Rising global ocean temperatures are influencing the incidences of extreme thermal events – including marine heatwaves (MHWs) – which, in turn, are affecting marine organisms physiologically. The consequence of acute stress depends, in part, on the temporal pattern of exposure. Such influence results as the molecular mechanisms triggered by stressful conditions are energetically costly, and therefore their activation can help or hinder an organism faced with additional stress depending on relative timing of exposure. Here we test the effect of consecutive MHWs separated by varying lengths of recovery on the shallow subtidal gastropod species, Monodonta labio. We exposed M. labio to two separate heatwave events (32 ºC) separated by either a 5-day, 10-day or 15-day recovery period (28 ºC) and assessed the effects on mortality, behaviour (attempt to right, success righting), and physiological performance (oxygen consumption rates) after each time period. While some mortality was experienced in the first heatwave event, it was significantly greater in the second event with a trend toward more mortality in the longer than shorter recovery period treatments. Over the duration of the experiment, longer recovery periods were associated with greater overall cumulative mortality. In terms of behaviour, although there was no change in attempt to right across the experimental periods, the proportion to successfully complete the behaviour decreased following the second heatwave. Interestingly, during the second heatwave no gastropods in the 5-day recovery treatment successfully righted, with some success observed for those with a longer recovery period. Despite these changes in mortality and behaviour, oxygen consumption rates remained consistent across treatments and the experimental period, indicating organisms did not adjust this process. Together, our results highlight that where a MHW event has sublethal effects, this could modify the response organisms show to a subsequent MHW. Moreover, the influence of this earlier MHW will be dependent upon the duration of the intervening recovery period afforded to organisms. Consequently, it will be important to consider repeat events, and the duration of recovery between them, when considering the effects of altered heatwave patterns driven by global climate change.

This study was supported by CUHK Direct Grant for Research (project 4053371)