A review on structure and functions of branchial gills in fishes

Fish gills represent one of the most functionally diverse and morphologically specialized organs among aquatic vertebrates, serving as the primary interface between internal physiology and the external aquatic environment. Beyond their classical role in respiration, gills perform multiple essential functions including ion and water regulation, acid–base balance, nitrogenous waste excretion, filter feeding, immune defense, and maintenance of overall homeostasis. These various physiological functions are supported by the structural complexity of the gills, which includes gill arches, gill rakers, filaments, and lamellae. Variations in gill morphology among species are strongly correlated with ecological niche, feeding strategies, and environmental conditions. Gill rakers, for instance, demonstrate remarkable adaptive diversity, ranging from elongated and densely packed structures in plankton feeders to short, spine-like projections in carnivores, reflecting the filtration and feeding mechanisms employed by the species . Similarly, the micro-anatomy of gill arches and filaments adapts to modulate water flow, optimize gas exchange efficiency, and reduce mechanical stress from the aquatic medium.

The gill is also a critical barrier organ whose epithelial surfaces form microbiological, chemical, physical, and immunological defense layers.

These surfaces are highly sensitive to fluctuations in temperature, salinity, ion concentration, pollution, and water quality. Environmental stressors, alongside dietary factors such as nutrient composition and feed formulation, strongly influence gill morphology and functional efficiency. Evidence indicates that nutritional modulation provides a more readily controllable strategy for enhancing gill integrity compared to environmental modification. This review synthesizes current scientific understanding of gill structure, function, and adaptive diversity, and critically evaluates the influence of feeding and environmental parameters on gill health. By integrating morphological, physiological, and ecological perspectives, the review underscores the significance of gill biology in fish growth, survival, and aquaculture productivity, providing a foundational framework for future research and sustainable fish health management.