Description: In a warming climate it is important to know the upper thermal tolerance limits of organisms, especially in coastal regions where climate change is expected to lead to an increase in weather extremes such as heat waves. Physiological studies can help to predict the effects of global warming by determining if a species lives currently at their upper thermal tolerance limit. In this study, the intertidal sea anemone Haliplanella lineata from West Java, Indonesia, was examined for its upper thermal tolerance limit and its reactions to heat stress in the range close to the identified limit. The upper thermal tolerance limit, here defined as the maximum temperature at which more than 50 % of the test individuals survived the duration of the experiment of 42 days, was reached at 36° C. A surprisingly strong increase in mortality was observed when temperatures rose by just 1° C above 36° C. A gradual elimination with rising temperatures above 36° C was expected. This limit was observed in a set up in which single individuals were exposed to four different temperate regimes for 42 days. They exhibited an optimum performance at 34° C. Some response variables such as habitus, heat shock protein levels and asexual reproduction revealed a pattern of failing physiological functions at temperatures higher than 34° C. This sea anemone lives in the inte1iidal and is adapted to a wide range of daily fluctuating temperatures which result in a good performance at 34° C. The high mortality is in agreement with the limited acclimatory capacity, since they already live close to their upper thermal tolerance limit as an intertidal and tropical population. In the context of global warming, however, they are presumably not threatened since the expected increase in temperature and heat waves in Indonesia will not exceed the identified limit much and H line at a can survive temperatures above its limit for short amounts of time and recovers during exposure to colder temperature during high tide. This study was performed in the framework of the GAME programme hosted by GEOMAR, the Helmholtz Center for Ocean Research in Kiel, in collaboration with the Institut Pertanian Bogar in Bogor, Indonesia.