Authors: Sanchali Sarkar, Dr. Anand Sur
Abstract: Fishes are often introduced as archetypal water breathers, yet a substantial fraction of extant species routinely combine branchial gas exchange with air breathing through lungs, modified swim bladders, buccopharyngeal surfaces, gut epithelia, or accessory organs. This dual (bimodal) respiratory strategy is widely recognized, but its functional significance is still unevenly explained across evolutionary history, comparative physiology, and ecology. A key gap in the literature is the tendency to treat air breathing as a simple hypoxia response rather than as a multi-trait adaptive complex that restructures ventilatory control, cardiovascular function, ionoregulation, and life history. This review synthesizes evidence on why dual respiration evolved repeatedly, how it operates mechanistically, and what tradeoffs it imposes. Using a comparative, concept-driven review framework, I integrate studies on (i) selective pressures that favor aerial supplementation (environmental hypoxia, hypercapnia, temperature, drought, and episodic habitat instability), (ii) physiological partitioning of oxygen uptake and carbon dioxide excretion between gills and air breathing organs, (iii) cardiovascular and blood oxygen transport adjustments that enable effective bimodal exchange, and (iv) ecological consequences including niche expansion, resilience to climate-driven deoxygenation, and evolutionary stepping stones toward amphibious lifestyles. The central argument is that dual respiration is best understood as an evolutionary solution to variable oxygen landscapes that is maintained by conditional benefits and constrained by costs such as surface predation risk, energetic demands of ventilation, and tensions between gill reduction and ionoregulatory capacity. The review proposes a conceptual model linking environment, organ design, control systems, and performance outcomes, and identifies priorities for future research in a rapidly warming and deoxygenating world.
DOI: https://doi.org/10.5281/zenodo.18812132
