Geographic variation in thermal traits in Digenea simplex and Champia parvula (Rhodophyta) in relation to present and glacial temperature regimes

Geographic variation in temperature responses (survival and growth) was inv estigated in two red algae: Digenea simplex (Wulfen) C. Agardh and Champia parvula (C. Agardh) Harvey. D. simplex has a tropical to warm temperate dis tribution; C. parvula extends from the tropics into the cold temperate zone . Ecoclinal variation was found in both species but was much stronger in C. parvula than in D. simplex. The former species showed variation in upper a nd lower tolerance limits as well as in the upper and lower limits for grow th. The latter species showed variation mainly in its lower tolerance limit . Ecoclinal variation was related to the amount of present and glacial sele ction pressure along the climate gradient. In both species, isolates from t he colder localities had insufficient cold tolerance to have survived low g lacial winter temperatures, so these locations must have been colonized aft er the end of the glaciation. Eastern Mediterranean and Atlantic population s were probably isolated during the glaciation by a thermal barrier at the entrance of the Mediterranean. In C. parvula, evidence existed for a trade- off between the performance at high and at low temperatures, which would en hance selection pressure in opposite directions at either end of the climat ic range. No evidence for such a trade-off was found in D. simplex.