Thermal tolerance of Echinolittorina species in Hong Kong: implications for their vertical distributions

Abstract

Intertidal rocky shores represent an extremely stressful physical environment dictated by the rise and fall of the tides. One of the major environmental stresses over this gradient is temperature, especially towards the upper reaches of the shore where species spend long periods out of water exposed to hot, desiccating conditions. As a result, the thermal tolerance of intertidal species is often positively correlated with their vertical distributions, and the physiological and molecular limits that drive such patterns have been the subject of recent research. Understanding these tolerance limits, from small (e.g. vertical distribution) to large (e.g. latitudinal) spatial scales, may provide information to predict species’ success under future climate change scenarios, and thus possible changes in community structure.

Given their abundance in the high shore, and well resolved taxonomy and phylogeography, the littorinids Echinolittorina malaccana, E. radiata and E. vidua are excellent models to investigate the relationship between thermal tolerance and spatial distribution patterns. These littorinids are widely distributed on Hong Kong shores and exhibit a distinct and consistent vertical distribution that ranges from temperate to tropical regions along the western Pacific coast. Field surveys in summer and winter at two moderately exposed shores (Stanley and South Bay, Hong Kong) showed that E. malaccana was distributed highest on the shore, followed by E. radiata and E. vidua respectively, and all the three species were found ~ 0.25m lower on the shore in summer than winter. Laboratory experiments, including determination of survival limits (LT50), Arrhenius breakpoint temperature of heart rate (ABT of HR) and activities of metabolic enzymes (MDH and LDH), were used to establish if the physiological attributes of the three species were related to their distribution patterns. The LT50 of E. malaccana were the highest of the three species (56.47oC), followed by E. radiata (55.5oC), and finally the lower shore species E. vidua (53.7oC); while ABT of HR in E. malaccana (48.2oC) was also higher than E. radiata (46.5oC) and E. vidua (46.6oC). The enzyme activities did not show any clear patterns. In terms of seasonal variation, LT50 and ABT of HR of all three Echinolittorina species were higher in summer than winter, which showed the potential for the littorinids to acclimate when environmental conditions become more severe.

The present study provided a fundamental understanding of how physiological, temperature tolerance may determine the spatial and temporal distribution patterns of Echinolittorina species at a local scale where strong environmental gradients vary vertically and also between seasons. Information on the tolerance limits of physiological traits such as LT50, heart rates and enzyme functioning may direct further investigations to identify the underlying causes of the survival limits of these species to temperature variation, and whether this tolerance is genetically or environmentally determined, for example through acclimation. Such studies will provide insights into how a species' physiology may limit their present-day distributions at multiple scales from local to biogeographical, but also enable predictions of how species may respond to changing temperature regimes.