Temperature responses of tropical to warm-temperate Atlantic seaweeds .2. Evidence for ecotypic differentiation in amphi-Atlantic tropical-Mediterranean species

Nineteen isolates of four amphi-Atlantic seaweed species with a tropical to warm-temperate/Mediterranean distribution (Microdictyon boergesenii, M. tenuis, Wurdenmannia miniata and Valonia utricularis) and one eastern Atlantic endemic (M. calodictyon) have been investigated for their temperature responses (tolerance limits and growth response curves). In all species, isolates survived undamaged at 30 degrees C but died at 33 or 35 degrees C. Growth occurred up to 30 degrees C in most isolates but no growth was observed at higher (33 degrees C) temperatures. Tolerance to low temperatures showed marked differences between and within species. In tropical isolates, damage generally occurred between 15 and 18 degrees C and plants grew at temperatures down to 18-20 degrees C. Isolates from warm-temperate localities (Canary Islands and Mediterranean) tolerated much lower temperatures (0-12 degrees C), and showed substantial growth at temperatures as low as 15-18 degrees C, which indicates ecotypic differentiation. It is hypothesized that these cold-adapted populations are descendants of populations which were isolated in the eastern Mediterranean during the last glacial episode and recolonized the western Mediterranean and Canary Islands at the onset of the interglacial.