Why Some Coral Reef Fishes are More Resilient to Heat than Others

By Muhammad OsamaReviewed by Laura ThomsonMar 12 2025

In an article recently published in the journal Global Change Biology, researchers comprehensively examined the physiological responses of coral reef fishes to extreme thermal variability in the Arabian/Persian Gulf, the hottest and most thermally variable coral reef habitat. Their goal was to analyze adaptations to rising sea temperatures and increasing thermal extremes, providing insights into the resilience of these species to climate change.

Impact of Climate Change on Marine Ecosystems

Coral reefs are essential ecosystems that support marine biodiversity, regulate nutrient cycling, and sustain fisheries. However, climate change threatens these ecosystems by driving rising sea temperatures and increasing thermal variability. Coral reef fishes have evolved under stable thermal conditions and function optimally within low-temperature ranges. As temperatures rise, many species become vulnerable, making it key to understand the physiological mechanisms that enable them to adapt to temperature fluctuations.

Fish's physiological responses to temperature changes often involve metabolic adjustments. Concepts like "plastic floors" and "concrete ceilings" describe how fish may exhibit flexibility in basal metabolic rates ("floors") while maintaining fixed maximum metabolic rates ("ceilings"). This study assesses whether coral reef fishes in the Arabian/Persian Gulf exhibit similar physiological responses to extreme thermal variability.

Investigating Different Reef Fishes

This paper assessed the thermal tolerance and metabolic performance of three tropical reef fish species, including Ecsenius pulcher, Scolopsis ghanam, and Cheilodipterus novemstriatus. These species were selected for their high local abundance and ecological diversity. The study compared populations from the Arabian/Persian Gulf, where temperatures range from 18 °C to 36.5 °C, with conspecifics from the Gulf of Oman, which experiences a more stable thermal range of 21 °C to 32 °C.

To investigate thermal physiology, researchers conducted field and laboratory experiments. Fish were collected from two reefs in each region, ensuring comparable environmental conditions. Intermittent flow respirometry was used to measure oxygen uptake rates, providing insights into standard metabolic rates (SMR), maximum metabolic rates (MMR), and aerobic scope (AS). Critical thermal maxima (CT_max) and minima (CT_min) were determined by gradually adjusting water temperatures until fish exhibited loss of equilibrium.

Key Findings and Insights

The outcomes showed significant differences in thermal tolerance between fish populations. Fishes from the Arabian Gulf exhibited higher upper thermal limits, with the Ecsenius pulcher demonstrating a critical thermal maximum of 39.55 °C compared to 39.10 °C in its Gulf of Oman counterparts. However, summer thermal safety margins (TSMs) were 1.47 °C lower in Arabian Gulf fishes, indicating increased vulnerability to further thermal extremes.

Interestingly, the authors found no significant changes in basal metabolic rates among Arabian Gulf fishes, contradicting the "plastic floors and concrete ceilings" hypothesis. This suggests that while these fishes can withstand higher temperatures, their metabolic flexibility remains limited, potentially affecting their long-term acclimation.

Additionally, Arabian Gulf fishes displayed broader thermal performance curves for aerobic scope, enabling them to maintain physiological function across a wider temperature range. However, some species, such as Cheilodipterus novemstriatus, exhibited reduced AS at peak temperatures, which may limit their energy intake and overall fitness. These results highlight potential biodiversity declines in the Arabian Gulf, where fish diversity is already lower than in the Gulf of Oman.

Practical Applications

This research has significant implications for coral reef conservation and management strategies. Understanding reef fishes' thermal limits and metabolic responses can help predict how these species might respond to future climate change. While some species exhibit adaptive capacities, the reduced thermal safety margins raise concerns about their vulnerability to extreme thermal events, such as marine heatwaves.

These findings highlight the need to integrate predicted thermal variability into conservation planning and management practices. Preserving thermal refuges, areas that provide stable conditions for marine life, could be crucial for enhancing the resilience of reef ecosystems. Continued research into the adaptive capacities of marine species will help inform policies to mitigate climate change's impacts on marine biodiversity.

Future Directions for Coral Reef Ecosystems

This study provides valuable insights into the thermal physiology of coral reef fishes in the Arabian/Persian Gulf. While some species exhibit enhanced thermal limits, their reduced thermal safety margins raise concerns about their long-term resilience to climate change. These findings highlight coral reef fishes' adaptive capacities and potential vulnerabilities as ocean temperatures continue to rise.

Future work should focus on genetic and epigenetic factors influencing thermal tolerance and the broader ecological implications for reef biodiversity. Long-term monitoring of these species under varying thermal conditions will be key to informing conservation efforts and developing strategies to protect marine life in an era of rapid environmental change.

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