From Gills to Tissues: An Oxygen-Cascade Perspective on Metabolic Scaling in Fishes

Data of experimental, comparative, and allometric studies was examined in relation to the wider oxygen transport cascade, including branchial ventilation, diffusion across the water–blood barrier, blood oxygen transport, muscle capillarization, and mitochondrial content. Experimental reductions in functional gill area are associated with lower maximal oxygen consumption and reduced critical swimming speed, whereas reversible gill remodelling may increase exposed lamellar surface under hypoxia or increased oxygen demand. Comparative evidence indicates that gill area accounts for a measurable, but incomplete, component of variation in metabolic rate and hypoxia tolerance. A descriptive comparison of ten published paired gill surface area (GSA) and standard metabolic rate (SMR) datasets did not reveal a consistently negative difference between gill surface area and standard metabolic demand across the species and experimental conditions examined. This comparison addresses maintenance metabolism but does not test oxygen-supply constraints during routine or maximal activity. The broader evidence reviewed includes large salmonids with cardiovascular compensation, cyprinids with reversible gill remodelling, haemoglobinless icefishes, and active pelagic taxa with high cardiorespiratory and locomotor investment. Collectively, these examples show that the functional significance of gill area depends on interacting branchial, cardiovascular, haematological, tissue-level, and ecological traits.