetrasporangia until the end of May. Here, at high tide, high water turbidity and heavy shading by brown algae reduced light to such an extent that it was below the threshold for light perception inR. purpureum for several hours. Since, on successive days, high water marks regularly fell c. 50 min later, very irregular light-dark regimes were the result. Days with two light and two dark periods (high tide at midday) are followed by days with one light and one, long, dark period (one high tide at dawn, the next at dusk). Experiments showed that these regimes are effectively short days forR. purpureum. In summer and autumn tetrasporogenesis in the midlittoral plants is blocked by high temperatures. Minimal daily temperature-values are often below the critical temperature, but this did not induce tetrasporangia. Probably average or maximal values block tetrasporogenesis and this was supported by experimental results. In one of the field samples a diminutive male gametophyte was found which had grown from a tetraspore that had germinated in situ, and this is the first report of a gametophyte ofR. purpureum from nature.

Light-dark regimes in the intertidal zone and tetrasporangial periodicity in the red algaRhodochorton purpureum