Predicting heat stress on seasonal tropical shores: why are “suboptimal” temperatures preferred by an intertidal snail?

This blog post is provided by Tommy Hui and tells the #StoryBehindThePaper for the paper “Why are “suboptimal” temperatures preferred in a tropical intertidal ectotherm?”, which was recently published in the Journal of Animal Ecology.
The authors are all part of the Tropical Intertidal Ecology group (tidehku.com) at the Swire Institute of Marine Science (SWIMS), The University of Hong Kong. Tommy Hui and Sam Crickenberger were Postdoctoral fellows during this work and whilst Sam has moved to take up teaching elsewhere, Tommy continues his research at SWIMS on the thermal ecology of intertidal animals. Jackson Lau was an undergraduate summer research fellow during the study and is now a postgraduate student in SWIMS. Gray A. Williams is the Director of SWIMS and a Professor in the School of Biological Sciences, HKU and has been working on Hong Kong shores for the past 30 years. This work was supported by General Research Fund 17138916 awarded to GAW from the Research Grants Council of Hong Kong.

Heatwaves have become global news headlines in recent years, highlighting the severe ecological and economic impacts of warming climates in both terrestrial and marine ecosystems. Our seashores are no exception, in these dynamic, transitional habitats, mass mortalities of oysters/mussels due to extreme heat events have recently been reported around the globe. Such extreme heat events are predicted to be more frequent and intense under future climate warming, which has already led to serious loss of both ecosystem functions and for aquaculture production. 

In Hong Kong, however, animals on rocky shores regularly experience such “die-offs”. Due to its location just ~ 120 km south of the Tropic of Cancer, Hong Kong experiences a “temperate” winter (cool and dry season) and a “tropical” summer (hot and wet season) driven by changes in the dominant monsoon systems. Unlike terrestrial habitats with vegetation cover, or marine habitats with the ocean as the giant thermal buffer, rocky shores are periodically exposed to direct solar radiation when emersed by the tide. During the hot and wet season rock surface temperatures can exceed 60 °C on south-facing shores in Hong Kong as, at this time of the year, low tides occur in the afternoon exposing the shores to intense solar radiation (Fig. 1). 

Fig. 1. Time series of rock surface temperatures on a south-facing rocky shore in Hong Kong at the splash zone where the high shore littorinid Echinolittorina malaccana is present (typically at ~ 3 – 4 m above Chart Datum) – reaching over 50 °C every day for at least a week and attain a maximum of 64.5 °C. Seawater temperature is approx. 28 °C and temperatures above this represent emersion periods when the tide is out.

This combination of seasonal and tidal regimes creates the perfect storm of thermal extremes, exceeding the thermal tolerances of many mobile and sessile rocky shore species (Figs. 2 & 3). As such, mass mortalities occur annually with the transition from the cool to hot season, and only species/individuals which are capable of behaviourally and/or physiologically ameliorating the extreme heat stress can survive.

Fig. 2. Mass mortality (open shell valves) of the mussel Mytilisepta virgata on a Hong Kong rocky shore during the annual “die-off” event. Photo credit: Gray A. Williams.
Fig. 3. (A) A mixed assemblage of dead gastropods and chitons in a rock pool during the Hong Kong summer. (B) Various species killed during the onset of summer in Hong Kong (from left to right, Liolophura japonica; Monodonta labio; Cellana toreuma and Siphonaria laciniosa; and Reishia clavigera). Photo credit: Gray A. Williams.

We investigated the thermal preference of a rocky shore snail, Nerita yoldii, to assess the contribution of behavioural thermoregulation to the snail’s ability to endure the thermally challenging Hong Kong summer (Fig. 4). This snail starts foraging as the tide falls and stops moving once the rock becomes hot and dry at low tide. At this time, body temperatures of the snails are correlated with the environmental conditions they experience where they stop. Therefore, choosing a suitable hiding location to survive emersion is a life-or-death decision, and individuals that choose the wrong place will die due to lethal thermal conditions.

To minimize such risk, snails choose cooler, “safe” locations on the shore as well as when given a choice in the laboratory. This choice, however, compromises both behavioural (measured using speed) and physiological (heart rate) performances of the snails, as the snails choose temperatures 7 – 14 °C lower than when these performances are at their peaks. Model simulations show that this risk-averse behaviour is consistent with the application of Jensen’s inequality, where the average performance is often lower than the performance at the average temperature. As a result, time-averaged performances of the snails are predicted to be lower and peak at cooler temperatures than measured performances in the laboratory.

Fig. 4. Nerita yoldii inactive on the rock surface during a low tide period. Photo credit: Sam Crickenberger.

By choosing cooler, “suboptimal” temperatures, the snails are effectively maximizing their time-averaged performances. This choice avoids severe performance losses that would occur if the snails were caught on hot rock surfaces which heated up rapidly during low tide. The decision of which temperatures (habitats) to choose requires the snails to be able to assess temporal changes in environmental conditions (i.e. to be able to assess their past thermal history). If the snails fail to do so, they will underestimate on-shore temperature extremes and make potentially lethal decisions. Risk minimization based on sufficient thermal history and choosing “suboptimal” temperatures are, therefore, effective strategies to survival thermally variable and extreme environments such as Hong Kong rocky shores.

Read the paper

Read the full paper here: Hui, T. Y., Crickenberger, S., Lau, J. W. & Williams, G. A. (2022). Why are ‘suboptimal’ temperatures preferred in a tropical intertidal ectotherm?. Journal of Animal Ecology, 00, 1– 16. https://doi.org/10.1111/1365-2656.13690

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