Cartilaginous fish respire using what structure?

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

Answer: (B)

Cartilaginous fish respire primarily through gills. Water flows over the gill filaments, where thin membranes with a rich capillary network enable gas exchange: oxygen diffuses into the blood and carbon dioxide diffuses out. The arrangement, often countercurrent, keeps a favorable oxygen gradient along the entire gill, making oxygen uptake efficient even when water doesn’t carry much dissolved oxygen. Lungs aren’t present in these fish, so they don’t use air-breathing organs. The skin can contribute a little gas exchange in some species, but it’s not the main source of respiration. Tracheae are part of insect respiration, not fish.