The temperature requirements for growth and survival of cold water seaweeds from both Hemispheres are compared and discussed in relation to the climatic history of the various regions and in relation to the origin of amphiequatorial distribution patterns. Endemic Antarctic species are most strongly adapted to low temperatures. In contrast, endemic Arctic macroalgae show higher temperature demands and correspond in their temperature responses to many Antarctic cold-temperate species. Arctic cold-temperate species show similar temperature requirements to cold-temperate species from southernmost South America. The temperature requirements of cold-temperate N. Atlantic species are somewhat higher than those of cold-temperate N.E. Pacific species. These differences are the result of the different times of exposure of these groups to low temperatures. The first steps in the adaptation of macroalgae to low temperatures are an increase in cold tolerance and an increase of growth and reproduction rates at low temperatures. Later, the ability to grow and reproduce at greater than or equal to 15 to 20° C and to survive temperatures greater than or equal to 20° C is lost. This temperature response type is exemplified in endemic Arctic and Arctic cold-temperate seaweeds exposed to low temperatures since about 3 My. The last steps in the adaptation to low temperatures include the loss of ability to grow and reproduce at greater than or equal to 5 or 10° C and a strong reduction in the upper survival temperatures (UST) down to 10-13° C. This temperature response type is typical for endemic Antarctic species exposed to cold waters for at least 14 My. Amphiequatorial filamentous green and brown algal taxa and microthalli of amphiequatorial brown algae mostly show UST's of 23 to 28.5° C, significantly higher compared to single Hemisphere taxa from the same regions. These findings strongly favour a migrationist jump across the equator to the other Hemisphere during Pleistocene lowering of the water temperatures in the tropics. Reproduction and growth during the passage across the equator would not have been possible in all species except Ectocarpus siliculosus due to the narrow temperature-reproduction and temperature-growth windows.
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