Ecotypic differentiation in thermal traits in the tropical to warm-temperate green macrophyte Valonia utricularis

Differentiation of thermal traits (i.e. growth, survival and reproduction) was studied in the green macrophyte Valonia utricularis (Roth) C. Agardh, which has a world-wide tropical to warm-temperate distribution. Ecotypic differentiation between northeast Atlantic/Mediterranean and Indo-west Pacific isolates occurred with respect to all investigated temperature characteristics. The Atlantic/Mediterranean group is more eurythermal and cold-tolerant compared to the Indo-west Pacific group, which is stenothermal and cold-sensitive. Isolates of Atlantic/Mediterranean origin show clearly higher growth rates at low temperatures (lower temperature limit: 5-8 vs. 18-20degreesC) and a much better tolerance to low temperatures than the Indo-west Pacific isolates (0-5 vs. 16degreesC). Large shifts towards low temperatures are accompanied by parallel but smaller shifts at high temperatures. Differences in upper survival temperatures amounted on average to 4degreesC (32-33degreesC vs. 34-37degreesC) and growth ceased in the Atlantic isolates at 30degreesC, whereas the Indo-west Pacific isolates still reached significant growth at 33degreesC. Additionally, temperature requirements for reproduction were shifted towards lower temperatures in the Atlantic/Mediterranean isolates [18-20 and 25degreesC vs. 28-30(33)degreesC]. The, cold-adapted Atlantic/Mediterranean ecotype retained a strong tropical imprint with high temperature tolerance and high growth rates at temperatures > 25degreesC. The northern distribution limit in the northeastern Atlantic Ocean at 39degreesN (at the 13degreesC February and 17degreesC August isotherms) is probably set by a growth and/or reproduction boundary. The northern distribution limit in the Pacific Ocean at 26degreesN (at the 21degreesC February and 29degreesC August isotherms) is probably set by low lethal winter temperatures. The different latitudes of these boundaries must be attributed to the occurrence of more cold-adapted populations in the northeastern Atlantic Ocean compared to the northwestern Pacific Ocean. The development of cold-adaptation in the northeastern Atlantic Ocean and its absence in the northwestern Pacific Ocean has been related to different impacts of Pleistocene glaciations.