A Review of Respiratory Adaptations in Aquatic and Terrestrial Chordates

Abstract

Respiration is a fundamental physiological process that enables organisms to meet their metabolic energy demands through efficient gas exchange with the environment. In chordates, respiratory adaptations have evolved in close association with habitat transitions from aquatic to terrestrial ecosystems, resulting in diverse structural and functional mechanisms for oxygen uptake. This review synthesizes current knowledge on respiratory adaptations across major chordate groups, highlighting the evolutionary modifications in respiratory organs, including gills, lungs, skin, and accessory air-breathing structures. Aquatic chordates primarily utilize gill-based respiration, characterized by extensive vascularized lamellae and countercurrent exchange systems that maximize oxygen extraction from water. Transitional forms such as amphibians exhibit dual respiratory modes, combining cutaneous, buccopharyngeal, and pulmonary respiration, reflecting their amphibious lifestyle. In contrast, terrestrial chordates, including reptiles, birds, and mammals, possess increasingly complex lung architectures and ventilation strategies adapted to aerial respiration and higher metabolic demands. Birds demonstrate highly efficient unidirectional airflow supported by air sacs, whereas mammals rely on alveolar lungs and diaphragmatic breathing. Comparative analysis reveals that respiratory evolution is closely linked to environmental oxygen availability, metabolic rate, and ecological specialization. Respiratory adaptations in chordates illustrate a progressive evolutionary trend that has facilitated successful colonization of diverse habitats, emphasizing the integral role of respiratory physiology in vertebrate adaptation, survival, and diversification.