What respiratory feature do jawless fish use?

• A. Gills with countercurrent exchange
• B. Lungs
• C. Skin diffusion
• D. Tracheae

Answer: (A)

Jawless fish rely on gills for respiration. Water flows over gill filaments, which have a large surface area for gas exchange. The blood in the gill capillaries moves in the opposite direction to the water, a setup known as countercurrent exchange. This arrangement maintains a favorable gradient for oxygen to diffuse into the blood along most of the gill surface, making oxygen uptake efficient even when water oxygen levels are relatively low, and it also helps expel carbon dioxide. They don’t have lungs or tracheae, which are found in other groups, and while some diffusion across the skin can occur in other animals, it isn’t the primary respiratory mechanism in jawless fish.