Temperature Responses and Evolution of Thermal Traits in Cladophoropsis Membranacea (Siphonocladales, Chlorophyta)

Temperature tolerances and relative growth rates were determined for different isolates of the tropical to warm temperate seaweed species Cladophoropsis membranacea (C. Agardh) Boergesen (Siphonocladales, Chlorophyta) and some related taxa. Most isolates of C. membranacea survived undamaged at 18 degrees C for at least 8 weeks. Lower temperatures (5 degrees-15 degrees C) were tolerated for shorter periods of time but caused damage to cells. All isolates survived temperatures up to 34 degrees C, whereas isolates from the eastern Mediterranean and Red Sea survived higher temperatures up to 36 degrees C. Growth occurred between 18 degrees and 32 degrees C, but an isolate from the Red Sea had an extended growth range, reaching its maximum at 35 degrees C. Struvea anastomosans (Harvey) Piccone & Grunow, Cladophoropsis sundanensis Reinbold, and an isolate of c. membranacea from Hawaii were slightly less cold- tolerant, with damage occurring at 18 degrees C. Upper survival temperatures were between 32 degrees and 36 degrees C in these taxa. Temperature response data were mapped onto a phylogenetic tree. Tolerance for low temperatures appears to be a derived character state that supports the hypothesis that C. membranacea originated from a strictly tropical ancestor. Isolates from the Canary Islands, which is near the northern limit of distribution, are ill adapted to local temperature regimes. Isolates from the eastern Mediterranean and Red Sea show some adaptation to local temperature stress. They are isolated from those in the eastern Atlantic by a thermal barrier at the entrance of the Mediterranean.