Clime and Tide

At low tide, marine organisms are exposed to thermal extremes,” said Professor Gray A Williams, researcher at SWIMS and Professor in the School of Biological Sciences. “The rocks can heat up from around 28 degrees Celsius, which is when they are in seawater, to over 55 degrees Celsius when the tide goes out and this can happen in two hours. The marine creatures living on the rock heat up and start to lose water – basically, they’re cooking on the hot rocks.

“This puts a huge physiological stress on these organisms, and we’re interested in the maximum heat they can tolerate, and asking what is the potential to increase that tolerance? How do they actually manage and cope with thermal stress?”

The study is not confined to Hong Kong’s shores: SWIMS has been collaborating with researchers from Singapore to Taiwan for nearly 10 years, looking at how high-shore snails use metabolic depression to combat high temperatures.

“A lot of data is building up now through these large-scale projects, linking different researchers throughout the region and also introducing these approaches to other areas such as South Africa, where we have deployed data loggers to measure shore temperatures and also recorded limpet thermal tolerance,” said Professor Williams.

“There is an assumption that places nearer the equator are the hottest, but we are finding that this is not necessarily so along the seashore. The pattern is not latitudinal but varies according to local conditions such as the time of day that the tide is low.”

The main focus of the research is on extreme high-shore organisms such as oysters, limpets and small snails called periwinkles which exist at the very top of the tide, “almost on land but splashed by the sea”.

“Our researchers are discovering that each of the three organisms deals with the heat in a different way,” said Professor Williams. “We call it fight or flight responses – the snails can move so one of their choices is flight, to move away from the heat stress and hide in cooler areas.

“Oysters are unable to move and limpets have limited movement, so their methods to beat the heat are different. Oysters form dense aggregations which we think helps them stay cool, and like the snails they can depress their metabolism during the hottest times, lowering their heart rates to save energy.

“Limpets are between the two, in that they can move but only do so in the winter when it is cool. To survive the summer, they basically shut themselves down for three months and go into aestivation – which is the summer equivalent of hibernation in winter – bears do it when the weather turns cold and limpets do it when the weather turns hot.”

Snail towers

Dr Sarah Lau, a researcher who studied the physiological and behavioural adaptations of periwinkles as part of the SWIMS research, said that, as well as flight, the snails employ simple fight behaviours to stay alive by regulating their temperature where they live, such as raising their bodies from the rocks and standing on top of each other to form ‘snail towers’.

“By doing this they are effectively reducing the amount of contact they have with the hot rocks,” said Dr Lau. “And then they’re also exposing themselves to moving air currents so that they can be cooled down more effectively. They can lower their body temperature from the surrounding rock by up to 10 degrees Celsius doing this, which can mean the difference between survival or death.”

Metabolic depression is another crucial survival trait for some intertidal species. Some periwinkles are able to depress their metabolic rates, so they can conserve energy to cope with prolonged, stressful periods of emersion. The collaborative project looked at how their use of metabolic depression varies across populations from Singapore to Taiwan, and to what extent such a trait is determined genetically and how much by environmental influences – that is, ‘nature versus nurture’.

With global temperatures continuing to rise, the next key question is to ask how effective these different strategies are. And in the future which of these species will be winners and survive, or losers and die off in hot areas? The findings of SWIMS researchers on the flexibility of these various strategies, such as conserving energy via metabolic depression, are optimistic: animals are not ‘prisoners’ to climate change and they are currently able to modify their behaviour and physiology to cope with the challenges.