Summary 1.Traditional approaches to understanding species responses to environmental conditions have focused on the isolated effects of single stressors, despite the fact that in nature organisms experience a variety of conditions. 2. In tropical monsoonal areas, intertidal animals can face hot desiccating conditions during emersion preceded, or followed by, intense rainfall. The combined effects of these stresses on physiological responses and protein profiles were investigated in a limpet, Cellana grata. 3. With short exposure (60 min) to single stressors, heat stressed limpets had elevated heart rates and more concentrated haemolymph and mantle water osmolalities than under normal temperatures or awash. Animals under rain had reduced haemolymph and mantle water osmolalities, but similar heart rates to unstressed animals. 4. After 120 min, unstressed animals did not differ in their physiological responses. Heat stressed limpets, however, had faster heart rates and more concentrated haemolymph and mantle water osmolalities, whilst those under rain had the lowest osmolalities, but similar heart rates to unstressed animals. 5. Limpets under rain followed by heat stress had faster heart rates, but lower haemolymph and mantle water osmolalities compared to animals under normal temperatures or heat stress. Limpets that were heat stressed, followed by rain, had similar heart rates to animals awash, under rain or normal temperatures but lower haemolymph osmolalities than other treatments, with the exception of limpets under rain. 6. There was a positive relationship between haemolymph and mantle water osmolalities, except for animals under rain, where mantle water osmolality was lower than the haemolymph, suggesting some isolation of body fluids from the external medium. 7. Haemolymph protein ⁄ peptide mass spectra of heat stressed animals (either before or after rain) were similar, while all other treatments differed, suggesting differential expression and regulation of proteins. 8. Heat stress invokes a more active physiological and protein level response than rain, but their combination had an interactive effect on limpets' metabolism. 9. Identifying the effects of multiple stresses at a variety of biological levels highlights the interactive effects which impact species, and provides a more complete understanding of how species may respond to environmental changes in their natural habitats.

Mobile species may actively seek refuge from stressful conditions in biogenic habitats on rocky shores. In Hong Kong, the upper intertidal zone is extremely stressful, especially in summer when organisms are emersed for long periods in hot desiccating conditions. As a result, many species migrate downshore between winter and summer to reduce these stressful conditions. The littorinids Echinolittorina malaccana and E. vidua, for example, are found on open rock surfaces high on the shore in winter but the majority migrate downshore in summer to the same tidal height as a common barnacle, Tetraclita japonica. In the laboratory, where environmental conditions could be controlled to approximate those occurring on the shore, we tested whether the downshore migration allowed littorinids to select barnacles as biogenic habitats to reduce stress and if this behaviour varied between seasons. In summer, littorinids demonstrated a strong active preference for the barnacles, which was not observed in the cool winter conditions, when animals were found on open rock surfaces even when barnacles were present. Littorinids, therefore, only actively select biogenic habitats during the summer in Hong Kong when they migrate downshore, suggesting that such habitats may play an important, temporal, role in mitigating environmental stress on tropical shores.

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Populations at the edge of their species' distribution ranges are typically living at the physiological extreme of the environmental conditions they can tolerate. As a species' response to global change is likely to be largely determined by its physiological performance, subsequent changes in environmental conditions can profoundly influence populations at range edges, resulting in range extensions or retractions. To understand the differential physiological performance among populations at their distribution range edge and center, we measured levels of mRNA for heat shock protein 70 (hsp70) as an indicator of temperature sensitivity in two high-shore littorinid snails, Echinolittorina malaccana and E. radiata, between 1°N to 36°N along the NW Pacific coast. These Echinolittorina snails are extremely heat-tolerant and frequently experience environmental temperatures in excess of 55 °C when emersed. It was assumed that animals exhibiting high temperature sensitivity will synthesize higher levels of mRNA, which will thus lead to higher energetic costs for thermal defense. Populations showed significant geographic variation in temperature sensitivity along their range. Snails at the northern range edge of E. malaccana and southern range edge of E. radiata exhibited higher levels of hsp70 expression than individuals collected from populations at the center of their respective ranges. The high levels of hsp70 mRNA in populations at the edge of a species' distribution range may serve as an adaptive response to locally stressful thermal environments, suggesting populations at the edge of their distribution range are potentially more sensitive to future global warming.

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