THERMAL ECOTYPES OF AMPHI-ATLANTIC ALGAE .1. ALGAE OF ARCTIC TO COLD-TEMPERATE DISTRIBUTION (CHAETOMORPHA-MELAGONIUM, DEVALERAEA-RAMENTACEA AND PHYCODRYS-RUBENS)

Three species of Arctic to cold-temperate amphi-Atlantic algae, all occurring also in the North Pacific, were tested for growth and/or survival at temperatures of −20 to 30°C. When isolates from both western and eastern Atlantic shores were tested side-by-side, it was found that thermal ecotypes may occur in such Arctic algae.Chaetomorpha melagonium was the most eurythermal of the 3 species. Isolates of this alga were alike in temperature tolerance and growth rate but Icelandic plants were more sensitive to the lethal temperature of 25°C than were more southerly isolates from both east and west. With regard toDevaleraea ramentacea, one Canadian isolate grew extraordinarily well at −2 and 0°C, and all tolerated temperatures 2–3°C higher than the lethal limit (18–20°C) of isolates from Europe. ConcerningPhycodrys rubens, both eastern and western isolates died at 20°C but European plants tolerated the lethal high temperature longer, were more sensitive to freezing, and attained more rapid growth at optimal temperatures. The intertidal species,C. melagonium andD. ramentacea, both survived freezing at −5 and −20°C, at least for short time periods.C. melagonium was more susceptible thanD. ramentacea to desiccation. Patterns of thermal tolerance may provide insight into the evolutionary history of seaweed species.