Practice question

Fish respiration primarily involves branchial gills supported by pharyngeal arches. Each gill arch bears filaments further divided into lamellae containing dense capillary networks. Water flows unidirectionally, driven by buccal-opercular pumping in osteichthyans or ram ventilation, counter to blood flow within lamellae, maximizing oxygen extraction via countercurrent exchange with efficiency exceeding 80%. Lungs are present only in dipnoan lungfishes for air breathing, skin respiration minor, tracheae characteristic of insects. Such branchial adaptation supports active swimming, reflecting aquatic oxygen availability and metabolic demands of ectothermic vertebrates living in water.