Thermal extremes and variability shape the performance of rocky shore littorinids

Abstract

The success of a species in an environment is largely determined by its ability to perform essential functions for survival and reproduction. In ectotherms, many performances are temperature-dependent and so determining how these are shaped by the thermal environment is key to understanding how species can be successful in thermally challenging environments. Typically, as a result of evolving to contrasting climatic regimes, species from tropical and temperate environments have conflicting characteristics in their thermal performances (e.g. performance breadth, thermal sensitivity, plasticity), which are shaped by the consistent extreme temperatures in tropical species, and by seasonal variation in temperatures in temperate species.

Many tropical areas like Hong Kong, however, experience variable climates due to the seasonal monsoons, and so species have to endure not only severe heat in the tropical-like summer, but also thermal variability with changes to the cool, almost temperate-like winter as a result of shifts in the monsoon systems. To investigate traits exhibited by these species to survive these supposedly conflicting thermal regimes, this thesis examined the behaviours (on-shore behavioural patterns and laboratory assessments of locomotory performance), physiology (resting metabolism measured as oxygen consumption) and simulations of the energetics of three coexisting, congeneric littorinid species (Echinolittorina malaccana, E. radiata and E. vidua) on Hong Kong rocky shores. Specifically, the adaptations of these three species were examined in the lights of the interplay of environmental challenges at a broad (i.e. thermal extremes and seasonal variation of the climatic system) and local scale (i.e. the energy limited and thermally dynamic intertidal environment).

Despite living in a thermally stressful environment, the three species were largely buffered from reaching their lethal limits due to their robust physiology and effective thermoregulatory behaviours on-shore. In addition, both high shore species exhibited strategies to minimize energy expenditure when emersed and inactive, and E. malaccana, in particular, exhibited strategies to maximize energy gain within the limited activity window available to feed. These strategies are particularly important for survival in the energy limited, thermally stressful rocky shore environment, as they enable the species to meet the substantial energy costs incurred from frequent exposure to high temperatures without compromising their energy balance.

To compensate for temperature variation across seasons, all three species demonstrated plasticity in their thermal performances. The extent of plasticity differed between behavioural (locomotion) and physiological (resting metabolism) thermal performances, most likely as a result of the different thermal regimes that have selected for these performances. In addition, the three species exhibited clear behavioural shifts across seasons, where the increased risk of wave dislodgement in winter appeared to constrain movement, resulting in reduced activity and increased exhibition of stress behaviours that may compromise their energy budgets.

Using littorinids living on seasonal tropical shores as a model group, this thesis demonstrated how the thermal environment selects for species’ thermal performances and plasticity, the suite of which ultimately underpins the success of species to survive both the thermal extremes and seasonal variability in environmental conditions experienced in the monsoonal tropics.