Stillman, Jonathon H., Adrienne B. Amri, Joe M. Holdreith, Alexis Hooper, Rafael V. Leon, Liliana R. Pruett, and Buck M. Bukaty. 2025. “Ecophysiological Responses to Heat Waves in the Marine Intertidal Zone.” Journal of Experimental Biology 228 (2): JEB246503.
The intertidal zone is one of the most dynamic environments on our planet, home to barnacles, mussels, sea stars, and many other resilient marine organisms. These organisms have developed and evolved resilience in order to survive in the constantly shifting temperature, moisture, and wave action as the tides ebb and flow in the intertidal. However, as heatwaves become more frequent and intense due to global warming, these fluctuations are becoming more extreme, putting intertidal life at risk.
Rising Temperatures, Rising Risks
Climate change has led to an increase in extreme temperatures events, known as heatwaves. In the ocean, marine heatwaves (MHWs) occur when sea surface temperatures are higher than 90% of past temperatures for at least five consecutive days. On land, atmospheric heatwaves (AHWs) are defined as abnormally hot temperatures that last at least two consecutive days. Both types of heatwaves have become more frequent and last for longer durations over the past century, posing major risks to biodiversity and ecosystem function, especially in the intertidal zones which can experience both MHWs and AHWs – often simultaneously.
Different species occupy distinct vertical zones within the intertidal habitat, and their exposure to heatwaves depends on their placement. Those higher up are more likely to endure AHWs because they spend more time exposed to air, while species lower in the zone are at greater risk from prolonged MHWs, as they remain immersed in warming waters. Regardless of their position, all intertidal organisms are now facing stress levels that could push their physiological limits beyond survival.
For example, during an extreme Pacific Northwest heatwave in June 2021, 74% of oysters and 81% of barnacles perished due to well above average air and water temperatures. Physiological stress from heat exposure can lead to weakened immune systems, making organisms more vulnerable to diseases and parasites. For instance, sea star wasting disease has been linked to MHW conditions. Additionally, behavioural impairments have been observed in marine gastropods, which react more slowly to extreme temperatures, potentially reducing their chances of survival in heatwave conditions.
Adaptation and Evolution Under Heat Stress
Despite extreme environmental fluctuations due to the heatwaves, some intertidal organisms show remarkable resilience. Many have evolved heat-hardening mechanisms, where exposure to high temperatures has strengthened their ability to tolerate future heatwaves. Studies on California mussels and limpets have shown that brief exposure to high temperatures can boost their thermal tolerance by activating protective heat shock proteins.
Extreme temperatures caused by heatwaves have been shown to disrupt cellular processes, increasing mutation rates and altering gene activity, which can lead to offsprings better adapted to heat stress than their parents who experiences heatwave conditions. Some species, such as tidepool corals, have already demonstrated superior survival abilities under heatwave conditions, indicating that natural selection may favour those best equipped to endure rising temperatures.
Threats to Reproduction and Ecosystem Balance
However, not all adaptations are beneficial in the long term. Sea star larvae that have been exposed to extreme temperatures develop faster during MHWs, but suffer higher mortality rates and abnormal body structures. This is just one example of how heatwaves can disrupt reproductive processes, leading to fewer, smaller, and lower-quality eggs and sperm – which could impact the survival of entire species. These reproductive challenges, combined with shifts in species distributions and predator-prey relationships, can restructure entire ecosystems.
The Future of Intertidal Life
As heatwaves become more frequent and severe, understanding their impact on intertidal organisms is a pressing scientific challenge. Researchers are working to predict when and where these extreme events will occur and how they might alter populations and ecosystems over time. While some species may adapt through heat hardening and genetic changes, others may struggle to survive. The survival of these ecosystems will depend on how well intertidal organisms can withstand the rising tide of climate change—or whether the heat will ultimately reshape life along the shore.
I am a PhD student at the University of Chicago and the Marine Biological Lab, currently studying germline development and regeneration in the amphipod crustacean, Parhyale hawaiensis. I did my undergraduate degree in India and did my Masters in Oceanography at the University of Massachusetts (during which I participated in multiple month-long research cruises out in the Pacific and Atlantic Ocean!). I am broadly interested in integrating ecology with developmental biology in marine organisms and I hope to comprehend the fundamental interconnectedness of the mysteries that swim in the Earth’s oceans. I am also an illustrator and a PADI certified diver.
