Thermal Tolerance in Marine Ectotherms

Marine ectotherms—from intertidal invertebrates to pelagic fishes—rely on external temperatures to regulate physiological processes. Thermal tolerance defines the range of temperatures within which these organisms can perform essential functions such as respiration, locomotion and reproduction. Upper tolerance limits are determined by the capacity to supply oxygen and maintain protein integrity under heat stress, while lower limits reflect the ability to sustain metabolic activity in cold waters. Species from variable habitats (for example intertidal zones) often exhibit broader tolerance windows than those from more stable thermal regimes (for example deep sea or polar environments). Acclimation and adaptation play key roles: some populations adjust their metabolic scope over weeks or generations, whereas others show limited plasticity and face increased risk as ocean temperatures rise. Understanding these limits is critical for predicting shifts in distribution, community structure and ecosystem services under climate change.

Research from Nature Portfolio

Recent studies have revealed that large predatory reef fishes adjust their feeding behaviour and meal size when confronted with elevated temperatures, yet these compensatory responses may be insufficient to meet soaring metabolic demands as prey biomass declines with coral degradation. Another study in polar waters demonstrated that several Antarctic teleost species possess narrow thermal safety margins; their gill ventilation rates and cardiac performance swiftly become constrained above modest warming, leading to altered swimming behaviour and habitat use. Together, these findings emphasise that even species capable of short-term acclimation may be threatened by the combined effects of increased temperature variability and diminished resource availability.

Thermal Tolerance in Marine Ectotherms publication trend

The graph below shows the total number of articles in thermal tolerance in marine ectotherms across all publications each year (not limited to Nature Index journals).

Technical terms

Critical thermal maximum (CTmax): The temperature at which an ectotherm loses coordinated movement, signalling the upper limit of acute tolerance.

Thermal safety margin: The difference between an organism’s CTmax and the highest temperature encountered in its habitat.

Aerobic scope: The difference between maximal and resting metabolic rates, indicating the energy available for activity beyond basic maintenance.

Oxygen- and capacity-limited thermal tolerance (OCLTT): A framework describing how warming narrows the performance window by impairing oxygen supply relative to demand.

Acclimation: Short-term physiological adjustments that modify thermal tolerance within an individual’s lifetime.

References

1. Marine heatwaves impact mortality of triploid Pacific oysters. Global Change Biology (2023).
2. Ecological traps in shallow coastal waters—Potential effect of heat-waves in tropical and temperate organisms. PLOS ONE (2018).
3. Large predatory coral trout species unlikely to meet increasing energetic demands in a warming ocean. Scientific Reports (2015).
4. Molecular Response to Extreme Summer Temperatures Differs Between Two Genetically Differentiated Populations of a Coral Reef Fish. Frontiers in Marine Science (2018).