Upper Temperature Limits of Tropical Marine Ectotherms: Global Warming Implications

Animal physiology, ecology and evolution are affected by temperature and it is expected that community structure will be strongly influenced by global warming. This is particularly relevant in the tropics, where organisms are already living close to their upper temperature limits and hence are highly vulnerable to rising temperature. Here we present data on upper temperature limits of 34 tropical marine ectotherm species from seven phyla living in intertidal and subtidal habitats. Short term thermal tolerances and vertical distributions were correlated, i.e., upper shore animals have higher thermal tolerance than lower shore and subtidal animals; however, animals, despite their respective tidal height, were susceptible to the same temperature in the long term. When temperatures were raised by 1 degrees C hour(-1), the upper lethal temperature range of intertidal ectotherms was 41-52 degrees C, but this range was narrower and reduced to 37-41 degrees C in subtidal animals. The rate of temperature change, however, affected intertidal and subtidal animals differently. In chronic heating experiments when temperature was raised weekly or monthly instead of every hour, upper temperature limits of subtidal species decreased from 40 degrees C to 35.4 degrees C, while the decrease was more than 10 degrees C in high shore organisms. Hence in the long term, activity and survival of tropical marine organisms could be compromised just 2-3 degrees C above present seawater temperatures. Differences between animals from environments that experience different levels of temperature variability suggest that the physiological mechanisms underlying thermal sensitivity may vary at different rates of warming.