Final answer:
Fish use countercurrent exchange in their gills to maximize oxygen exchange with water.
Step-by-step explanation:
The type of breather that uses a countercurrent exchange mechanism is fish. Fish use countercurrent exchange in their gills to maximize the exchange of oxygen and carbon dioxide with water.
In countercurrent exchange, the water flow and blood flow in the gills move in opposite directions. As a result, oxygen-rich water flows past blood vessels that are depleted in oxygen, allowing for the efficient exchange of gases.
This mechanism is important for fish because it allows them to extract as much oxygen as possible from water, enabling them to live and thrive in aquatic environments.