Abstract

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. 18–20 °C) and a much better tolerance to low temperatures than the Indo-west Pacific isolates (0–5 vs. 16 °C). Large shifts towards low temperatures are accompanied by parallel but smaller shifts at high temperatures. Differences in upper survival temperatures amounted on average to 4 °C (32–33 °C vs. 34–37 °C) and growth ceased in the Atlantic isolates at 30 °C, whereas the Indo-west Pacific isolates still reached significant growth at 33 °C. Additionally, temperature requirements for reproduction were shifted towards lower temperatures in the Atlantic/Mediterranean isolates [18–20 and 25 °C vs. 28–30(33) °C]. The cold-adapted Atlantic/Mediterranean ecotype retained a strong tropical imprint with high temperature tolerance and high growth rates at temperatures > 25 °C. The northern distribution limit in the northeastern Atlantic Ocean at 39 °N (at the 13 °C February and 17 °C August isotherms) is probably set by a growth and/or reproduction boundary. The northern distribution limit in the Pacific Ocean at 26 °N (at the 21 °C February and 29 °C 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.