THERMAL ECOTYPES OF AMPHI-ATLANTIC ALGAE .2. COLD-TEMPERATE SPECIES (FURCELLARIA-LUMBRICALIS AND POLYIDES-ROTUNDUS)

Two species of cold-temperate algae from the North Atlantic Ocean, Polyides rotundus and Furcellaria lumbricalis, were tested for growth and survival over a temperature range of -5 to 30-degrees-C. In comparisons of eastern and western isolates, both F. lumbricalis, a North Atlantic endemic, and P. rotundus, a species having related populations in the North Pacific, were quite homogeneous. F. lumbricalis tolerated -5 to 25-degrees-C and grew well from 0 to 25-degrees-C, with optimal growth at 10-15-degrees-C. P. rotundus tolerated -5 to 27-degrees-C, grew well from 5 to 25-degrees-C, and had a broad optimal range of 10-25-degrees-C. Both species tolerated 3 months in darkness at 0-degrees-C. In neither case could any geographic boundary be explained in terms of lethal seasonal temperatures, suggesting that these species are restricted in distribution by strict thermal and/or daylength requirements for reproduction. The hypothesis that northern species are more homogeneous than southern taxa in terms of thermal tolerance was supported. A second hypothesis, that disjunct cold-temperate species should be more variable than pan-Arctic species, was not supported.