Physiological Mechanisms in Aquatic Vertebrates

Aquatic vertebrates exhibit a suite of specialised physiological adaptations that support life in water environments with variable oxygen, temperature and salinity. Central to these adaptations is the regulation of acid–base balance and respiratory gas transport, achieved through dynamic control of intracellular and extracellular pH, modulation of haemoglobin–oxygen affinity and coordinated action of ion transporters across gill, renal and vascular epithelia. Cardiovascular control mechanisms, including selective branchial ventilation, modulation of cardiac shunting and morphological remodelling of heart and vasculature, further optimise oxygen delivery under fluctuating environmental conditions.

Oxygen transport is fine‐tuned by molecular properties of haemoglobin that exhibit pH sensitivity, such as the Bohr and Root effects, allowing fishes to unload oxygen efficiently during stress or visual tasks. Cellular acid–base sensors and transporters, including soluble adenylyl cyclase and membrane proton pumps, integrate CO₂ and bicarbonate signals to regulate intracellular pH and drive oxygen release in red blood cells and specialised tissues like the retina. Developmental plasticity of mitochondrial function in reptilian embryos and juveniles also reflects the enduring impact of early hypoxic exposure on metabolic efficiency and organ performance.

Advances in our understanding of these mechanisms are vital for conservation of wild stocks, prediction of species responses to climate‐driven hypoxia and the optimisation of aquaculture practices. By elucidating the interplay between molecular sensors, haemoglobin function and cardiovascular control, researchers are uncovering unifying principles that govern physiological resilience across diverse groups of aquatic vertebrates.

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Physiological Mechanisms in Aquatic Vertebrates publication trend

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Technical terms

Bohr effect: The reversible decrease in haemoglobin–oxygen affinity in response to lower pH or higher CO₂ concentration, promoting oxygen release in tissues.

Root effect: An extreme pH‐dependent reduction in both haemoglobin–oxygen affinity and carrying capacity, enabling high‐level oxygen secretion in specialised organs.

Soluble adenylyl cyclase (sAC): A cytoplasmic enzyme activated by bicarbonate that regulates intracellular pH and downstream transport processes.

Vacuolar‐type H⁺‐ATPase: A proton pump that acidifies cellular compartments or extracellular microenvironments, crucial for pH‐driven oxygen release.

Intracellular pH (pHi): The pH within cells, regulated by transporters and enzymes to ensure optimal enzyme activity and ion balance.

References

1. Soluble adenylyl cyclase is an acid‐base sensor in rainbow trout red blood cells that regulates intracellular pH and haemoglobin–oxygen binding. Acta Physiologica (2024).
2. Developmental plasticity of mitochondrial function in American alligators, Alligator mississippiensis. AJP Regulatory Integrative and Comparative Physiology (2016).
3. Root Effect Haemoglobins in Fish May Greatly Enhance General Oxygen Delivery Relative to Other Vertebrates. PLOS ONE (2015).
4. A novel acidification mechanism for greatly enhanced oxygen supply to the fish retina. eLife (2020).