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Mid-tidal levels on Hong Kong's exposed shores are dominated by sessile barnacles (Tetraclita squamosa) and mussels (Septifer virgatus). Such shores experience dramatic elevations in air and rock temperatures in mid summer that have been causally linked to mortalities of these species. Tetraclita squamosa and S. virgatus had lower body temperatures relative to their rock substratum. Mussel inhabitants of high- and low-zoned pools were in temperature equilibrium with contained water. On the rocks, body temperatures of T. squamosa were higher than those of S. virgatus. Middle shore individuals had higher body temperatures than lower shore ones. Individuals in the sun had higher body temperatures than those in the shade. Factors possibly affecting body temperature are orientation to the incident sunlight, height on the shore and the contact area between the individual and the substratum.

Mean heat coma temperature (HCT) values for submerged Septifer virgatus were 45.0d̀C and 44.5d̀C for individuals of between 10-20mm and 30-50mm shell length, respectively. Mean median lethal temperature (LT50) values for submerged S. virgatus were the same for both size classes (47.0d̀C). A longer exposure in air resulted in a lower LT50 value; a shorter exposure in air resulted in a higher LT50 value.

Water content of Septifer virgatus was between 89.21–90.26%, i.e. including the water from the mantle cavity. In air temperatures of between 30–32d̀C and after long periods of exposure, Septifer virgatus showed significant water loss, i.e. of between 13.87-15.52% body water content. In air temperatures of between 33–36d̀C and after relatively short exposures, Septifer virgatus showed small water losses, i.e. between 1.92-5.74% body water content.

On Hong Kong's exposed rocky shores, sessile components of the middle shore environment are thermally stressed in summer when low tides occur in the afternoon in combination with periods of cloudless sunny days and when rock temperatures may climb to ≅ 50d̀C through solar insolation. Lower-zoned individuals and high-zoned pool residents are buffered from the adverse effects of such environmental extremes. Mortality occurred mainly in the sun-exposed shore populations of both Tetraclita squamosa and Septifer virgatus and is particularly evident when the above combination of climatic and tidal factors first occurs in July.