What is the respiratory system of bony fish?

• A. Lungs
• B. Gills
• C. Skin
• D. Tracheae

Answer: (B)

Bony fish breathe primarily through gills. Water flows into the mouth, passes over gill filaments covered by lamellae, and exits through the opercular openings. The gill filaments provide a large surface area rich in capillaries, allowing oxygen from the water to diffuse into the blood and carbon dioxide to diffuse out. The flow of water over the gills is typically countercurrent to the blood flow, which maximizes oxygen uptake even when water oxygen is low. While some fish can approximate respiration through the skin or have lung-like organs in specific lineages, the main respiratory organ for most bony fish is the gills. Lungs belong to air-breathing vertebrates, tracheae are part of insect respiration, and skin-based respiration is only supplementary in some species.