Ecology and distribution of the subtidal red alga Acrosymphyton purpuriferum (J. AG.) Sjöst. (Rhodophyceae, Cryptonemiales)

Abstract

The causal factors for the spatial and temporal distribution of the subtidal red alga Acrosymphyton purpuriferum (J. Ag.) Sjöst. (Rhodophyceae, Cryptonemiales) have been analysed through a combination of climatological data and cultures studies. A. pupuriferum occurs in the warm temperate Medite and 30 m. The erect branched gametophyte is a summer annual, the crustose tetrasporophyte is unknown in the field. Culture experiments have shown that temperature is a critical factor for maturation of the gametophyte and thet temperature, irradiance and daylength are critical factors for maturation of the tetrasporophyte. These responses to environmental factors could be correlated with the seasonal periodicity in the gametophyte as observed in the field. Young gametophytes are found in spring, because tetraspores are formed only under winter and early spring conditions. Mature gametophytes are found at the end of summer because gametangia are formed only under summer conditions. Gametophytes disappear in winter, probably because vegetative growth stops once maturation has set in. The culture experiments suggest that the tetrasporophytes are perennial. Temperature bounds delimit A. pupuriferum's geographical distribution. The species occurs between the 17°C summer isotherm and the 18°C winter isotherm, the region where its life history can be completed in an “average” year. An approximately uniform reproduction rate of the tetrasporophyte throughout the species' depth range has been postulated on the basis of the observed response to irradiance and daylength. The reproductive rate remains similar when both daylength and irradiance decrease, and this is exactly how these factors change with increasing depth.